ACTAUNIVERSITATIS

UPSALIENSISUPPSALA

2020

Digital Comprehensive Summaries of Uppsala Dissertationsfrom the Faculty of Science and Technology 1886

Towards atomically resolvedmagnetic measurements in thetransmission electron microscope

A study of structure and magnetic moments in thinfilms

HASAN ALI

ISSN 1651-6214ISBN 978-91-513-0830-2urn:nbn:se:uu:diva-398561

Dissertation presented at Uppsala University to be publicly examined in Room 80101,Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, Friday, 7 February 2020 at 09:15 forthe degree of Doctor of Philosophy. The examination will be conducted in English. Facultyexaminer: Professor Peter Van Aken (Max Planck Institute for Solid State Research, Stuttgart,Germany).

AbstractAli, H. 2020. Towards atomically resolved magnetic measurements in the transmissionelectron microscope. A study of structure and magnetic moments in thin films. DigitalComprehensive Summaries of Uppsala Dissertations from the Faculty of Science andTechnology 1886. 83 pp. Uppsala: Acta Universitatis Upsaliensis. ISBN 978-91-513-0830-2.

The magnetic properties of thin metallic films are significantly different from the bulk propertiesdue to the presence of interfaces. The properties shown by such thin films are influenced bythe atomic level structure of the films and the interfaces. Transmission electron microscope(TEM) has the potential to analyse the structure and the magnetic properties of such systemswith atomic resolution. In this work, the TEM is employed to characterize the structure of theFe/V and Fe/Ni multilayers and the technique of electron magnetic circular dichroism (EMCD)is developed to obtain the quantitative magnetic measurements with high spatial resolution.

From TEM analysis of short period Fe/V multilayers, a coherent superlattice structure isfound. In short period Fe/Ni multilayer samples with different repeat frequency, only the TEMtechnique could verify the existence of the multilayer structure in the thinnest layers. Themethods of scanning TEM imaging and electron energy loss spectroscopy (EELS) results wereused and refined to determine interdiffusion at the interfaces. The confirmation of the multilayerstructure helped to explain the saturation magnetization of these samples.

Electron magnetic circular dichroism (EMCD) has the potential to quantitatively measure themagnetic moments of the materials with atomic resolution, but the technique presents severalchallenges. First, the EMCD measurements need to acquire two EELS spectra at two differentscattering angles. These spectra are mostly acquired one after the other which makes it difficultto guaranty the identical experimental conditions and the spatial registration between the twoacquisitions. We have developed a technique to simultaneously acquire the two angle-resolvedEELS spectra in a single acquisition. This not only ensures the accuracy of the measurementsbut also improves the signal to noise ratio as compared to the previously used methods. Thesecond important question is the effect of crystal orientations on the measured EMCD signals,considering the fact that the crystal orientation of a real crystal does not remain the samein the measured area. We developed the methodology to simultaneously acquire the EMCDsignals and the local crystal orientations with high precision and experimentally showed thatthe crystal tilt significantly changes the magnetic signal. The third challenge is to obtain EMCDmeasurements with atomic resolution which is hampered by the need of high beam convergenceangles. We further developed the simultaneous acquisition technique to obtain the quantitativeEMCD measurements with beam convergence angles corresponding to atomic size electronprobes.

Keywords: Transmission electron microscope, thin films, magnetic moments, electron energyloss spectroscopy, simultaneous acquisition, electron magnetic circular dichroism

Hasan Ali, Department of Engineering Sciences, Applied Materials Sciences, Box 534,Uppsala University, SE-75121 Uppsala, Sweden.

© Hasan Ali 2020

ISSN 1651-6214ISBN 978-91-513-0830-2urn:nbn:se:uu:diva-398561 (http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-398561)

Dedicated to my parents

List of Papers

This thesis is based on the following papers, which are referred to in the text by their Roman numerals.

I M. van Sebille, A. Fusi, L. Xie, H. Ali, R. A. C. M. M. van

Swaaij, K. Leifer, and M. Zeman, "Shrinking of silicon nanocrystals embedded in an amorphous silicon oxide matrix during rapid thermal annealing in a forming gas atmosphere," Nanotechnology, vol. 27, pp. 365601-365601, 2016.

II S. A. Droulias, G. K. Pálsson, H. Palonen, A. Hasan, K. Leifer, V. Kapaklis, B. Hjörvarsson, and M. Wolff, "Crystal perfection by strain engineering: The case of Fe/V (001)," Thin Solid Films, vol. 636, pp. 608-614, 2017.

III A. Frisk, H. Ali, P. Svedlindh, K. Leifer, G. Andersson, and T. Nyberg, "Composition, structure and magnetic properties of ultra-thin Fe/Ni multilayers sputter deposited on epitaxial Cu/Si(001)," Thin Solid Films, vol. 646, pp. 117-125, 2018.

IV H. Ali, T. Warnatz, L. Xie, B. Hjörvarsson, and K. Leifer, "Quantitative EMCD by use of a double aperture for simultaneous acquisition of EELS," Ultramicroscopy, vol. 196, pp. 192-196, 2019.

V H. Ali, J. Rusz, T. Warnatz, B. Hjörvarsson, and K. Leifer, “Simultaneous mapping of EMCD signals and local crystal orientations in a transmission electron microscope”. Submitted

VI H. Ali, D. Negi, T. Warnatz, B. Hjörvarsson, J. Rusz and K. Leifer, “Atomic resolution electron probe magnetic circular dichroism measurements enabled by patterned apertures”, Sub-mitted

VII H. Ali, J. Eriksson, H. Li, S. H. M. Jafri, M. S. S. Kumar, J. Ögren, V. Ziemann, and K. Leifer, "An electron energy loss spectrometer based streak camera for time resolved TEM measurements," Ultramicroscopy, vol. 176, pp. 5-10, 2017.

Reprints were made with permission from the respective publishers.

Additional papers/conference contributions

I. G. Omanakuttan, O. M. Sacristán, S. Marcinkevičius, T. K. Uždavinys, J. Jiménez, H. Ali, K. Leifer, S. Lourdudoss, and Y.-T. Sun, "Optical and interface properties of direct InP/Si heterojunction formed by corrugated epitaxial lateral overgrowth," Optical Materials Express, vol. 9, pp. 1488-1488, 2019.

II. T. Warnatz, F. Magnus, S. Sanz, H. Ali, K. Leifer and B. Hjörvarsson, “The preliminary title is: Long-range interlayer exchange coupling in Fe/MgO[001] multilayers”, to be sub-mitted

III. H. Ali, T. Warnatz, L. Xie, B. Hjörvarsson, and K. Leifer, "Towards Quantitative Nanomagnetism in Transmission Elec-tron Microscope by the Use of Patterned Apertures," Micros-copy and Microanalysis, vol. 25, pp. 654-655, 2019.

IV. H. Ali, L. Xie, M. v. Sebille, A. Fusi, R. e. A. C. M. M. v. Swaaij, M. Zeman, and K. Leifer, "TEM analysis of multi-layered nanostructures formed in the rapid thermal annealed silicon rich silicon oxide film," Poster presentation , The 16th European Microscopy Congress (EMC), Lyon, France, 2016.

V. H. Ali, T.Warnatz, L. Xie, B. Hjörvarsson and K. Leifer, “Towards a quantitative EMCD analysis”, Digital poster presentation, 19th International Microscopy Congress (IMC), Sydney, Australia, 2018.

VI. H. Ali, T.Warnatz, L. Xie, B. Hjörvarsson and K. Leifer, “Use of custom-fabricated aperture to acquire the EMCD signals with high precision and improved S/N”, Oral presentation, 4th International Workshop on TEM Spectroscopy in Materi-als Science (TEMSpec), Uppsala, Sweden, 2019.

VII. H. Ali, T.Warnatz, L. Xie, B. Hjörvarsson and K. Leifer, “Towards quantitative determination of magnetic moments in transmission electron microscope”, Oral presentation, Euro-pean Congress and Exhibition on Advanced Materials and Processes (EUROMAT), Stockholm, Sweden, 2019.

Contents

1  Introduction and motivation ................................................................ 13 

2  Instruments and techniques .................................................................. 15 2.1  TEM sample preparation ................................................................. 17 2.2  Imaging and diffraction techniques ................................................. 19 2.3  Electron energy loss spectroscopy (EELS) ..................................... 23 2.4  Details of EMCD experiments and analysis ................................... 26 

2.4.1  Setting up the EMCD experiments on TEM .......................... 26 2.4.2  Data acquisition ..................................................................... 28 2.4.3  Post processing ...................................................................... 29 

2.5  Summary ......................................................................................... 31 

3  Magnetic thin films .............................................................................. 33 3.1  Deposition and characterization techniques used by the collaborators ............................................................................................. 34 3.2  Structural characterization in the TEM ........................................... 35 3.3  Summary ......................................................................................... 37 

4  Electron magnetic circular dichroism (EMCD) ................................... 39 4.1  EMCD measurements in a TEM ..................................................... 41 

4.1.1  Why electrons instead of X-rays ............................................ 43 4.1.2  EMCD sum rules ................................................................... 44 

4.2  Development of the EMCD technique ............................................ 45 4.3  Major challenges ............................................................................. 48 4.4  Summary ......................................................................................... 49 

5  Towards quantitative EMCD with atomic resolution .......................... 50 5.1  Quantitative EMCD by simultaneous acquisition of EELS ............ 50 

5.1.1  q-E experimental setup .......................................................... 53 5.1.2  Acquisition of EMCD signals using a double aperture.......... 55 

5.2  Simultaneous mapping of EMCD signals and local crystal orientations ............................................................................................... 57 5.3  Enabling the EMCD measurements with atomic resolution electron probes ......................................................................................... 58 5.4  Summary ......................................................................................... 62 

6  Time resolved magnetic measurements ............................................... 64 6.1  Currently applied time resolved TEM setups .................................. 64 6.2  Time resolved measurements in TEM using a streak camera ......... 65 6.3  Summary ......................................................................................... 68 

7  Conclusions and outlook ...................................................................... 69 

Sammanfatting på Svenska ........................................................................... 71 

Acknowledgements ....................................................................................... 73 

References ..................................................................................................... 75 

Abbreviations

Annular dark field (ADF) Charge coupled device (CCD) Double aperture (DA) Double signal 2 holes (DS2) Double signal 7 hole (DS7) Electron energy loss spectroscopy (EELS) Electron magnetic circular dichroism (EMCD) Electron volt (eV) Energy dispersive X-ray spectroscopy (EDS) Energy filtered TEM (EFTEM) Gatan imaging filter (GIF) High angle annular dark field (HAADF) Milliradians (mrad) Monolayer (ML) Quadruple aperture (QA) Scanning transmission electron microscope (STEM) Selected area aperture (SAA) Selected area electron diffraction (SAED) Single signal 8 hole (SS8) Spectrometer entrance aperture (SEA) Spectrum imaging (SI) Transmission electron microscope (TEM) Two beam condition (2BC) X-ray absorption spectroscopy (XAS) X-ray diffraction (XRD) X-ray magnetic circular dichroism (XMCD) X-ray photoelectron spectroscopy (XPS) X-ray reflectivity (XRR) Zero loss peak (ZLP) Zone axis (ZA)

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1 Introduction and motivation

With the advances in the field of nanotechnology, the size of the devices has considerably shrunk over the last few decades. The developments in fabrica-tion technologies of magnetic materials have resulted in a significant reduc-tion in the size of magnetic devices such as magnetic hard drives and mag-netic sensors. According to the international data corporation (IDC), the global datasphere will grow from 33 zettabytes in 2018 to 175 zettabytes by 2025, with an annual increase of 61%. This huge increase in the world data demands a rapid progress in the storage capacities. As the size of the devices decrease, the storage capacity increases and this is what the researchers are aiming for the future storage devices. Recently scientists have even been able to demonstrate the use of single atoms for reading and writing the data [1, 2]. The continuously shrinking size of the devices demands for the development of characterization techniques which could investigate the structure and the magnetic properties of such devices at atomic scale.

Magnetic thin films and multilayers are important candidates for data storage devices in computers. As the thickness of a magnetic film reduces to a few (or a few tens of) atomic layers, the role of interfaces becomes important. The effects like spin-orbit coupling [3] and symmetry breaking [4] can pro-duce novel magnetic phenomena at the interfaces [5-7]. On the other hand, the quality of interfaces such as roughness [8, 9] and defects [10] at the inter-faces can significantly influence the magnetic properties of the system. The understanding of these phenomena can lead to the fabrication of novel mag-netic devices with reduced sizes but it requires the characterization tech-niques with sufficient spatial resolution to explore these effects. The trans-mission electron microscope (TEM) is a powerful analytical tool to charac-terize the materials with resolution down to sub-nm scales. Over the years, TEM has been used as an important instrument to determine the magnetic properties of materials at nanometer scales using techniques like Lorentz microscopy [11, 12], electron holography [13-15], differential phase con-trast (DPC) [16, 17] and the newly introduced technique of electron magnet-ic circular dichroism (EMCD) [18-22].

In this doctoral work, TEM has been employed to characterize the structures, interfaces and magnetic moments in magnetic thin films and multilayers. The main focus of the research work presented here is to develop the EMCD

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technique for quantitative measurements of magnetic moments with the goal to obtain magnetic measurements from single atomic columns. The major challenge with the EMCD technique is the requirement of two electron ener-gy loss spectra (EELS) which are serially acquired in a TEM. The serial acquisition of the spectra makes it challenging to guaranty the spatial regis-tration and that the same experimental conditions are maintained between the two acquisitions, thus making it nearly impossible to obtain the EMCD measurements with a spatial precision of single atoms. We take up this chal-lenge in this work and develop strategies to simultaneously acquire multiple signals in single acquisition. We implement the strategy to not only acquire the EMCD signals in single acquisition but further develop it to map the EMCD signals and local crystal orientations simultaneously.

The motivation for this work is to obtain the magnetic measurements with atomic resolution. One of the key inspirations for the discovery of EMCD in the presence of well-established XMCD technique is the prospect to achieve superior spatial resolution in the TEM, in principle the atomic resolution. To obtain the EMCD signals from single atomic columns, the measurements need to be done with large beam convergence angles in zone axis (ZA) ge-ometry. Both of these requirements are challenging to achieve, as the strength of EMCD signals become very weak at large convergence angles and the higher dynamical diffraction effects make the things complicated in ZA geometry. We take this challenge and in this work, we are able to obtain the quantitative EMCD signals in ZA with beam convergence angles suffi-ciently high to reach the atomic resolution.

