2014-Phys.Stat.Solidi-Synthesis&characterization arc depo mag .pdf

8/9/2019 2014-Phys.Stat.Solidi-Synthesis&characterization arc depo mag .pdf

1/4

2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Phys. Status Solidi RRL8 , No. 5, 420423 (2014) /DOI 10.1002/pssr.201409087

www.pss-rapid.com

p s s

Synthesis and characterizationof arc deposited magnetic(Cr,Mn) 2AlC MAX phase filmsAurelija Mockute*, 1, Per O. . Persson1, Fridrik Magnus2, 3, Arni Sigurdur Ingason1, Sveinn Olafsson3,Lars Hultman1, and Johanna Rosen 1 1 Department of Physics, Chemistry and Biology (IFM), Linkping University, 58183 Linkping, Sweden2 Department of Physics and Astronomy, Uppsala University, P.O. Box 530, 75121 Uppsala, Sweden3 Science Institute, University of Iceland, Dunhaga 3, 107 Reykjavik, Iceland

Received 20 February 2014, revised 30 March 2014, accepted 2 April 2014Published online 7 April 2014

Keywords MAX phases, arc deposition, epitaxial thin films, magnetism

* Corresponding author: [email protected] , Phone: +46 13 281279

2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction Mn+1AXn (MAX) phases (M = earlytransition metal, A = A-group element, X = C or N, andn = 13) constitute a family of inherently nanolaminatedmaterials. For n = 1, the hexagonal structure gives a MXMAMXMA atomic layer stacking in thec-direction ([0001]). MAX phases combine metallic andceramic characteristics [1, 2]. For example, they are goodelectric and thermal conductors, while also being oxidationand thermal shock resistant. Recently, the already uniqueset of properties was expanded by the observation offerromagnetism in the theoretically predicted and experi-mentally synthesized (Cr,Mn)2GeC [3, 4] and Mn2GaC [5].A magnetic ground state has also been theoretically pre-dicted for (Cr,Mn)2AlC, though this MAX phase is anti-cipated to enable tuning of the magnetic state by eithervariation of the Mn concentration or by different Cr Mnconfigurations on the M -sublattice [6]. The nanolaminatedstructure and anisotropic nature of MAX phases, comple-mented with the recently suggested potentially tunablemagnetic properties, suggests a functional material with potential for various technological applications, such as forsensors or spintronics.

(Cr,Mn)2AlC was recently synthesized both as bulkmaterial [7] and as thin films by magnetron sputtering[8] with resulting compositions of (Cr 0.94Mn0.06)2AlCand (Cr 0.84Mn0.16)2AlC, i.e., with a Mn content of 3 at%and 8 at%, respectively. Vibrating sample magnetometry(VSM) measurements of (Cr 0.94Mn0.06)2AlC showed nomagnetic response, which is not surprising at the relativelylow Mn content. The magnetic characterization of the(Cr 0.84Mn0.16)2AlC thin films was not performed due to presence of minor amount of magnetic impurity phases.Thus, the magnetic properties of (Cr,Mn)2AlC thin filmsremain unexplored. Theoretical calculations indicate thatthe magnetic moment per formula unit increases with in-creasing Mn content [6]. To increase the Mn incorporation,a highly energetic plasma flux provided in arc thin filmdeposition might enhance the solubility limit of Mn inCr 2AlC, while also allowing for a reduced synthesis temperature.

In this Letter, we report on the successful increase ofthe Mn content in (Cr,Mn)2AlC thin films compared to previous work [8], as realized by pulsed cathodic arc depo-sition using a Cr/Mn compound cathode. Furthermore, ex-

(Cr 1 xMn x)2AlC MAX phase thin films were synthesized bycathodic arc deposition. Scanning transmission electron mi-croscopy including local energy dispersive X-ray spectros-copy analysis of the as-deposited films reveals a Mn incorpo-ration of as much as 10 at% in the structure, corresponding

to x = 0.2. Magnetic properties were characterized with vibrating sample magnetometry, revealing a magnetic responseup to at least room temperature. We thus verify previoustheoretical predictions of an antiferromagnetic or ferromag-netic ground state for Cr 2AlC upon alloying with Mn.


2/4

Phys. Status Solidi RRL 8, No. 5 (2014) 421

www.pss-rapid.com 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Rapid

Research Letter

perimental evidence is presented for theoretically predictedmagnetic properties.

2 Experimental details Thin films of (Cr,Mn)2AlCwere synthesized using a high current pulsed cathodic arcat a base pressure of 5.81 10 7 Torr. A compound Cr/Mncathode of composition 50/50 at%, as well as elemental Aland C cathodes were used in alternating mode at a rate of10 Hz. Pulse lengths were set to 450, 250, and 1000 s forCr/Mn, Al, and C cathodes, respectively. The films weredeposited on Al2O3(0001) substrates cleaned in acetone,methanol, and isopropanol ultrasonic baths for 5 min eachand degassed for 10 min at the growth temperature of625 C. The selected temperature was chosen based onoptimization of structural quality, identified throughX-ray diffraction (XRD). A pulse sequence of 4:1:4:20

Cr/Mn:C:Cr/Mn:Al was used, mimicking the atomic MXMA layering in M2AX phases. A pulse ratio C:Alof 1:20 has previously been used in Ti2AlC synthesis. Theamount of Cr/Mn pulses in the sequence has been deter-mined through growth rate calibration followed by adjust-ments based on XRD results.

A reference film of (Cr,Mn)5Al8 for VSM analysis wasdeposited by dc magnetron sputtering from elemental tar-gets on an Al2O3(0001) substrate kept at 600 C.

XRD and X-ray reflectivity (XRR) measurements were performed using a Panalytical Empyrian MRD equippedwith a line focus Cu K source ( = 1.54 ) and a hybridmirror optics on the incident beam side. Scanning electron

microscopy (SEM) analysis was conducted using a LEO1550 SEM. A cross-sectional sample for scanning trans-mission electron microscopy (STEM) analysis was pre- pared by conventional mechanical methods followed bylow-angle Argon ion milling with a final fine-polishingstep at low acceleration voltage. STEM imaging and en-ergy dispersive X-ray spectroscopy (EDX) elementalanalysis was performed in the doubly corrected LinkpingTitan3 60300 equipped with a Super-X EDX detectorfor elemental analysis. Magnetic measurements werecarried out in a Cryogenic Ltd. VSM in the temperaturerange 5 280 K with the magnetic field parallel to the film plane.

3 Results and discussion Figure 1 shows the XRD 2 scan revealing the presence of (000n) MAX phase peaks. A c-lattice parameter of 12.86 was calculated us-ing Braggs law, which is consistent with previously reported magnetron sputtered (Cr,Mn)2AlC thin films [8].

An SEM overview image of the epitaxial (Cr,Mn)2AlCfilm surface is shown in Fig. 2. The film is constituted of partly coalesced islands with a substrate surface coverageof 72%. Each irregularly-shaped island exhibits flat sur-faces and the same height such that fringes could be ob-served in the XRR measurement, estimating the film thick-ness to 21 2 nm. Uniform island height and flat surfacescan be attributed to preferred basal-plane growth of MAX phases [9] enabled here by a high mobility of adatoms

I n

t e n s

i t y

[ a r b . u

n i t s

]

(0002)

(0004)(00010)

(00012)

(0006)Al2O3 Al2O3

2 []20 40 60 80 100

Figure 1 XRD 2 scan from an epitaxial (Cr,Mn)2