This thesis is organized as follows: Chapter 2 contains the introduction to the transmission electron microscope and the experimental techniques ap-plied to produce the results presented in this thesis. The results of Paper I are included in this chapter where I employed for the first time the TEM spectroscopic and imaging techniques to understand the thin film structures. Chapter 3 is linked to the results of Papers II and III. It gives an introduc-tion to magnetic thin films, the deposition and characterization techniques used by the collaborators and describes how the TEM structural characteri-zation contributed to the understanding of these samples. Chapter 4 gives a summary and a short history of the EMCD technique for the readers to get familiar with it. In Chapter 5, the main EMCD results of this work related to Papers IV, V and VI are presented and discussed. Chapter 6 describes the concept of the time-resolved TEM setup published in Paper VII. Finally concluding remarks and an outlook for future work are given in Chapter 7.

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2 Instruments and techniques

In this doctoral work, all the experiments have been carried out on the transmission electron microscope (TEM). This chapter gives an introduction to TEM and describes the experimental techniques used in this work. Fur-thermore it includes the experimental details of the EMCD experiments.

The transmission electron microscope is a vital analytical tool with multidi-mensional analysis capabilities. It can be used for imaging the features with a spatial resolution down to single atoms. It can provide with structural in-formation in the form of electron diffraction patterns. It can be used to obtain chemical information about the specimen by electron energy loss spectros-copy (EELS) and energy dispersive x-ray spectroscopy (EDX). In the TEM equally physical properties of materials such as magnetism can be measured by techniques like EMCD, DPC, electron holography and Lorentz microsco-py. The standard TEM characterization techniques are briefly discussed in this chapter [23, 24].

In a TEM, the electrons interact with the atoms of the sample while they are transmitted through the thin specimen and the signals resulting from this interaction are used for imaging and analysis. The electrons are emitted from a filament (typically tungsten or lanthanum hexaboride) connected to a few hundred kV high voltage source. The emission of the electrons can be ob-tained by heating the filament (thermionic emission), by applying a strong electric field gradient (field emission) or a combination of both (Schottky emission). The emitted electrons are focused and directed to the specimen with the help of electromagnetic lenses and apertures. The specimen should be thin enough to allow the transmission of incident electrons (typically less than 100 nm). The transmitted electrons are used to create image and diffrac-tion pattern below the specimen. Just like the optical convex lenses, the magnetic lenses have back focal and image planes where the diffraction pat-terns and images produced by the transmitted electrons appear respectively. The diffraction patterns give very useful and quantitative information about the crystallographic structure of materials such as defects, perfection of lay-ers in thin films, sizes of nano-objects and structural coherence between different grains of the sample. The images, on the other hand, can be ob-tained from various contrast mechanisms like diffraction contrast, chemical contrast or phase contrast and provide both qualitative and quantitative spa-

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tial information about structural features such as the phase distributions, the point defects and dislocations, the layering structure, the interfacial width and the diffusion between two adjacent layers in a multilayer structure.

The first TEM was built by Ruska and Knoll in the 1930’s [25] and the mo-tivation behind this invention was to achieve higher spatial resolution as compared to the optical microscopes. Soon it was realized that apart from getting the specimen images with improved spatial resolution, the electron microscope can be applied as an analytical tool to study the chemical and physical properties of materials. Electrons due to their mass and charged nature interact strongly with materials producing a magnitude of useful sig-nals which can be utilized to obtain valuable information about the speci-men. From the time of their invention, TEMs have undergone extensive pro-gress. The development of better electron emission systems, advanced com-puter control systems and sophisticated cameras and detectors have signifi-cantly improved the technique. More recently, the developments of aberration correctors for lenses and electron monochromators have broken the spatial and energy resolution limits not only for this instrument but also in general. The vacuum systems, TEM specimen holders and goniometers have been developed allowing for close to atomic stability of sample move-ment.

The TEM has proven itself an invaluable instrument to study the structure and properties of materials in the nano regime. One of the limitations of TEM is that the analysis volume is usually very small (a few tens of na-nometers) therefore it is always better to use complementary techniques to understand the microstructure of the material before going for TEM studies. Of course the information obtained by TEM would then solve many myster-ies and questions which were not possible to answer by other techniques. For example, in the Papers I, II and III, TEM in combination with other charac-terization techniques helped to understand the essential scientific questions about the structure of the materials. As mentioned above, the beauty of TEM is that it can be used as a powerful analytical instrument to map the physical properties such as magnetism with high spatial resolution. For the Pa-pers IV-VI, this feature of TEM has been used to develop a quantitative measurement technique for the determination of magnetic moments of mate-rials.

The work presented in this thesis has been carried out on two different TEMs. A JEOL 2000FX equipped with an LaB6 filament operating at 200 kV acceleration voltage and attached with a post column Gatan imaging filter (GIF) 2002 has been used for the first EMCD work (Paper IV). This old microscope served us as a platform to open the entrance aperture of the GIF and build custom-made apertures into it. The successful experience of

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building the aperture in this microscope encouraged us to continue the work on a relatively new microscope. Additionally the time resolved setup (Pa-per VII) is built on this microscope. The rest of the work has been done on an FEI Tecnai F30ST (scanning) TEM assembled with a field emission gun (Schottky emitter) operating at 300 kV acceleration voltage. The microscope is equipped with a Gatan tridiem spectrometer and a 2k x 2k CCD camera. This instrument has been applied for advanced analysis using HRTEM, EFTEM, EELS and STEM spectrum imaging (SI) techniques.

Figure 2.1. JEOL 2000FX equipped with a GIF2002 (left) and FEI Tecnai F30ST equipped with GIF Tridiem (right) used in this thesis work. Both microscopes are located in the Ångström laboratory, Uppsala University.

2.1 TEM sample preparation One of the key steps in the TEM experiments is to prepare a good quality TEM sample. The choice of the sample preparation method may influence the resulting analysis and bring artifacts if care has not been taken. Therefore it is important to choose a sample preparation technique which does not change the properties of the sample that we aim to study. In the work pre-sented in this thesis, classical sample preparation techniques have been used. TEM samples were prepared in either cross-sectional or plan-view geometry in different experiments.

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For cross sectional preparation, two small pieces of the sample are attached with the films facing to each other using the epoxy glue. The resulting sand-wich like structure is clamped inside a 3 mm Ti-ring and embedded in epoxy glue. The 3 mm diameter is important because this is the size of sample stage in most of TEM sample holders. The resulting structure is then coarsely pol-ished from both sides to reach a thickness of 80-100 µm. In the next step, the thickness is further reduced by dimple grinding the sample from both sides using a Gatan dimple grinder. For plan-view samples, a 3mm disc of the material is cut using a Gatan ultrasonic disc cutter which is subsequently polished and dimple grinded from the substrate side. A thickness in the range of 5-30 µm is achieved at the thinnest parts of dimple grinded samples. In the last step, the samples are subjected to Ar ion milling [26] in a Gatan pre-cision ion polishing system (PIPS). For the sample preparations in this work, the incident angle of the ions was set to 6o and the energy of the ions was set to 4 keV at the start and changed to 1.5 keV when the perforation was near. For the EMCD experiments, the samples were directly transferred from PIPS to the TEM to minimize the oxidation. For more details in a specific case, see description in the corresponding papers.

The sample preparation is accomplished when a hole starts to appear either at the interface of two pieces (in cross-section) or in the center of the disc (in plan view). The TEM sample has a wedge shape in the thinnest region with the thickness increasing linearly (rough estimation) as a function of distance from the hole. Normally, the hole shape is longitudinal in a cross-sectional sample and circular in a plan-view sample as shown in Figure 2.2. The color fringes in plan-view sample appear due to the interference of light in the optical microscope which changes periodically with the varying thickness of the sample [27] and can be used to estimate thickness of sample by a simple formula 2 [27], where m is the number of dark minima starting from the hole, n is the refractive index of the material and λ is the wave-length of light.

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Figure 2.2. Optical microscope images of TEM samples prepared in (a) cross-section and (b) plan view geometry.

2.2 Imaging and diffraction techniques The TEM provides with a variety of imaging and diffraction techniques which can be combined with a spectroscopic technique to get worthwhile information about structure and composition of materials down to atomic scale. Broadly speaking TEM can be operated in either TEM or STEM mode. In TEM mode a parallel electron beam illuminates a wide region on the sample and the electrons transmitted through the specimen are detected by a CCD camera to produce an image or a diffraction pattern. The transmit-ted electrons can either directly pass through the specimen or scatter at dif-ferent angles determined by the type of interaction with the material. Figure 2.3 shows an illustration of the interaction of incident electron beam with a TEM sample. Electrons, due to electromagnetic interaction are strongly scat-tered by materials. This interaction can lead to different types of scattering processes and the production of a variety of useful signals that can be meas-ured in a TEM. In the next few paragraphs, the transmitted electrons will be generally divided into two categories: the directly transmitted electrons pass-ing straight through the specimen and the scattered electrons which are de-flected to different angles on exit from the specimen. The scattering angles of the electrons in the TEM are normally very small and are measured in milliradians (mrad).

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Figure 2.3. A schematic illustrating the interaction of incident electrons with a TEM sample which generates a variety of signals. Only the transmitted electrons have been used for the work presented in this thesis.

An objective aperture below the specimen can be used to select either direct-ly transmitted or scattered electrons to obtain a bright field (BF) or a dark field (DF) image respectively. The contrast in BF or DF images is mainly governed by the mass or thickness and the diffracting power of the specimen and can give valuable information about compositional and structural chang-es within a specimen. DF images are especially useful to get crystallographic and grain contrast and this feature has been used to confirm the existence of crystalline Si particles in Si rich Si oxide films in Paper I.

The electrons behave both as particles and waves. When the electrons are considered as waves, they possess an amplitude and a phase. In case of a scattering event both amplitude and phase of an electron wave can change. When no or a bigger objective aperture is used below the specimen, both the directly transmitted as well as the scattered electrons contribute to the for-mation of the image. In the case of a thin specimen, the amplitude of the electron wave does not change and this superimposition of electron waves with different phases gives rise to the so called phase contrast and the result-ing images then phase contrast images. This technique is also called high resolution TEM (HRTEM) because images with atomic resolution can be obtained by this technique. A combination of HRTEM images and selected area electron diffraction (SAED) can give valuable information about local structural features e.g. point defects, dislocations, grain boundaries and strain. An example of the use of HRTEM and SAED is given in Chapter 3 (Figure 3.1).

In STEM mode, the electron beam is converged to obtain a well-focused spot called electron probe which is scanned across a desired region on the

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sample and the scattered electron are collected below the sample with the help of semi-conductor detectors to produce the images. The detectors used in STEM mode to collect the scattered electrons have an annular shape giv-ing them the name of annular dark field (ADF) detectors.

Figure 2.4. A schematic showing the shape of HA(ADF) detector which collects the scattered electrons transmitted through the specimen. The collection angle of the detector can be adjusted by changing the camera length; hence ADF and HAADF images can be produced using the same detector.

Depending on the scattering angles of the collected electrons, the resulting ADF images may contain diffraction as well as Z-number contrast. The elec-trons-nuclei interaction (the so called Rutherford scattering) results in a high angle scattering and the images produced by these electrons are called high angle annular dark field (HAADF) images [24]. For a typical transition met-al at an acceleration voltage of 200 keV, the angles used for HAADF imag-ing are typically larger than 50 mrad. The contrast in (HA)ADF images is proportional to the nth power of the atomic number (Zn) of the scattering atoms where the value of n depends on the collection angle of the detector. The generally accepted values of n are bounded between 4/3 ≤ n ≤ 2 with higher values of n for larger inner angle of the detector [28]. Due to this Z-dependence of contrast, the heavier atoms appear brighter than the lighter ones in (HA)ADF images. The ADF and HAADF images have been used in Papers I-III to obtain useful information about the materials.

DF and ADF imaging techniques were applied to study the crystalline state of Si in Paper I. Two SiOx samples with embedded Si nanocrystals annealed in forming gas (N2+H2) with different annealing times were analyzed. The primary purpose was to show the existence of Si nanocrystals in the films annealed in forming gas atmosphere. The ADF and DF TEM images clearly showed that Si is crystallized in the form of nanoparticles in both annealed

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samples (Fig. 1, Paper I). The collection angle of ADF detector was set to a value where a diffraction contrast in the ADF images showed the presence of high mass crystal particles in the amorphous film. The particles were then attributed to crystalline Si by creating the DF TEM images using the Si-specific reflections in the diffraction pattern.

A combination of STEM-ADF with a spectroscopic technique such as EELS or EDX can significantly enhance the obtained information. An EDX or EELS spectrum can be acquired at each beam position during the scanning resulting in a 3D spectrum image (SI) dataset where the X and Y dimensions represent the image and the Z-dimension contains the spectra [29]. Once acquired, SI datasets can be post processed to obtain compositional as well as magnetic information as implemented in Papers III, V and VI.

Figure 2.5. A schematic illustrating the acquisition of EELS spectrum imaging da-taset. In STEM mode, a fine electron probe is scanned across the sample and an EELS spectrum is acquired at each scan point in parallel with the acquisition of a reference ADF image. (The picture has been taken from www.gatan.com with due permissions)

Another way to obtain elemental maps where the intensity of an image di-rectly reflects the compositional modulation is energy filtered TEM (EFTEM) imaging. In TEM mode, an energy selecting slit (from a few eV up to 50 eV) built in GIF can be used to filter out the transmitted electrons that have lost a specific energy to produce a final EFTEM image. Thus, the intensity of an energy filtered image will highlight the areas in the specimen where the selected energy loss took place, hence precisely giving the ele-mental distribution. Like STEM-SI, a three dimensional data cube can also be acquired in EFTEM mode by moving the energy loss over a small slit and acquiring energy filtered image at each step [30, 31]. Such EFTEM-SI con-

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tains a 3D stack of 2 dimensional images with each image at a specific ener-gy loss.

The EFTEM technique was used to analyze the two samples annealed in forming gas studied in Paper I. The results were presented in a conference [32]. The goal of this analysis was to investigate the nanostructural change in Si nanocrystals in the two samples with different annealing times in forming gas. The TEM samples were prepared in a cross-sectional geometry. A 3D EFTEM spectrum image (SI) in the range of -4 - 40 eV was acquired for the two samples. The width of the energy slit was set to 2 eV and a step size of 1 eV was used. Si plasmon images were extracted from the SI by fitting a Gaussian with peak position at 16.7 eV [33] shown in Figure 2.6. The bright contrast in these images represents crystalline Si. It can be observed that the density of crystalline Si is reduced in the film annealed for longer time. An alternating bright and dark contrast in left image indicated the existence of multilayer structure which seems to be distorted in the right image where Si appears to form bigger nanoparticles. It may be concluded that the size of Si crystallites increases with increase in annealing time but the overall density is reduced due to an Ostwald ripening process.

Figure 2.6. Si plasmon images for two Si-rich SiOx samples annealed for (a) 210 s (b) 270 s in forming gas environment. The images show a reduction in the density of crystalline Si with longer annealing time.

2.3 Electron energy loss spectroscopy (EELS) Throughout this doctoral work, EELS [33-35] has been used as the main spectroscopic technique for composition analysis as well as to determine magnetic moments. When the fast incident electron hit the thin TEM speci-men, most of them are directly transmitted or scattered without losing any energy. This type of transmission is called elastic scattering. Some of the electrons transfer or lose a fraction of their energy while passing through the specimen, a process that is called inelastic scattering. The incident electron

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can lose its energy by exciting atomic vibrations (phonons), collective oscil-lations of the free electron gas (plasmons) in the specimen or it can transfer energy in ionizing an atom by exciting a core shell electron. In all these cas-es, the energy loss suffered by the incident electrons is characteristic to ele-ments and thus gives direct information about the composition of the speci-men. The energy losses of the transmitted electrons can be determined by using an electron energy loss (EELS) spectrometer. The spectrometer can either be built in-column or post-column in different TEMS, the latter being used in this work. Figure 2.7 shows a cross-sectional schematic of EELS spectrometer.

Figure 2.7. A schematic of EELS spectrometer showing major parts of the instru-ments.

The main parts of an EELS spectrometer are the entrance aperture which is used to set the collection angle of the energy loss electrons, the magnetic prism which splits the electrons based on their energy losses along an energy dispersive axis, an energy-selecting slit to filter the electrons of specific en-ergy loss and a couple of lenses to focus the electrons to create an image or EELS spectrum on the CCD camera. With the feature of the energy slit in-cluded, the spectrometer is also called an image filter.

A typical EELS spectrum has energy loss [eV] on horizontal axis and the intensity or the electron counts on vertical axis. The spectrum is mainly di-vided into low loss and core loss regions with a dividing line between the two regions in the vicinity of 50 eV. The low loss region contains elastically scattered electrons in the form of zero loss peak (ZLP) and plasmons loss peaks whereas the core loss region contains energy loss edges resulting from inner-shell ionizations. The phonons are normally buried into ZLP but it is possible to separate in modern TEMs by the use of a monochromator [36, 37]. The intensity in the core loss region is much lower than the low loss

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region as can be seen in Figure 2.8 where the core loss region is magnified by 50 times.

Figure 2.8. (a) low loss and (b) core loss parts of an EELS spectrum are shown. This spectrum was acquired on Tecnai F30 TEM for one of the FeNi multilayer samples analyzed in III. The characteristic energy loss edges of Fe, Ni and Cu are highlighted by arrow heads.

EELS spectroscopy is sensitive to the chemical environment of elements. Unlike EDX where the energy resolution is normally between a few tens of eV and 150 eV, an EELS spectrum has energy resolution in the range of 0.01-2 eV depending on the degree of monochromatisation of the TEM. This excellent energy resolution of EELS makes it possible to detect a change in the valence state of elements by a small energy shift or a change in the shape of the energy loss edge [38]. Another application of EELS is the measure-ment of local thickness of a TEM sample. The thickness of the area under the electron beam can be obtained by acquiring a low loss EELS spectrum and comparing the intensity of the ZLP (I0) to the intensity of the whole spectrum (It) by following equation [33].

(1)

Where is the thickness and is the inelastic mean free path of electrons passing through the material. The value of is material dependent and can be calculated by equations given in [33] . This method is called log-ratio method and was used to determine the thicknesses of the samples in EMCD experiments.

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2.4 Details of EMCD experiments and analysis An EMCD signal is obtained by taking the difference of two angle-resolved EELS spectra acquired at specific scattering angles in the diffraction plane where the momentum transfer of the two spectra are reverse to each other. A short theoretical description of the EMCD technique is given in Chapter 4 and the EMCD results related to Papers IV, V and VI are presented in Chapter 5. This section describes the important steps involved in all the EMCD experiments. The EMCD experiments and analysis can be divided into three steps excluding the sample preparation: 1) setting up the pre-acquisition experimental conditions 2) data acquisition 3) post processing of the experimental data

2.4.1 Setting up the EMCD experiments on TEM Due to the use of custom made apertures, all the EMCD experiments require special experimental conditions which need to be fulfilled before the acquisi-tion of data. There are three major steps in setting up the experimental condi-tions for all the EMCD experiments 1) installing the aperture in the spec-trometer entrance aperture (SEA) in correct orientation 2) aligning the dif-fraction pattern with respect to the aperture holes 3) tuning the spectrometer

To build the apertures in the SEA, specific apertures were fabricated on round titanium discs with the diameter of the disc equal to the aperture hole in the SEA. The aperture disc was mounted in the relevant hole of the SEA and rotated to align the aperture holes perpendicular to the energy dispersive axis of the spectrometer which is parallel to the SEA arm. The mask aperture in the SEA was used as a reference to align the apertures and optical images of the mounted apertures were taken to make sure the correct alignment. A Cu spring ring was used for holding the aperture to avoid it falling inside the spectrometer as shown in Figure 5.9. The SEA holder was inserted back inside the spectrometer and the alignment of the apertures was re-assured by acquiring the CCD images of the apertures. In both the TEMs used in these experiments, it was possible to evacuate the spectrometer without disturbing the column vacuum. The pumping time to reach good vacuum values in the spectrometer after installing the apertures was about half an hour.

The second step to align the diffraction patterns with the aperture holes in the desired orientation was carried out by rotating the TEM sample. The unavailability of a double tilt rotation holder made this process quite tough. A single tilt rotation holder was available but it was hard to orient the sample to perfect 2BC or ZA with single tilt. Consequently a double tilt non-rotational sample holder was used. The TEM sample was rotated manually inside the sample holder stage, then inserted in the TEM whereafter the dif-

27

fraction pattern (DP) was acquired to find the misalignment. To adjust the rotation of the sample, the holder was taken out, the sample was rotated in the desired direction by rotation degrees estimated by the misalignment and then inserted back into the TEM. From the DP the residual misalignment could be found. The process was re-iterated until the best possible alignment is achieved. An example of this diffraction pattern alignment process is shown in Figure 2.9 where it was required to align the position of the 2 dif-fraction spots with the positions of the two smaller holes in a quadruple aper-ture (QA). For details, see Section 5.2.

Figure 2.9. The steps to align the diffraction pattern with respect to the quadruple aperture (QA) are shown (a) the image of the QA is taken as the reference image and the position of the two smaller aperture holes is marked with red circles (b) the cross-sectional TEM sample of Fe(MgO) is inserted and the diffraction pattern (DP) is viewed with respect to the aperture positions (red circles), the purpose here is to align the 002-axis of the MgO (Fe) to the axis passing through the red circles. For this purpose, the sample is taken out and rotated manually in the desired direction (c) the sample is inserted back and the DP is viewed with respect to the aperture positions, the 002-axis is now closer to the apertures axis but still not coincided with it, so the sample is again taken out and rotated more in the same direction (d) the sample is inserted back and the DP is viewed w.r.t. the aperture positions, the 002-axis is now very close to the aperture axis with a very small misalignment. The sample is again taken out and very finely rotated in the same direction. (e) The sam-ple is inserted back and the DP is viewed, now the 002-axis is perfectly aligned to the apertures axis and the 0 and 002 (Fe) diffraction spots are seen through the smaller aperture holes of the QA as shown in (f).

Once the aperture and the diffraction pattern are aligned to the desired orien-tation, the next step before starting the acquisition is to tune the spectrome-ter. As the aperture shapes are non-standard for the spectrometer, the auto-matic tuning routine does not work perfectly and a manual tuning of the

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lenses is needed to get the best possible alignment. In most of the cases, the current in FX, FY, SX and SY lenses of the spectrometer was changed to get the best resolution. In the case of the quadruple aperture, a few lenses in the service mode were also tuned to increase the vertical extension of the EELS spectrum on the CCD camera to obtain better resolution along qy scattering angles. Despite the manual tuning, it is hard to perfectly align the spectrome-ter, especially when the apertures lie on different x-coordinates on the CCD camera (the case of QA and VA). The residual aberrations were corrected by a MATLAB script in the post processing as described in Section 2.4.3.

2.4.2 Data acquisition In all the EMCD experiments described in Papers IV-VI , the data were acquired in the form of 2D EELS images where the EELS spectra appear as intensity traces with the energy loss edges as bright vertical lines as shown in Figure 5.6. The collection angle of the apertures is an important parameter influencing the quality of the obtained EMCD signal, especially the EMCD signal strength varies significantly by changing the inner/outer collection angles of the ventilator apertures. The collection angles were set to the opti-mum values (predicted by simulations in the case of ventilator apertures) by changing the camera length. Below is a brief description of the acquisition conditions used in different experiments. The detailed experimental condi-tions can be found in the relevant papers. - In Paper IV, the EMCD experiments were carried out on JEOL 2000FX

equipped with a GIF 2002 spectrometer. The spectrometer was installed under the microscope in house a few years back and it is not directly cou-pled with the microscope HV and projector lenses. There are two conse-quences of this uncoupling 1) the image/DP seen on the viewing screen of the microscope will be magnified by x50 times on the CCD camera of the GIF. In the case of Tecnai F30ST used in the next experiments where the EELS spectrometer is coupled to the microscope, the image/DP on the viewing screen is squeezed in advance to compensate the increase in mag-nification and the same magnification is achieved on GIF CCD as seen on the viewing screen. This difference in magnification between the spec-trometer CCD camera and the viewing screen on the JEOL creates un-pleasant circumstances and even with the smallest available camera length on the microscope, the collection angle for the EMCD experiments on the GIF CCD is very small. To overcome this problem, the microscope was operated in FREE LENS CONTROL and the magnification on the viewing screen was reduced by changing the current in intermediate and projector lenses. 2) The second effect of uncoupled spectrometer is that the specific energy loss range of electrons was selected by changing the current in drift tube instead of changing the electron gun offset as used in a coupled mi-croscope. This change in drift tube current leads to a change in the focus of

29

the image/DP. For the EMCD experiments, this was compensated by re-focusing the DP at an energy loss of 600 eV. The EMCD experiments were performed in the TEM mode. The electron beam was converged to an area of ~100 nm and an image of the CCD in the spectroscopy mode was ac-quired both for the double aperture and the qE modes. The experiments were repeated several times to ensure the reproducibility. The TEM sample was prepared in plan view geometry and the sample thickness was ~25 nm at the region of measurements.

- In Paper V, the EMCD experiments were carried out in microprobe STEM mode with a beam semi-convergence angle of 1.6 mrad on the Tecnai TEM. The TEM sample was prepared in plan view geometry with the thickness of magnetic (Fe) film being 20 nm. A 100x100 nm2 area was scanned with a step size of 4 nm and in the spectroscopy mode a 2D CCD image was acquired at each scan point, producing a 4D dataset. The beam dwell time was set to 5 s per scan point. Each 2D image in the 4D dataset contains four spectral traces, two conjugate EELS spectra for the EMCD measurements and two from the diffraction discs.

- In Paper VI, the EMCD experiments were carried out in STEM mode with the electron probe size defined by the beam convergence angle. The EMCD signals were obtained for three different convergence semi-angles of 5 mrad, 7.5 mrad and 10 mrad. In all the experiments, the electron probe was scanned across the TEM sample in a line and a 2D EELS image was acquired at each scan point using the diffraction acquisition software. Both the plan view and cross sectional TEM samples were used for the meas-urements in [001] and [110] zone axes respectively. The maximum but safe criterion was used to choose the dwell time per scan point i.e the maximum time for which the sample does not undergo any apparent beam damage (5 s in this case). The purpose of using the high dwell times was to obtain EMD signals with good S/N ratio from as small area as possible.

2.4.3 Post processing This section describes the steps involved in post processing of the acquired data to obtain the EMCD signals. - The first step is to correct the energy drift due to which the intensity peaks

move a few pixels to and fro in the 2D EELS images at different scan points. This was done by using the ‘Align SI by Peak’ option under the ‘SI’ menu in digital micrograph (DMG). A peak position at one of the scan points is selected as reference by inserting a window and the data at all the other scan points are aligned with respect to that. This step was not needed in Paper IV as the data were already in the form of single CCD images.

- In the ventilator aperture work, after the peak alignment the 2D EELS im-ages at many scan points were integrated to obtain one image for the data obtained for different type of apertures. The number of integrated scan

30

points is mentioned in the figure captions of Paper VI. The integrated 2D EELS images still contain some spectrometer alignment aberrations which need to be corrected before extracting the EELS spectra. These aberrations are the most severe when the holes of the aperture lie at different x-coordinates on the CCD such as the SS8 or DS7 aperture. A MATLAB script was used to convert the 2D EELS image into an EELS spectrum im-age such that the spectrum ‘Picker tool’ in DMG recognizes each row in the 2D CCD image as an EELS spectrum. The same peak alignment rou-tine mentioned above was used to align the maxima of the peak in each row as shown in Figure 2.10.

Figure 2.10. (a) 2D EELS image acquired for SS8 aperture (b) the image is convert-ed into an EELS SI with the help of a MATLAB script which converts each row into an EELS spectrum (c) the peaks at all the rows are aligned to obtain the corrected 2D EELS image.

- The EELS spectra were extracted by drawing intensity profiles along the spectral traces and integrating over a suitable number of pixels. The selec-tion of scattering angles along qy axis was decided on the base of a separate analysis where the EMCD signal strength was analyzed for different qy values. In most of the cases, the maximum EMCD signal strength was found at higher qy scattering angles considering qy = 0 at the center of the 2D EELS image (i.e. the upper and lower parts of the spectral traces). It is concluded that the weakening of EMCD signal at lower qy values might be due to higher interference of high intensity Bragg reflections.

Figure 2.11. A 2D EELS image acquired using DS2 aperture. The EELS spectra for the EMCD measurement are extracted from the areas marked with red rectangles.

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- In case of quadruple aperture work, after the peak alignment step, the 4D dataset was exported to MATLAB. A MATLAB script created two 3D EELS SI datasets from the 4D dataset by extracting the EELS spectra from the upper and lower spectral traces respectively. After background subtrac-tion, another MATLAB script normalized the post edge of EELS spectra at each pixel of the two 3D datasets. Finally a MATLAB script was used to calculate the relative difference at L3 and L2 edges of the EELS spectra at each pixel of the two post edge normalized datasets and produce L3-difference and L2-difference maps shown in Paper V. A similar script summed up the intensities of the 0 and g beams from the two spectral trac-es produced by the beam apertures and produced a map with a value of Ig/I0 at each pixel.

- In all the cases, the energy loss axis of the extracted C+ and C- EELS spec-tra was calibrated for standard Fe L2,3 energy loss edges. The background of the EELS spectra was subtracted by using a power law model. The post-edge of the background subtracted spectra was normalized and an energy window 740-790 was used for normalization. The post-edge normalized spectra were then normalized to the maximum intensity of the L3-edge to simplify the comparison. Finally the EMCD signal was obtained by sub-tracting the C- spectrum from C+ spectrum.

- It is important for the quantitative EMCD measurements that the S/N ratio and the EMCD signal strength at both the energy loss edges is sufficient to extract the reliable mL/mS values. We used the bcc Fe in our experiments as the magnetic parameters of Fe are well known and it can be used as a test criterion to check the accuracy of the measurements. The mL/mS ratios were measured using

Ω4

, , 2 ∗ , , (4)

- . The L3 and L2 intensities of the EMCD signal were extracted by integrat-ing the intensities of the difference signal on L3 and L2 edges. The meas-ured mL/mS values are presented in Papers IV, V and VI. The mL/mS vales measured in all the cases are in good agreement to the already reported values [39-42].

2.5 Summary The transmission electron microscope has been used as the experimental instrument in the work presented in this thesis. The experimental techniques such as TEM sample preparation and the imaging and diffraction techniques

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used for the structural characterization of the thin films are discussed. A brief description of electron energy loss spectroscopy is given which is used as the main spectroscopic technique in this work for the composition analy-sis as well as for the magnetic measurements. The dark field, ADF imaging and the energy filtered TEM imaging results related to Paper I are dis-cussed. Finally the experimental details including experimental setup, data acquisition and post processing for the electron magnetic circular dichroism experiments are described.

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3 Magnetic thin films

A layer of any material with thickness ranging from a fraction of nanometer to several micrometers may be termed as a thin film. A very common exam-ple of the use of thin films is a household mirror where a very thin metal coating is deposited on the backside of a glass sheet to a produce reflection. With the advances in deposition technologies during 20th century, thin films have revolutionized for their use in many technological fields such as mag-netic storage devices [43, 44], magnetic and optical sensors [45-47], thin film batteries [48, 49], semiconductor devices [50, 51] and thin film solar cells [52, 53].

Magnetic thin films and multilayers are mostly used in electronic devices such as hard drives and computer memory chips. The physical properties of a magnetic thin film may be significantly different from the bulk material. The reduced dimensionality of a thin film can give rise to many interesting phenomena. These include the dominance of surface properties due to large surface-to-volume ratio, the crystal symmetry breaking and spin-orbit cou-pling at the interfaces. Magnetic multilayer structures have attracted a lot of interest due to the possibility to manipulate and tailor the magnetic proper-ties through the interlayer exchange coupling [54]. The properties of a mag-netic thin film or multilayer are highly dependent on its structure. The real structure of a thin film is significantly influenced by the growth parameters [55, 56]. A deep understanding about the structure not only makes it easy to explain the properties shown by the thin film but also pro-vides with a way to optimize the growth conditions. In the case of magnetic thin films and multilayers, the structural features such as the interfacial roughness, interfacial diffusion, epitaxial relationship of the layers to the substrate, the strain in the layers, structural coherence of the layers, crystal-line quality and structural defects can play a vital role to modify and tune the obtained properties [8, 9, 57, 58]. A thorough investigation of the structure is thus essential to fabricate devices with desirable properties. TEM is an im-portant instrument to look at the nanoscale structural features in the magnetic thin films and often this understanding is very helpful to explain and tune the properties shown by the device.

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3.1 Deposition and characterization techniques used by the collaborators

The magnetic thin films and multilayers used for structural analysis and EMCD experiments were fabricated and analysed using different x-ray tech-niques by the collaborators. All the films were deposited by direct current magnetron sputtering. The growth process is carried out in a high vacuum chamber. In sputtering, ions are accelerated towards a target material where they knock out the atoms from the target. The sputtered atoms travel towards the substrate and condensate forming a layer. The Fe films used in the EMCD experiments were capped with a few nanometers layer of either MgO or Al2O3 to avoid oxidation of the film.

The microstructural characterization of the thin films is mainly carried out using x-ray based techniques. X-rays due to their small wavelength ap-proaching the lattice spacings are diffracted by most of the crystals and this feature makes them a suitable candidate to investigate the structure of the materials. X-ray diffraction (XRD) is non-destructive and is the most com-monly used characterization technique by the thin film community. The pri-mary information obtained by XRD is the lattice parameters and the quality of the crystal but it also gives information about film thickness, multilayer structure and its periodicity, roughness of the film etc. XRD has been used as a main technique for the characterization of structures in Paper III (Fig. 2). XRD is also a fantastic tool to determine how much the crystal deviates from perfect crystal structure, the so called mosaicity. This feature has been used to determine the crystal quality in Paper II (Fig. 7).

Another approach to use X-rays in thin film studies is x-ray reflectivity (XRR). The basic principles of XRD and XRR are same with the exception that much smaller incident angles of X-rays are used in XRR. The primary information obtained by XRR is about the interfacial properties and it is more applied in case of multilayer and superlattice structures. It gives infor-mation about the roughness of interfaces and the thickness of individual layers in a multilayer structure. The interaction of grazing incidence X-rays with a multilayer sample results in a typical characteristic spectrum as shown in Fig. 3 of Paper II where the constructive interference of x-rays reflected from the top and bottom interfaces of a layer (as well as full film) produces oscillations (Kiessig fringes). In most cases, the results from XRR are fitted using a theoretical model and then used to determine the layers thicknesses, superlattice period and so on. XRR has been applied to determine the indi-vidual layers thickness, bilayer period and roughness of the interfaces in Papers II and III.

35

X-ray photoelectron spectroscopy (XPS) is an x-ray based composition anal-ysis technique which is broadly applied to determine the binding structure and the valence of atoms as well as the stoichiometry of materials in the near surface region (~10nm). The high energy x-ray photons are irradiated on sample surface and the electrons escaping from the surface are detected by a spectrometer where the kinetic energy of the electrons is measured and con-verted back to the binding energy. The binding energy is characteristic to materials and thus gives direct information about the presence of specific elements. As XPS is a surface sensitive technique, it was used in combina-tion with Ar ion sputtering to get depth information and produce composi-tion profiles shown in Fig. 7 of Paper III. The depth resolution of XPS was not sufficient to resolve the layers in the samples with lower periodicities which were further investigated in the TEM.

3.2 Structural characterization in the TEM Although the applied techniques discussed above give very useful and often sufficient information to understand the structure and properties of thin films but in many cases TEM studies are needed to get a deep insight into the structure which could be missing in other studies due to a limited resolution. In the TEM, the spatial features such as the individual layers in a multilayer structure, the roughness of the layers, the defects and the interdiffusion at the interfaces can be observed in the real space with nanometer down to atomic resolution. The TEM contributions in Papers II and III are briefly de-scribed here.

In Paper II, STEM-HAADF image of Fe/V (4/28 ML) confirmed the exist-ence of a coherent multilayer structure in the real space. The HAADF image shows that the layers are flat and no waviness is present at a relatively larger length scale (Fig. 2, Paper II).

In addition to the HAADF image published in Paper II, more work using HRTEM and SAED was carried out on the same sample for internal verifica-tion of the data collected using other techniques. The HRTEM image and the superlattice reflections in SAED pattern shown in Figure 3.1 verify the pres-ence of a coherent superlattice structure. The bilayer repetition period meas-ured from the superlattice reflections is 45.4 ± 2.2 Å which is quite close to 45.9 Å measured with XRR. The positions of Fe and V layers in the HRTEM image were found by measuring the lattice plane spacings in the growth direction.

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Figure 3.1. (a) HRTEM image and (b) SAED pattern for Fe/V 4/28(monolayers) superlattice sample

In Paper III, a series of five FeNi multilayer samples was grown with vary-ing periodicity. The purpose of the study was to investigate how the layer thicknesses affect the composition and magnetic properties of such multi-layers and what is the lower limit of layer thickness where the multilayer structure still exists. The depth profiling XPS showed the existence of layers down to 16/16 ML (monolayers) of Fe/Ni and it appeared that the samples were completely intermixed with no multilayer structure remaining for 8/8 and 4/4 MLs. Thanks to TEM which can easily solve such ambiguities. Three samples with 16/16, 8/8 and 4/4 ML thickness of Fe/Ni were chosen to investigate in TEM. The ADF images could clearly resolve and confirm the presence of multilayer structure in all the samples (Fig. 5, Paper III).

Figure 3.2. An ADF image of FeNi 16/16 ML sample.

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To determine the extent of interdiffusion between the layers, STEM-EELS was employed in combination with the ADF images and 3D spectrum image (SI) datasets were acquired across the layers for the three samples. The da-tasets were analyzed and the composition maps of Fe, Ni and Cu were ex-tracted in the way described in Paper III. An example of composition maps extracted for 16/16 ML sample is shown below. Figure 3.3 (a) shows the ADF image which was used as a survey image. The green box shows the area where the electron beam was scanned to acquire a 3D EELS-SI. The yellow box contains the reference image for spatial drift correction. Figure 3.3 (b), (c) and (d) show the Fe, Ni and Cu maps extracted from the SI. The Fe and Ni layers are clearly resolved in the maps. Similar composition maps were produced for 8/8 and 4/4 ML samples which confirmed the existence of Fe and Ni layers in those samples too. To determine the interfacial width and interdiffusion between the layers, composition profiles were extracted across the layers by integrating the intensity in each row (Fig. 6, III).

Figure 3.3. (a) An ADF survey image showing the scanned region in green box (b),(c),(d) composition maps of Fe, Ni and Cu extracted from the EELS SI-dataset respectively.

The composition profiles showed that Fe and Ni are heavily diffused into each other and the interdiffusion is asymmetric i.e. Fe diffuses more into Ni and vice versa. This trend was in agreement with the phase diagrams of Fe and Ni which report very low solubility of Ni into bcc Fe [59]. The TEM results confirming the existence of layers in 4/4 and 8/8 ML samples helped to understand the unknown extra reflections seen in XRD and it was estab-lished that Fe exists in an fcc phase in 4/4 and 8/8 ML samples, and relaxes to bcc in the samples with higher periodicity. Coming to the magnetic prop-erties of especially 4/4 ML sample, it was hard to explain the value of satura-tion magnetization obtained for this sample without knowing that the layers of Fe and Ni still exist.

3.3 Summary A brief introduction to magnetic thin films is given and the deposition and characterization techniques used by the collaborators are described. The

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TEM results for the structural characterization of Fe/V and Fe/Ni multilayer samples are discussed. The high angle annular dark field (HAADF) images show a flat and coherent layering structure in Fe/V multilayer sample with a periodicity of 4/28 monolayers respectively. The selected area electron dif-fraction (SAD) pattern confirms the coherent superlattice structure and the bilayer periodicity is measured from the superlattice reflections in the SAD. The HAADF images clearly resolve the multilayer structure in Fe/Ni multi-layer samples with periodicities of 16/16 monolayers, 8/8 monolayers and 4/4 monolayers respectively. The EELS composition mapping resolves the layers in all the samples and shows that the layers are interdiffused into each other in an asymmetric way i.e. Fe is diffused more into Ni layers and Ni is diffused less into Fe layers. The confirmation of the layering structure in the sample with 4/4 monolayers periodicity helps to explain the measured satu-ration magnetization of the sample.

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4 Electron magnetic circular dichroism (EMCD)

Electron magnetic circular dichsoism (EMCD) is an electron energy loss spectroscopy (EELS) based technique to measure the magnetic moments in a transmission electron microscope. The EMCD technique has resemblance to the well-established X-ray magnetic circular dichroism (XMCD) technique. The following few sections contain a brief description of the EMCD theory. The readers interested in detailed theoretical descriptions are referred to [60, 61].

In optics, a material is called dichroic if the light rays with different polariza-tion are absorbed differently in it [62] and this property of materials is called dichroism. If the incident light rays are circularly polarized then the differ-ence in absorption is called circular dichroism. It was predicted in 1975 [63] that magnetic materials irradiated by X-rays would show magnetic dichroism i.e. the absorption of left and right circularly polarized X-ray photons for a magnetic material would be different and sensitive to magnetic transition. The phenomenon, called as X-ray magnetic circular dichroism (XMCD) was experimentally demonstrated in 1987 [64]. In XMCD measurements, two absorption spectra using left and right circularly polarized photons are rec-orded and their difference gives the XMCD signal which has a typical signa-ture of opposite signs at L3/L2 absorption edges for magnetic transition met-als (M5/M4 for rare earth metals) as shown in Figure 4.1. By applying sum rules on the XMCD signal [65, 66], precise values of relative orbital to spin magnetic moments can be obtained [42].

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Figure 4.1. An XMCD signal acquired on Fe L2,3 absorption edges. The red and blue curves are the absorption spectra acquired with left and right circularly polar-ized photons. The XMCD signal (black curve) is obtained by taking the difference of the two absorption spectra. The figure is adopted from [67] with due permissions.

The similarities between X-ray absorption spectroscopy (XAS) and electron energy loss spectroscopy (EELS) have been known for a long time [68, 69]. Initially it was assumed that spin polarized electrons are needed for magnetic dichroic measurements in a TEM but later it was realized that it is not true. It was shown that under certain assumptions, the polarization vector for a po-larized photon is equivalent to the momentum transfer for a scattered elec-tron [70]. Within certain approximations, there is a strong resemblance in the equations describing the scattering process in EELS and the absorption pro-cess in XAS.

The scattering of an electron in EELS is described by double differential scattering cross section (DDSC) given by

Ω

4 1| | . | |

,

(2)

Where is the momentum transfer is the position vector, and i and f are the initial and final states of the target electron with energies Ei and Ef respectively. The absorption cross section for XAS is given by

∝ | | .,

| | (3)

Where ω is the angular frequency and e is the polarization vector of the pho-ton. From the comparison of equations ovan and ovan, it is clear that within

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dipole approximations, the polarization vector e of photon in XAS is equiva-lent to the momentum transfer q of inelastically scattered electron. This indi-cates that the dichroic measurements should be possible with electrons by measuring angle resolved EELS spectra using the direction of momentum transfer as a parameter. Following the approximations used in [70] and [61], the double differential scattering cross section described by equation ovan can be written for two coherent electron waves inside the specimen as

Ω4

, , 2 ∗ , , (4)

The DDSC now becomes a sum of two direct terms and one interference term. The first two direct terms are called dynamic form factors (DFF) and they individually describe the inelastic scattering of an incident plane wave into an outgoing plane wave with wave vector transfer (or ′) and energy loss E. They describe the directional dependence in angle resolved EELS spectra for anisotropic materials. The third term describes the interference between the incident and the scattered wave due to dynamical diffraction and is called mixed dynamic form factors (MDFF). This term is responsible for the magnetic signal that is measured in EMCD experiments and is used in the calculations to simulate the magnetic signal distribution under different dynamical diffraction conditions.

4.1 EMCD measurements in a TEM Based on the equivalence between polarization vector of a photon and mo-mentum transfer of a scattered electron, Hebert and Schattschneider in 2003 [70] proposed the detection of EMCD signals in a TEM under certain geometrical conditions. The proposal was based on the fact that the superpo-sition of two coherent electron waves gives rise to a magnetic signal at spe-cific scattering angles in the diffraction plane as described ovan. To measure these magnetic signals, four basic conditions need to be fulfilled. 1) Two coherent electron waves must exist to produce two simultaneous momentum transfers 2) they must not be in phase (with a phase shift of pi/2 yielding the strongest EMCD signal) 3) the two momentum transfers must not be parallel to each other (ideally should be perpendicular to each other) 4) the helicity of the measurements must be changed. The two coherent electrons waves can be produced by different ways but the most convenient way is to use the crystal itself to split the incident electron beam into two out-of-phase coher-ent electron beams. The resulting diffraction geometry is called the two beam condition. To fulfill conditions (3) and (4), the acquisition of two

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EELS spectra is required at scattering angles defined by the vector products ′ and ′ where ⟘ ′. These conditions can be fulfilled by ac-

quiring the two EELS spectra at two conjugate scattering angles located on the so called Thales circle positions with the 0 and the G beams defining the diameter of the circle. According to Thales theorem, the end points of the diameter of a circle always subtend a right angle at any point on the circle. Based on these suggestions, Hebert and Schattschneider proposed [70] the following acquisition geometry for the detection of EMCD signals.

Figure 4.2. The acquisition geometry for classical EMCD experiments. The sample is tilted to 2BC with 0 and G beams activated and the EELS spectra are acquired at two conjugate positions defined by the vector products of momentum transfer vec-tors.

The word chiral has been used in literature for the two scattering angles where EMCD measurements are performed. Chiral indicates something which is non-superimposable on its mirror image. Its origin is Greek cheir (hand) as the human hands are the simplest examples of chirality. Subsequently the EELS spectra acquired at the chiral scattering angles are called chiral plus (C+) and chiral minus (C-) spectra. This terminology will be followed throughout this thesis.

Although EMCD has many similarities with XMCD, the determination and interpretation of EMCD signals is far more complex than XMCD. The rea-sons behind are the dynamical diffraction effects that modify electron scat-tering in a complex way. The strength and distribution of EMCD signals not

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only depends on magnetic properties of the material but also depends on dynamical diffraction coefficients. The value of dynamical diffraction coef-ficient depends on different parameters like the acceleration voltage, the thickness of crystal, the extinction length of a reflection, the specific orienta-tion of crystal and the size of the detector. The size of the detector is defined by the collection angle of the aperture used to acquire the chiral EELS spec-tra. Instead of being located only at specific scattering angles defined by the Thales circle, the EMCD signals spread out over a large range in reciprocal space due to dynamical diffraction effects therefore a bigger detector can help to increase the strength of the measured signal [71].

4.1.1 Why electrons instead of X-rays Although XMCD is a well-established technique to determine the magnetic moments of materials quite precisely, it has a few limitations which motivat-ed the discovery of EMCD. The spatial resolution obtained in XMCD meas-urements, in general, is few tens of nanometer (nm) [72-75]. Many interest-ing magnetic phenomena due to quantum confinement effects and the pres-ence of interfaces arise in the range of few atomic layers to a few nanome-ters. For example, the magnetic behavior may significantly change at the interfaces in magnetic thin films and multilayers due to symmetry breaking and spin-orbit coupling. This change usually spans over single to a few atomic layers. An in depth understanding of such magnetic phenomena may lead to fabrication of devices with novel properties but it demands character-ization techniques with spatial resolution sufficient enough to explore such effects. Electrons due to their charged nature can be focused into atomic sized probes by using magnetic lenses in a TEM and are suitable candidates to achieve this spatial resolution. Another limitation of XMCD is the lack of depth sensitivity. The depth probed by XMCD is usually in the range of few nanometers. EMCD instead probes the full depth of a TEM specimen which is usually a few tens of nm, making it possible to have a full quantum object in the analysis volume. Moreover, with the complementary analysis tech-niques in a TEM, it is possible to simultaneously obtain the magnetic and structural information about the specimen and correlate them at nm-scale. Last but not least, XMCD needs synchrotron source which is not easily available all the times. EMCD measurements can be performed in any stand-ard TEM equipped with an EELS spectrometer which is widely available in many of the TEM labs and is less expensive than a synchrotron source.

Despite many advantages of EMCD described above, the technique has a few limitations. The need to obtain well defined coherent electron beams limits the technique to specimens that are single crystalline at the place of analysis. Some efforts have been done to carry out EMCD measurements for amorphous samples by the use of a single crystal overlayer as a beam splitter

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[76] but it is difficult to interpret the resulting EMCD signals. There is also a work to obtain the EMCD signals without the need to follow a specific dif-fraction geometry [77] but it is a non-trivial and lengthy procedure to extract the EMCD spectra. Due to dynamical diffraction effects, the EMCD signals are quite sensitive to specimen thickness and the choice of detector. There-fore it is often required to carry out simulations beforehand to find out opti-mum specimen thickness and the size and position of the detector for the best obtainable EMCD signals.

4.1.2 EMCD sum rules The difference spectra in XMCD and EMCD measurements for magnetic materials have opposite signal strengths at two absorption or energy loss edges. If the experimentally obtained spectra follow this specific signature, it qualitatively confirms the detection of magnetic moments in the material as shown in Figure 4.3.

Figure 4.3. An EMCD spectrum obtained for bcc Fe shows opposite difference in-tensities at L3,2 energy loss edges. The EELS spectra were acquired on JEOL 2000FX with an acquisition time of 30 s.

To obtain quantitative information about the orbital and spin moments, the relation between the intensities extracted from the difference spectrum at the two edges and the element-specific quantitative values of magnetic moments need to be established. It can be shown that the magnetic transitions follow the so called sum rules (the key element to establish the above mentioned relation) which show that the EMCD intensities are related to the spin and orbital magnetic moment ratio. The sum rules for XMCD technique had been derived and well established but those sum rules could not be directly

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applied to EMCD spectra due to dynamical diffraction effects encountered in EMCD. In 2007, the analytical form of EMCD sum rules taking into account the dynamical diffraction effects was derived [78] which made it possible to calculate the magnetic parameters with element specificity [79]. According to the EMCD sum rules, the mL/mS ratio can be calculated by the following equation.

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Δ Δ

Δ 2 Δ

(5)

The equation ovan can be used to calculate the ratio of orbital to spin mo-ments and it was used to determine the mL/mS values reported in IV-VI. To separately obtain the values of orbital and spin magnetic moments, dynam-ical diffraction coefficients should be quantitatively calculated.

4.2 Development of the EMCD technique

The first EMCD signals were detected by Schattschneider et al. in 2006 [80] for a bcc Fe crystal. The big challenge coming up was the very poor signal to noise ratio (S/N) in the experimentally detected EMCD signals as compared to the simulated as well as to the XMCD spectra. In the classical EMCD experimental setup, two EELS spectra are acquired at two conjugate detector positions and the difference of these two EELS spectra is the EMCD signal which is further used for quantitative analysis. The following paragraphs contain a brief description of different experimental setups and methods used to improve the S/N ratio of the technique as well as the progress to achieve better spatial resolution in the measurements. For a more comprehensive review on the progress of EMCD, the reader is referred to [81]. Considering the serial acquisition of two EELS spectra needed in EMCD measurements, an alternative experimental setup was introduced in 2008 [82] which could acquire two EELS spectra simultaneously. An im-provement in the spatial resolution reaching to the 10 nm scale was also reported in this paper. In the same year, Verbeeck et al. [71] showed by sim-ulations that the EMCD signal is distributed across a wide range in the recip-rocal space and the use of relatively bigger acquisition apertures can help to improve the S/N ratio. In 2009, Lidbaum et el. acquired a series of energy filtered diffraction patterns (EFDIF) in the energy loss range of Fe L3, L2 edges [39]. They constructed the reciprocal space maps of the resulting EMCD signals as well as mL/mS ratios. This was the first quantitative work based on EMCD measurements and the S/N ratio was significantly improved as compared to previously reported measurements. Lidbaum et al. [83] also

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produced the first real space maps in EMCD experiments and highlighted that the EMCD measurements are sensitive to the orientation of TEM sam-ple.

Figure 4.4. Experimentally acquired reciprocal space maps of EMCD signals and the extracted mL/mS values. This was the first quantitative determination of magnetic moments using EMCD after its discovery. The figure is adopted from [39]with due permissions.

Another approach to improve the S/N ratio in EMCD measurements was to use higher energy electrons to reduce the plural scattering as well as dynam-ical diffraction effects. Tatsumi et al. [84] used an ultra-high voltage TEM with an acceleration voltage of 1 MeV and reported an increased S/N ratio as compared to standard TEM operating at 200 keV. As described in Sec-tion 4.1, the strength and distribution of EMCD signals is dependent on dy-namical diffraction conditions in addition to the magnetic properties of mate-rials. An optimization of dynamical diffraction conditions in the acquisition may lead to a significant improvement in the resulting S/N ratio as reported by Wang et al. [85].

One of the fundamental reasons motivating the development of EMCD tech-nique is the spatial resolution which is expected to be orders of magnitude better than what could be achieved with XMCD. The power of EMCD can be realized by answering the scientific questions which are out of scope of XMCD and other related techniques due to their limited spatial resolution. So the second most important point of focus after the S/N ratio in the past years has been to improve the achieved spatial resolution of EMCD analysis. Traditionally the EMCD experiments are required to be carried out in TEM mode using a parallel illumination to obtain two coherent electron waves. Then the spatial resolution obtained in the TEM mode is defined by selected area aperture (SAA) which is a few 100 nm in size or when the SAA is re-moved, it is the diameter of the electron beam. A resolution below 50 nm was shown by converging the beam in the TEM mode and changing the z-height of the sample, the so called LACDIF method [82]. To improve the spatial resolution below this extent would be possible by reducing the beam

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diameter and would naturally call for scanning TEM (STEM) mode where the spatial resolution is defined by the size of scanning electron probe.

In STEM mode, on the other hand, the Bragg spots in the diffraction plane become disks with the diameter of the disks defined by the beam conver-gence angle. Particularly for convergence angles needed for atomic resolu-tion, the disks would overlap. In such a situation, the EMCD signal at the Thales circle positions spreads out and weakens thus putting a fundamental limit on spatial resolution. Nevertheless, attempts started to get EMCD sig-nals in STEM mode. Schattschneider et al. were the first ones who obtained EMCD signals in STEM mode. Due to the natural assumption made about the harms of large convergence angles on EMCD detection, they used mi-croprobe STEM to limit the beam semi-convergence angle below 2 mrad which was sufficient to produce a beam under 2 nm diameter [86]. It was the first breakthrough in the achieved spatial resolution. The work produced a hope that it might be possible to detect EMCD signals with even larger con-vergence angles. Thersleff et al. in 2016 used semi-convergence angles of 8 mrad resulting in a probe diameter of 2 Å and detected EMCD signals for an Fe film [87]. Although the spatial resolution for detectable EMCD signals was limited to a few nanometers due to spatial drift and poor S/N ratio but the work opened a road for the detection of EMCD signals with convergent beams.

Figure 4.5. Classical EMCD maps acquired with sub-nm convergent beams in STEM mode. The figure is taken from [87] with due permissions..

There is one recent work by Rusz et al. [88] where the group obtained the EMCD signals from single atomic planes. The work is carried out in STEM mode with convergence angles resulting in atomic plane resolution but the experimental set up is different from what is used in the classical EMCD measurements. They manipulated the phase of the electron probe by precise shifts across the atomic planes and acquired EELS spectra for two conjugate phase shifts which were further processed to obtain EMCD signals. Atomic plane resolution has also been shown with parallel beam conditions in EFTEM mode by the use of an achromatic aberration corrected TEM [89].

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Nevertheless, it is still a challenge to obtain atomic resolution EMCD maps in STEM mode with the best ever demonstration shown in Figure 4.5. In addition to the developments made in S/N ratio and spatial resolution domains, EMCD has been applied both as a qualitative and quantitative technique to analyze the magnetic properties of materials [18-22, 90-99]. The technique has been utilized to measure site specific magnetic properties in a crystal [100]. The technique has been extended to get in-plane [101] and depth resolved magnetic measurements [102].

4.3 Major challenges Despite the efforts made to improve S/N ratio and spatial resolution of the EMCD technique, there are still challenges which make the quantitative EMCD analysis cumbersome and limit the achievable spatial resolution of measurements. As mentioned before, two EELS spectra at conjugate scatter-ing angles are needed to obtain an EMCD signal. In most of the experi-mental work, these two EELS spectra are acquired in a serial fashion with one acquisition following the other. This may lead to a change in experi-mental conditions as well as make it hard to ensure the spatial registration between the two acquisitions, thus making the high spatial quantitative EMCD analysis difficult. Although an alternative setup for acquiring the two conjugate EELS spectra in one acquisition was introduced but it has its own challenges (explained in Section 5.1.1) making it unsuitable for quantitative EMCD measurements. Another factor to consider, when working in STEM mode, is the local orientation of TEM sample. No crystal is perfectly orient-ed in a thin TEM lamella and small changes in the orientation of the sample may occur in the measured area which may lead to big changes in the result-ing EMCD signals. A major challenge to achieve atomic resolution EMCD is that the measurements should be carried out in zone axis (ZA) orientation with beam convergence angles being high enough to reach atomic resolution. Both of these requirements contain challenges in themselves. It is not easy to get EMCD signals in ZA due to high dynamical diffraction effects and EMCD signals get very weak at higher beam convergence angles. At higher convergence angles, the Bragg spots are converted into disks overlapping with each other as well as with the EMCD signal positions. Under these conditions, the weak EMCD signal may easily be hidden under the intensity of these Bragg disks which is much higher than the EMCD signal, thus mak-ing the EMCD detection difficult.

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4.4 Summary A short theoretical background of the electron magnetic circular dichro-ism (EMCD) technique is described. EMCD is an electron energy loss spec-troscopy (EELS) based technique to measure the magnetic moments of the materials in a transmission electron microscope. The technique has resem-blance with the X-ray absorption spectroscopy (XAS) based technique XMCD where the difference of the absorption spectra acquired with left and right circularly polarized photons is used to measure the magnetic moments. Whereas in the EMCD technique, the difference of two angle-resolved EELS spectra at two conjugate scattering angles in the reciprocal space is used to measure the magnetic moments, and this difference signal is known as the EMCD signal. The conjugate scattering angles are found by the theoretical comparison of the scattering process in EELS and the absorption process in XAS. Afterwards a review on the development of the EMCD technique is given and the major challenges of the technique are discussed.

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5 Towards quantitative EMCD with atomic resolution

In this chapter are described, the EMCD experiments carried out in this doc-toral work as well as the obtained results. The results discussed in this chap-ter belong to Papers IV, V and VI. The objective of this work is to establish EMCD as a quantitative technique with the goal to obtain magnetic meas-urements with atomic resolution. The most important feature of the work presented here is the simultaneous acquisition of angle resolved spectroscop-ic signals to acquire EMCD signals with high precision.

5.1 Quantitative EMCD by simultaneous acquisition of EELS

The basic experimental geometry followed in most of the classical EMCD experiments is the one that was proposed and used by Schattschneider et al. [80] in the first detection of EMCD signals. In the first step, TEM sample is tilted away from a high symmetry zone axis to reach a systematic row condition where only one diffraction spot, say g, fulfills the Bragg criterion and thus its intensity is much higher as compared to the rest of the diffraction spots. This condition is called 2-beam condition (2BC) where thus the zero beam and the reflection g are excited. When the sample is titled by an additional small angle such that g and –g reflections are excit-ed with equal intensity, this acquisition geometry is called 3 beam condition. The experiments done in this doctoral work used 2 beam diffraction condi-tions (2BC). An image of the viewing screen of the TEM containing a dif-fraction pattern of bcc Fe in 2BC is shown in Figure 5.1. The diffraction pattern contains two strongly excited spots whereas all the other reflections are very weak in intensity. The diffraction pattern is produced by the elec-tron beam converged to ~100 nm diameter in the TEM mode. The purpose of this image is to highlight the prominent features seen in the 2-beam diffrac-tion condition such as the Kikuchi band and the circular ring shape produced by higher order Laue zone (HOLZ) reflections which are otherwise difficult to observe on the CCD camera. The Kikuchi line is very useful experimental parameter to set the 2BC as the position of the Kikuchi line with respect to the diffraction spots shows the deviation from the exact Bragg

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conditions [103]. For 2BC, the Kikuchi line should symmetrically pass through the 2 strongly excited reflections.

Figure 5.1. A foto of the viewing screen of Tecnai F30 TEM showing the diffraction pattern for Fe with the sample tilted in 2BC. Only one systematic row of reflections is activated in 2BC with the direct and one diffracted beam strongly excited. The upper strongly excited spot is g=002 and the lower spot is 0-beam in this case.

Once the 2BC is fulfilled, an objective aperture or spectrometer entrance aperture (SEA) is used to acquire the EELS spectra at the two positions marked by C+ and C– in Figure 5.2. The two acquisitions are carried out one after the other by either moving the aperture or the diffraction pattern be-tween the two acquisitions. In all the experimental results shown in this the-sis, the SEA was used as a signal selecting aperture and the diffraction pat-tern was moved to switch between the two acquisition positions (for serial acquisitions). The collection angle and position of the SEA are important experimental parameters which should be optimized to obtain good quality EMCD signals. As the EMCD signals spread out in the reciprocal space due to the dynamical diffraction effects instead of being localized at fixed scat-tering angles, a bigger aperture can help to acquire more signal improving the overall S/N ratio. On the other hand, too big aperture can also create problems increasing the CCD readout noise. The optimum collection angles can be estimated from simulations of the inelastic scattering of the fast elec-trons in a magnetic sample. The collection angles for a fixed SEA can be changed by changing the camera length. In the EMCD experiments using the circular apertures, a semi-collection angle in the range of 3-4 mrad was used. Other factors that can affect the strength of the measured EMCD signals are the specific diffraction conditions and the thickness of the specimen. In all the EMCD experiments using 2BC, g = 002 for Fe was used instead of g = 110 because for g = 002 the magnetic signal strength is weakly influ-enced by a change in crystal thickness between 10-30 nm [83]. The thickness

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of the magnetic layer was 25 nm and 20 nm in the double aperture (DA) and the quadruple aperture (QA) experiments respectively.

Figure 5.2. A representation of classical EMCD acquisition setup (a) The TEM sample is tilted to fulfill 2BC with g = 002 for Fe (b),(c) two EELS spectra are ac-quired at C+ and C- positions (d) the background subtracted and post edge normal-ized EELS spectra are subtracted to obtain the EMCD signal. These data are ac-quired on the JEOL 2000FX TEM.

The two conjugate EELS spectra acquired at C+ and C- positions are back-ground subtracted and post-edge normalized. The resulting spectra are sub-tracted from each other to obtain the EMCD signal which should have oppo-site signal strength at two energy loss edges as shown in Figure 5.2 (d)..

One major drawback of the classical EMCD experimental setup described above is the serial acquisition of two conjugate EELS spectra required for EMCD measurements. The change of aperture position between the two acquisitions takes some time due to which it is difficult to guaranty identical experimental conditions between the two acquisitions. For example, the measured area of the sample might change between the acquisitions due to spatial drift. The sample structure may be modified by the electron beam [104] in first acquisition, making it different for the second acquisition. The problems of the spatial drift and beam damage become severe in the STEM mode when one acquires full maps of EELS spectra at two positions and each map may take a few tens of minutes. Thersleff et al. [87] reported a change in the sample morphology after the first map acquisition in STEM mode but they could not conclude which was the effect of this beam damage on the resulting EMCD signal. They used a sub-nm electron probe in the scanning mode but the final spatial resolution for EMCD detection was in the range of few nanometers due to spatial drift and limited acquisition time per scan point. Another fact to note is that many EMCD papers in the litera-ture have reported EMCD measurements on magnetic oxides. It has been

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reported in literature that the transition metal oxides are prone to beam dam-age and change their oxidation states under the influence of electron irradia-tion in the TEM [105-107]. A change in oxidation state of a transition metal oxide leads to a change in the L3/L2 ratio for the energy loss edges of the metal [108] which may contribute to the measured EMCD signals. In addi-tion, the instrumental instabilities may also contribute to a change between the two acquisitions.

5.1.1 q-E experimental setup

To avoid the problems arising due to serial acquisition of the two conjugate EELS spectra needed in EMCD measurements, Schattschneider et al. [82] proposed a new acquisition setup for EMCD experiments based on q-E dia-gram. To obtain a q-E diagram, the beams (in 2BC) are oriented on the CCD camera in such a way that the axis passing through them is parallel to the energy dispersive axis of the EELS spectrometer. Switching to spectroscopy mode in this condition replaces the horizontal (qx) axis of the reciprocal space to energy loss axis whereas the q dependence of the different EELS spectra along the vertical (qy) axis is preserved. The image of the CCD cam-era in spectroscopy mode is thus converted into a q-E image with qy as the vertical and E as the horizontal axis as shown in Figure 5.3. The EELS spec-tra at the two conjugate scattering angles can be extracted by drawing line profiles in the upper and lower regions of the q-E image. The EMCD meas-urements thus can be accomplished in a single acquisition ensuring the same experimental conditions for both conjugate spectra.

Figure 5.3. q-E acquisition setup is shown (a) the 2 beams aligned parallel to the energy dispersive axis of the spectrometer (b) an image of the CCD camera in spec-troscopy mode produces a q-E diagram (c),(d) EELS spectra extracted from two conjugate positions (e) the background subtracted and post edge normalized EELS spectra are subtracted to obtain the EMCD signal. The data is acquired on the JEOL 2000FX TEM.

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Despite the advantage of obtaining two EELS spectra simultaneously, the q-E mode has several drawbacks. In the q-E mode, the EELS spectra are produced by integrating the signal across full CCD camera including the areas where the magnetic signal is either very weak or even does not exist. This results in an overall increase in the noise of the EMCD signals. As well spurious, unwanted intensities could contribute to the spectral intensities. The q-E image contains high intensity Bragg spots whose intensity is thou-sands of times higher than the intensity at the classical EMCD positions. This high Bragg intensity limits the experimental dwell time (or beam cur-rent) to very low values to avoid CCD saturation. It becomes hard to get a good S/N ratio with such low acquisition times. Figure 5.4 shows an exam-ple of a q-E image acquired on the JEOL microscope with an acquisition time of 10 s where the CCD camera is already saturated by high intensity in the Bragg reflections.

Figure 5.4. A q-E image acquired with an acquisition time of 10 s. The high intensity of Bragg spots saturates the CCD camera.

Even if the sample is perfectly tilted to 2BC with 0 and G beams excited, there still exists a weak magnetic signal between 0 and –G which has oppo-site magnetic strength compared to the one between 0 and G [83]. If the beams are centred on the CCD camera in 2BC, this opposite signal is sub-tracted from the desired signal and the overall signal strength is reduced as shown in Figure 5.5. To avoid the interference of this opposite signal, the 0 beam can be positioned at the edge of the CCD camera but it will result in the acquisition of even more noisy signal on the other end (as explained in Fig. 6 of Paper IV).

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Figure 5.5. An illustration of magnetic signal distribution in 2BC. A weak magnetic signal exists between the direct and the weakly excited beam –G which is opposite in sign to the strong magnetic signal between 0 and G beams.

Moreover it has been shown that the magnetic signal from different structur-al components in the same TEM sample can appear at different scattering angles between and around the 2 beams [92]. In a q-E image, all such signals would be integrated together, thus making the purity of the resulting EMCD signal uncertain.

5.1.2 Acquisition of EMCD signals using a double aperture

The objective of this project was to obtain EMCD signals with improved S/N ratio and better signal selectivity in a single acquisition. A new experi-mental setup is proposed by combining the concepts of classical EMCD and q-E EMCD. For this purpose, an aperture strip containing two circular holes, the double aperture (DA), is built at the spectrometer entrance aperture’s (SEA) level keeping the axis joining the two aperture holes perpendicular to the energy dispersive axis of the spectrometer. The two beams are aligned in q-E orientation as described in Section 5.1.1. Switching to spectroscopy mode and acquiring the CCD image with the DA inserted produces two spectral traces in the obtained image resulting from the intensity in the two aperture holes. The intensity profiles extracted along each trace produces the two conjugate EELS spectra required for EMCD measurements. The EMCD signal can be obtained by taking the difference of post-processed EELS spectra.

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Figure 5.6. A representation of DA EMCD acquisition setup. (a) An aperture slit with two circular holes is placed in the diffraction plane with the two apertures positioned on C+ and C- positions (b) an image of the CCD is acquired in spectros-copy mode (c) The EELS spectra are extracted from the upper and lower spectral traces. The EMCD signal is obtained by taking the difference of post processed EELS spectra. The data is acquired on the JEOL 2000FX TEM.

The DA-EMCD acquisition setup offers a variety of advantages compared to the previous acquisition setups. - The first and the most important outcome of the DA-EMCD is the simulta-

neous acquisition of two EELS spectra, making the EMCD acquisitions (or scans in STEM mode) possible in a single acquisition. This can greatly help to minimize the problems related to specimen and energy drift, conse-quences of beam damage etc. The DA-EMCD thus ensures the spatial reg-istration and the same experimental conditions for the acquisition of two components of EMCD signal.

- A comparison of Figure 5.6 and Figure 5.4 immediately makes the value of the double aperture clear. The DA is filtering out the unnecessary high intensity of the Bragg spots, thus allowing for longer acquisition times or high beam currents to improve the S/N ratio of the acquired EMCD signal. The DA EELS image shown in Figure 5.6 (b) is obtained with an acquisi-tion time of 60 s.

- Using circular apertures, the acquisition area on the CCD can be limited to scattering angles where useful or strong magnetic signals exist. This results in an increase in the signal strength and a decrease in overall noise of the acquired EMCD signals. A comparison of the DA-EMCD signal and the qE-EMCD signal acquired with equal dwell time shows a significant im-provement in S/N ratio of the former compared to the latter (see Fig. 3 in Paper IV).

- The circular apertures can be well positioned at scattering positions pre-dicted by theory to acquire the desired EMCD signal, filtering out spurious magnetic signals which may arise from other structural components or may exist on the opposite side of the 0-beam (as shown in Figure 5.5).

The above mentioned advantages in combination with single-acquisition EMCD signals lead to minimize the problems of the extraction of quantita-tive information from the EMCD. This in turn would improve the accuracy and reliability of the quantitative measurements.

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5.2 Simultaneous mapping of EMCD signals and local crystal orientations

The classical EMCD experiments demand to fulfill the strict diffraction con-ditions (2BC or 3BC). From an experimental point of view, it is not always trivial to tilt the specimen in a 2-beam orientation. It is naturally assumed in most of the EMCD experiments that the intensity of the 0 and the g beams would be equal at 2BC but it is not the case. Depending on the thickness and the extinction length of the specimen, the intensity of the g-beam may be higher or lower than 0-beam at a perfect 2BC. Moreover, considering the fact that ideal crystals do not exist, the orientation of the crystal throughout the measured area may not be the same. Especially in the STEM mode, the crystal orientation may vary from one scan point to the next. It has been previously shown that a small change in crystal orientation may cause a big change in the measured EMCD signals [83] but no systematic study is done in this regard. One of the challenges in carrying out such an experiment would be the acquisition of the EMCD signals and the diffraction patterns one after the other. It is difficult to assure the spatial registration and the same experimental conditions between the two serial acquisitions making the point by point analysis cumbersome. We took this challenge and developed an acquisition strategy for simultaneously mapping the EMCD signals and local crystal orientations in a single scan.

We built a quadruple aperture (QA) which can simultaneously acquire four angle resolved spectroscopic signals in a single acquisition including the two conjugate EELS spectra needed for the EMCD measurements and the inelas-tic intensities of the 0 and the g beams. With the help of simulations, we show that the intensities of the 0 (I0) and the g (Ig) beams vary with a change in orientation from the perfect 2-beam orientation and each value of (Ig/I0) can be assigned to a crystal tilt with respect to the 2BC. In the STEM mode, we acquired a 4D dataset with a 2D spectroscopic image at each scan point (details in Section 2.4Fel! Hittar inte referenskälla.). Three maps of Ig/I0, L3 difference signal and L2 difference signal were produced by integrating the intensities in different spectral traces in each 2D image which are presented in Paper V. The maps correlate to each other in a good manner. The results show that a mistilt from the 2-beam orientation can significantly affect the measured EMCD signals. By analyzing the EMCD signals for different ori-entation intervals, we conclude that the EMCD signal strength decreases with a tilt away from the 2-beam orientation. The simulations showed that this decrease in the EMCD signal strength is not symmetric for the plus and minus tilts from the 2BC and the signal stays relatively stable for a tilt from the 2-beam orientation towards the 3-beam orientation. The experimental results verified the simulations and we obtained similar mL/mS values for the EMCD spectra within this tilt range.

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Figure 5.7. An illustration of the QA-EMCD acquisition. (a) The sample is tilted to 2BC (b) The QA is inserted in the diffraction plane with the smaller aperture holes centered on the diffraction spots (c) A 2D spectroscopic image showing four spectral traces originating from the four aperture holes.

5.3 Enabling the EMCD measurements with atomic resolution electron probes

The most competitive advantage of EMCD on other comparable techniques is the possibility to obtain quantitative magnetic information from single atomic columns within an analysis volume. Although EMCD has the poten-tial to get magnetic measurements with atomic resolution but it is challeng-ing due to several issues as described in Section 4.3. The large beam con-vergence angles are needed to reach atomic size electron probes and the dif-fraction spots are converted into overlapping discs making the q-selection of the EMCD signals difficult. Another limitation is the inherently used 2BC diffraction geometry in which only one kind of atomic planes are parallel to the electron beam, thus losing the resolution of individual atomic columns. There is one previous work where the authors reported EMCD measure-ments in zone axis (ZA) [109] but they carried out the experiments in paral-lel illumination conditions and the EMCD signals were obtained by moving the aperture to different positions in between the Bragg spots guided by the simulations. A standard methodology is needed to acquire EMCD signals both in ZA and with large convergence angles. Meanwhile Negi et al. [110] proposed the use of custom-made patterned apertures to acquire EMCD sig-nals in ZA with convergence angles resulting in atomic resolution. They named these apertures as ventilator apertures. This proposal opened a door for the goal of atomic resolution EMCD measurements.

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Figure 5.8. Simulations of (a) non-magnetic and (b) magnetic component of the inelastic scattering cross-section at the Fe-L3 edge. An electron beam with a semi-convergence angle of 5 mrad accelerated by 300 keV is passing through [110]zone axis of 20 nm bcc Fe crystal. (c) The ventilator aperture is superposed on the mag-netic signal. Courtesy to Jan Rusz.

After the publication of ventilator apertures, we took up the challenge to experimentally verify the simulations. We fabricated the ventilator apertures as well as new apertures with optimized design. As we used a variety of apertures in this project, therefore we assigned them distinct names for the sake of convenience. The ventilator apertures shown ovan can acquire single component of magnetic signal in one acquisition and have 8 holes, so we call them single signal 8 hole (SS8) aperture. To acquire the two components of the EMCD signal (i.e. two conjugate EELS spectra), either the aperture or the diffraction pattern should be rotated by 22.5o between the two acquisi-tions. This adds up one level of complexity to the experimental setup. The aperture may be rotated by building a piezo-electric motor connecting the aperture but the limited space for inserting the SEA holder in the spectrome-ter is a constraint. On the other hand, the diffraction pattern may be rotated by either rotating the sample or by changing the current in intermediate and projector lenses, both of which have the risk to change the experimental conditions between the two acquisitions. Luckily we have two smaller aper-ture holes (1 mm and 2 mm) in the SEA in addition to the 5 mm aperture which is mainly used for imaging and diffraction. This technical aspect led to us to an idea of building two SS8 apertures simultaneously with a 22.5o rotation offset. With this setting, the two magnetic components could be acquired just by switching the aperture between the two acquisitions. An EMCD signal acquired using SS8 apertures, with a beam semi-convergence angle of 5 mrad is presented in Paper VI.

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Figure 5.9. A picture of the SEA holder showing the positions of 5 mm, 2 mm, 1 mm and mask apertures (from right to left) used in normal operating modes. Two SS8 apertures are inserted in 1 mm and 2 mm aperture positions in complementary ori-entations with a rotation offset of 22.5o.

Considering the complications of serial acquisition of two EMCD compo-nents as described in Section 5.1, the acquisition of EMCD signals from the same area under the same experimental conditions is not trivial. The chal-lenge to ensure spatial registration between the two acquisitions becomes demanding when hunting for atomic resolution. To cope with this challenge, a mirrored ventilator aperture, here called double signal 7 holes (DS7) aper-ture was used to simultaneously acquire the plus and minus components of EMCD signal. It is worth mentioning here that we optimized the design of DS7 aperture as compared to the mirror ventilator aperture proposed in ref-erence [110]. The proposed and the optimized apertures are shown in Figure 5.10. In the optimized design, the aperture holes were cut in such a way to minimize the effects of first order Bragg reflections. A clear EMCD signal was detected using the DS7 aperture which is shown in VI.

Figure 5.10. (a)The proposed mirrored ventilator aperture for simultaneous acquisi-tion of two EMCD components (b) the optimized design of (a), here called DS7 aperture. The fig (a) is adopted from Ref. [110] with permissions.

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The ventilator apertures discussed above have a few limitations. The major limitation is the need to design a new aperture for every different crystal symmetry as well as for each different orientation of the same crystal. The other challenge is the difficulty to tune the spectrometer with these multihole apertures with each signal lying at different x and y coordinates on the CCD camera. Although the data was acquired in a 4D format which allows some of the post-acquisition alignment corrections as described in Sec-tion 2.4.3 but it is still difficult to guaranty an artifact free data. For already weak EMCD signals, a small misalignment of the spectral trace may lead to a big change in the measured values. To counter these problems, we pro-posed the use of a two hole aperture (here called double signal 2 hole DS2) for the acquisition of EMCD signals. The DS2 aperture, in principle, can be used to obtain EMCD measurements for multiple crystal symmetries and orientations. At the same time, it is much easier to tune the spectrometer with two holes located at the same x-coordinates on the CCD camera, elimi-nating many of the artifacts and difficulties encountered for ventilator aper-tures.

Figure 5.11. The EMCD signals (b, d, f) acquired in [110] zone axis of bcc Fe with beam semi-convergence angles of (a) 5 mrad (c) 7.5 mrad and (e) 10 mrad respec-tively. The acquisition was carried out using the DS2 aperture with the aperture holes oriented parallel to 110 axis in the diffraction plane. The inner and outer collection angles of the aperture were set to 12.5 mrad and 25 mrad respectively.

We used the DS2 aperture to acquire EMCD signals for two different orien-tations of the same TEM sample (bcc Fe) and it equally works well for the both. We acquired clear EMCD signals with beam semi-convergence angles of 5 mrad, 7.5 mrad and 10 mrad as shown in Figure 5.11.

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Figure 5.12. EMCD signals extracted by integrating different number of scan points (SP) in the line scan acquired with a beam semi-convergence angle of 10 mrad. A clear EMCD signal at both the L3 and L2 edges can be observed by inte-grating the data of 10 SPs.

At a beam semi-convergence angle of 10 mrad, the diffraction limited elec-tron probe size is 1.2 Å ignoring the spherical aberration [111]. This probe size is already smaller than the lattice plane spacings in many magnetic ma-terials, thus our measurements enable the detection of EMCD signals with beam convergence angles corresponding to atomic resolution probes. It is a significant step towards atomic resolution EMCD measurements as the ma-jor problem to obtain atomically resolved EMCD signal is the need of large beam convergence angles which make the detection of weak EMCD signals difficult. After the proposal of Negi et al. [110], the first needed step was to establish a methodology with which EMCD signals could be detected using convergent electron beams. We have established this methodology and have showed that the detection of quantitative EMCD signals is possible with beam convergence angles sufficient to reach atomic resolution. In the next step, the same setup should be applied to an aberration corrected TEM with an optimization of effects like beam damage to obtain atomically resolved EMCD maps.

5.4 Summary The EMCD signals is obtained by taking the difference of two conjugate angle-resolved EELS spectra which are acquired serially in the classical EMCD experiments. The serial acquisition produces challenges in maintain-ing the identical experimental conditions and the spatial registration between the two acquisitions. A technique for the simultaneous acquisition of the two angle-resolved EELS spectra is developed by the use of a double aperture. The double aperture ensures the acquisition of EMCD signals under identical experimental conditions as well as the perfect spatial registration in the

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measurements. In addition, it offers better signal to noise ratio as compared to the previously used qE acquisition setup. The technique of the simultane-ous acquisition of the angle-resolved spectroscopic signals is extended to measure the EMCD signals and the local crystal orientations in a single scan. With the use of a quadruple aperture, four spectroscopic signals are simulta-neously acquired, two of which are the conjugate EELS spectra required for the EMCD measurements whereas the remaining two signals give the inelas-tic intensities of the 0 and the g beams. The influence of a mistilt from the exact 2-beam condition (2BC) is studied and a safer way to tilt the sample into 2BC is introduced where the effects of small misorientations are mini-mum. Finally the simultaneous acquisition technique is applied to detect the EMCD signals with large beam convergence angles under a high symmetry zone axial diffraction geometry. The quantitative EMCD signals are detected for a beam semi-convergence angle of 10 mrad which correspond to a dif-fraction limited electron probe size of 1.2 Å. The work thus enables the de-tection of EMCD signals with atomic sized electron probes.

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6 Time resolved magnetic measurements

This chapter describes a novel TEM setup to obtain time resolved infor-mation at nanoscale. The time resolved measurements are important to un-derstand the dynamic phenomena that occur at time scales much lower than the scope of an ordinary TEM. The life time of these phenomena can be in the range of few microseconds to few femtoseconds and even lower. It is not possible to visualize these dynamics in a conventional TEM where the tem-poral resolution of two adjacent data frames depends on the readout time of the CCD camera which is typically in the range of a few milliseconds. In this work specifically, we are interested to explore the magnetic transitions in the materials happening in the time range of few microseconds to few nanosec-onds. The current time-resolved TEMs are described and our time resolved set up presented in Paper VII is discussed.

6.1 Currently applied time resolved TEM setups The current time-resolved TEMs use a so called ‘pump probe method’ to obtain information about structural and physics dynamics happening on sub-micrometer scales. The electron gun of such TEMs is replaced by a laser-driven photocathode which is used to generate ultrafast and short electron pulses [112-121]. In some experiments, the TEM sample is excited by using another short laser pulse. The excitation of the sample by a laser pulse is called ‘pump’ whereas the electron bunch produced by the photocathode is called a ‘probe’. The pump and probe are coupled to each other by a fixed time delay which defines the time resolution of the observed phenomenon.

The pump probe time resolved TEMs can work either in stroboscopic or in single shot mode. In stroboscopic mode, the electron packets consist of sin-gle or a few electrons. The electron pulse interacts with the excited material at a fixed delay time in a relatively long time cycle and the cycle is repeated few hundred (or even thousands) times to get sufficient intensity on the cam-era. The advantage of using single or few electrons in a packet is the im-proved spatial and temporal resolution [122], but it is applicable only to re-peatable dynamic processes. To study the non-repeatable dynamic processes such as melting, single shot experiments are needed [123, 124]. In such ex-periments, a large number of electrons are used in the electron packet (or

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electron bunch) to carry high intensity sufficient to obtain the images with adequate S/N ratio. The image of the dynamic changes is thus acquired in a single shot at a fixed delay time. The disadvantage of using many electrons in the electron bunch is a reduced spatial and temporal resolution typically in the few nanometers and few nanoseconds regime [125]. This is because the electrons in a pulse spread in time and space due to significant coulomb re-pulsion.

6.2 Time resolved measurements in TEM using a streak camera

The time resolved TEM setups discussed above need to build a special elec-tron source in a TEM to generate the pulses of electrons. In the stroboscopic experiments, the dynamic phenomena at only one or a few time delays in a full time cycle are studied. In the single shot experiments, the dynamic changes occurring only at one specific time delay after the excitation can be studied as the structure of the sample changes after the excitation. Here we propose and build a time resolved setup which does not need to manipulate the electron gun and is capable to track the full cycle of dynamic changes in one measurement. It equally can be driven in stroboscopic mode. The setup requires an ordinary TEM equipped with an EELS spectrometer.

The idea of the time resolved setup proposed here has been borrowed from the field of optics. In optics, the optical streak cameras are used to obtain temporally resolved measurements in the sub-micrometer timescales [126]. It was surprising for us that no significant effort had been put to implement the streak cameras in electron microscopes, with a little work done by Bos-tanjoglo et al. [127]. We built the streak camera setup inside the EELS spec-trometer attached with JEOL 2000FX TEM. The experimental strategy and the sample excitation mechanism are briefly described below. The detailed description can be found in Paper VII.

The time-resolved measurements are achieved by building a streak camera inside the EELS spectrometer. A streak camera consists of two parallel cur-rent conducting plates separated by a dielectric (vacuum in this case). A high voltage (HV) is applied to the plates and the polarity of the HV is reversed in alternating cycles. The electron beam passing through the plates is deflected towards the positively charged plate, with the deflection angle increasing with growing charge on the plates. This effectively results in a sweep of the electron beam from point I (initial) to F (final) on the CCD camera.

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Figure 6.1. A schematic showing the concept of streak camera. Two metal plates are charged and discharged alternatively in subsequent cycles, causing the electron beam passing through them to sweep across the CCD camera. The plates are posi-tioned in front of the CCD camera through the TV camera port in the GIF 2002.The distances in the schematic are not according to the scale.

The polarity of the plates is reversed in the next cycle. This reverse charging of the plates will cause the electron beam to sweep across the CCD camera in opposite direction (from F to I) overriding the data captured in cycle 1 which is of course undesirable. To avoid this reverse sweep, a mechanism is needed to blank the electron beam in every even cycle. We use the drift tube in the EELS spectrometer to blank the beam. For this purpose, we built a switching circuit between the drift tube power supply (1 kV) installed in Gatan instrumentation bin (GIB) and the drift tube. The switch is closed during every even cycle applying 1kV to the drift tube, resulting in moving the electron beam away from the CCD camera. In the odd cycles, the switch is opened with 0 V applied to the drift tube, allowing the normal sweep of the electron beam.

Figure 6.2. A drawing illustrating the charging/discharging cycles of the metal plates, the electron beam is allowed to sweep across the CCD camera in odd cycle (from I to F) whereas it is blanked in even cycles (from F to I).

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The TEM sample should be physically excited to induce dynamic changes in one of its properties which would be studied in the time-resolved measure-ments. The excitation mechanism depends on the type of properties to be studied. In this case, we are interested to study the collective magnetic exci-tations of the sample and for this purpose the TEM sample should be mag-netically excited and de-excited while getting the measurements during the de-excitation cycle. The objective lens of the TEM should be switched off to remove the external magnetic field and the TEM should be operated in the so called Lorentz mode [128]. We modified an existing TEM sample holder in a way that it could produce the magnetic field around the TEM sample in specified time intervals synchronized with the beam sweeping cycles. Two conducting wires are installed inside the sample holder with the input ports outside the sample holder rod and the outputs going to the TEM sample stage. The inputs are connected to an external power supply whereas the outputs are connected to a small current loop (CL) built around the TEM sample.

Figure 6.3. A drawing of the modified TEM sample holder with two wires assembled in the holder rod to reach the sample stage.

The CL is built on a 3 mm Si/SiO2 disc with a 100 µm hole in it. The disc is positioned above the TEM sample with the hole centered on the area of in-terest. The CL is then connected to the output of the wires installed in the sample holder.

Figure 6.4. Schematics showing the (a) design of current loop fabricated on a 3 mm disc (b) the positioning of the current loop on TEM sample in the TEM holder

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In this initial setup, we expect the beam sweep times in the microseconds to nanoseconds range due to the hardware limitations (such as the time con-stants of beam sweeper plates and the beam blanker switch, the switching time of the HV power supply etc.). Depending on the sweeping times, the pixel-.pixel time resolution on the CCD camera can be much smaller. For example, if we consider 1 µs beam sweep time for a 2k camera, the temporal resolution between two neighboring pixels would be 500 ps. The streak camera based time-resolved setup can be used to acquire one dimensional time-resolved images, diffraction patterns and EELS spectra with the time resolved axis perpendicular to the energy dispersive axis. Con-sidering the acquisition of EELS, the setup should be able to obtain time-resolved EMCD measurements although the measurements in this case would take much longer time seeing the weak intensity of the EMCD sig-nals.

6.3 Summary A streak camera based setup is proposed for time resolved measurements in the transmission electron microscope. The streak camera is built in the EELS spectrometer just before the CCD camera. The conducting plates of the streak camera are charged with opposite polarities in alternating cycles. The electron beam passing through the plates is deflected towards the positively charged plate where the deflection angle increases with increasing charge on the plates. This results in a sweep of the electron beam from the negatively charged towards the positively charged plate on the CCD camera, thus mak-ing the sweep axis effectively a time-resolved axis. The electron beam is swept across the CCD camera in one direction and is blanked in the opposite direction to avoid overriding of the data. The TEM sample holder is also modified by building a current loop around the TEM sample which can magnetically excite the sample. This setup should be able to carry out the single shot as well as the stroboscopic time-resolved measurements.

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7 Conclusions and outlook

As the dimensions of the thin films are reduced, the role of interfaces be-comes important to control the magnetic properties. The effects like the symmetry breaking and spin-orbit coupling at the interfaces can significantly alter the magnetic properties of the thin films compared to the bulk proper-ties. It is very important to understand and control the structure of such sys-tems to produce the devices with desired magnetic properties. At the same time, the characterization techniques are needed which could analyse the structure and the magnetic properties with a spatial resolution in the range of nanometers down to atomic resolution. Transmission electron micro-scope (TEM) is one of the most suited instruments which can characterize the structures and the magnetic properties of the materials at these length scales.

The TEM has been employed to characterize the structures of Fe/V and Fe/Ni magnetic multilayer systems. The STEM-HAADF images of Fe/V with a periodicity of 4/28 monolayers (ML) showed the real space structural coherence of the layers. The HRTEM images and the SAD pattern confirm the existence of coherent superlattice structure. STEM-HAADF was applied to confirm the existence of multilayer structures in Fe/Ni thin films with periodicities of 16/16 ML, 8/8ML and 4/4 ML. The Fe/Ni interfaces in all the three multilayer systems were analyzed using STEM-EELS spectrum imaging. The results the STEM images and EELS maps showed that the Fe and Ni layers exist in all the samples but are heavily interdiffused. The diffu-sion is asymmetric with Ni diffused more into the Fe layers and Fe diffused less into the Ni layers. The confirmation of the existence of multilayer struc-ture in Fe/Ni 4/4 monolayers sample helped to understand and explain the measured saturation magnetization for this sample.

To analyse the magnetic properties of such thin films using TEM, we have developed the methodology to simultaneously acquire the angle-resolved electron energy loss spectra (EELS) in a single acquisition to establish elec-tron magnetic circular dichroism (EMCD) as a quantitative analysis tech-nique with high spatial resolution. First, we built a double aperture in the TEM to simultaneously acquire the two conjugate EELS spectra needed for the EMCD measurements. The simultaneous acquisition ensures the identi-cal experimental conditions and spatial registration for the two EELS spec-

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tra, thus improving the accuracy of the measurements. The EMCD signals obtained by the use of double aperture acquisition show better signal to noise (S/N) ratio as compared to the previously used qE acquisition setup. We further developed the technique to simultaneously acquire the EMCD signals and the local crystal orientations with high accuracy in a single scan. This work helped to experimentally understand the influence of crystal tilt on the measured EMCD signals. Finally we developed the simultaneous acquisition methodology to acquire the quantitative EMCD signals in a high symmetry zone axis with beam convergence angles corresponding to atomic resolution electron probes. This work enables the detection of the EMCD signals with atomic sized electron probes.

Although we demonstrate the acquisition of the EMCD signals with large beam convergence angles which correspond to atomic resolution electron probes, we cannot obtain the real space EMCD maps with atomic resolution. The reason is the presence of spherical aberration which limits the minimum probe size in the range of a few Ångströms on the Tecnai TEM. To obtain the EMCD measurements with atomic resolution, the first key step is to es-tablish a methodology which could acquire the EMCD signals with large beam convergence angles which cause the diffraction disks in the reciprocal space to overlap. We have addressed this challenge and have demonstrated the EMCD acquisition with such beam convergence angles. The next step would be to employ the same acquisition setup on an aberration corrected TEM and obtain the atomic resolution EMCD maps. This would further need optimizations of factors such as beam damage and spatial drift.

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Sammanfatting på Svenska

Dagens tunnfilmsteknik tenderar att bygga funktionella filmer med tunnare och tunnare delskikt. När dimensionerna på de tunna filmerna reduceras blir gränsytorna allt viktigare för de magnetiska egenskaperna. Effekter som symmetribrytning och spinn-ban-koppling i gränsytorna kan väsentligt för-ändra de tunna filmernas magnetiska egenskaper jämfört med bulkegenskap-erna. Det är väldigt viktigt att förstå och kontrollera strukturen för att kunna tillverka komponenter och system med önskade magnetiska egenskaper. Samtidigt behövs karakteriseringstekniker som kan analysera strukturen och de magnetiska egenskaperna med en rumsupplösning på nanometernivå eller ända ner till atomupplösning. Transmissionselektronmikroskop (TEM) är ett av de mest lämpade instrumenten som kan karakterisera materialens struk-turer och magnetiska egenskaper vid dessa längdskalor.

TEM har använts för att karakterisera strukturerna i de magnetiska fler-skiktssystemen Fe/V och Fe/Ni. STEM-HAADF-bilderna av Fe/V med en periodicitet av 4/28 monolager (ML) visade den verkliga rymdstrukturella koherensen hos skikten. HRTEM-bilderna och SAD-mönstret bekräftar fö-rekomsten av en sammanhängande supergitterstruktur. STEM-HAADF an-vändes för att bekräfta förekomsten av flerskiktsstrukturer i tunna Fe/Ni-filmer med periodiciteter av 16/16 ML, 8/8ML och 4/4 ML. Fe/Ni-gränsytor i alla de tre flerskiktssystemen analyserades med användning av STEM EELS-spektrumavbildning. Resultaten från STEM-bilderna och EELS-kartorna visade Fe- och Ni-skikt i alla prover men de är inte rena metaller. Diffusionen är asymmetrisk, där Ni diffunderar mer in i Fe-skikten medan Fe diffunderade mindre in i Ni-skikten. Denna bekräftelse av förekomsten av flerskiktsstruktur i Fe/Ni 4/4 ML hjälpte till att förstå och förklara den upp-mätta mättnadsmagnetiseringen för detta prov.

För att analysera magnetiska egenskaper hos den här typen av tunna filmer med TEM har vi utvecklat metodik för att samtidigt ta upp vinkelupplösta electron energy loss spektra (EELS) i en enda mätning för att etablera electron magnetic circular dichroism (EMCD) som en kvantitativ analystek-nik med hög rumslig upplösning. Först byggde vi en dubbel bländare i TEM för att samtidigt ta upp de två konjugerade EELS-spektra som behövs för EMCD-mätningarna. Samtidig upptagning garanterar identiska experimen-tella förhållanden och rumslig registrering för de två EELS-spektra, vilket

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förbättrar mätningens noggrannhet. EMCD-signalerna erhållna genom an-vändning av tekniken med dubbla bländare uppvisar bättre signal/brus (S/N)-förhållande jämfört med den tidigare använda qE-tekniken. Vi vidareutveck-lade tekniken för att samtidigt mäta EMCD-signaler och lokala kristallorien-teringar med hög noggrannhet i en enda mätning. Detta arbete hjälpte till att experimentellt påvisa inverkan av kristallernas orientering på de uppmätta EMCD-signalerna. Slutligen utvecklade vi tekniken med samtidig upptag-ning till att kunna kvantifiera EMCD-signalerna i en zonaxel med hög sym-metri med strålkonvergensvinklar som motsvarar mätning med atomupplös-ning. Detta arbete möjliggör upptäckt av EMCD-signaler med atomär upp-lösning.

Även om vi demonstrerar att mätning av EMCD-signaler med stora strål-konvergensvinklar som motsvarar med atomär upplösning kan vi inte mäta upp reella EMCD-kartor med atomär upplösning. Anledningen är förekoms-ten av sfärisk aberration som begränsar minsta sondstorlek till några få Ång-ström på Tecnai TEM. För att erhålla EMCD-mätningar med atomupplös-ning var det första nyckelsteget att upprätta en metod som kunde mäta EMCD-signaler med stora strålkonvergensvinklar som får diffraktions-mönstret i det reciproka rummet att överlappa varandra. Vi har antagit denna utmaning och lyckats genomföra EMCD-mätningar med sådana strål-konvergensvinklar. Nästa steg skulle vara att använda samma mätteknik på ett brytningsfelskorrigerat TEM för att kunna mäta upp atomärt upplösta EMCD-kartor. För detta skulle ytterligare optimering behövas för att und-vika faktorer som strålskador och drift. Dessutom har vi byggt olika kompo-nenter för tidsupplöst TEM och de första experimenten borde kunna genom-föras inom en nära framtid.

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Acknowledgements

First of all, I would like to express my deepest gratitude to my supervisor Prof. Klaus Leifer who introduced me to the world of electron microscopy and nanoscience. He is a real mentor and guided me through every phase of my PhD studies. The best thing about Klaus is that I can anytime go to his office and talk to him. The skills and knowledge I acquired during my PhD have a significant contribution from Klaus. I would simply say that I respect and adore you Klaus.

I extend my gratitude to my co-supervisor Prof. Björgvin Hjörvarsson. Although we had very few meetings but the impact of the meetings is very high. After every meeting with him, I felt a potential change in my thoughts and personality. I want to thank my second co-supervisor and a colleague Dr. Ling Xie who always welcomed me whenever I asked for a help. He has a very friendly and supportive nature. I should not forget to thank Dr. Thomas Thersleff (Tom), my former co-supervisor for all the help and guidance.

I would like to thank the present and former members of ELMiN group Dr. Hu Li, Tianbo Duan, Omer Sher, Farnaz Ghajeri, Sharath Kumar, Anumol Ashok, Ishtiaq Hassan Wani, Yuanyuan Han, Aqib Hayat and Sylvester Wambua for all the nice time spent together. Thanks to all the colleagues from MIMS, Tribo and MST groups for being kind during all this time. Thank you Aqib for reviewing my thesis. I am thankful to Jona-tan Bagge and Urban Wiklund for their help to write the Swedish sum-mary.

Here I want to specially thank my teacher, colleague and a good friend Dr. Hassan Jafri who helped me to get the funding for my PhD studies and introduced me to Klaus. He produced a turning point in my life and I will never forget that. You will always find me humble and obedient, sir. I am also thankful to Erasmus Mundus Experts consortium for providing the financial support for my PhD studies.

The PhD life would be very boring without the friends and colleagues who keep meeting me in the corridors or at coffee breaks. I am thankful to friends Imran Aziz for arranging regular coffee breaks, Luimar Correa Filho for

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nice discussions during smoking in the cold, Nishant Saini for Punjabi chats in the corridors and Lee Fowler for sharing short stories.

I am very thankful to the people in the administrative staff Jonatan Bagge, Sara Rudqwist, Ingrid Ringård and Maria Skoglund who always re-sponded me in a kind way whenever I approached them with a question. I pay my gratitude to the MSL staff Fredric Ericsson, Amit Patel, Victoria Sternhagen and Lars Riekehr for all the technical helps. I don’t want to skip former MSL staff member Jan Åke whose help was very valuable for me to accelerate in my first TEM project. I am grateful to Eric Lindholm who fabricated the apertures for me in short durations. He has a key role in the success of my projects. From the name of Eric, I remember Johan Eriksson who introduced me to Eric. Johan is a very nice guy, speaks very softly and is always ready to help you of his best.

I acknowledge the contributions of my collaborators from the department of Physics Volker Ziemann, Tobias Warnatz, Jan Rusz and Devendra Negi. Without their efforts I could not have done all this work.

Here I would pay my deep tribute to my mother Yasmin Tahira and my father Abdul Latif Bhatti. Today I am here just because of them, they struggled throughout their life but continued my education. I salute you Ami and Abu. Many thanks to my brothers, sisters and friends in Pakistan for their best wishes.

My final thanks goes to my dear wife Aimen Imtiaz and my sweet daughter Ayza Ali. I am really grateful to you Aimen for giving me adequate space to concentrate on my studies. I know it was not easy for you to manage every-thing with the baby and I appreciate your efforts. Your love has always been a source of satisfaction for me. Thank you my lovely Ayza, you are the peace of my heart. Hasan Ali Uppsala Sweden December 2019

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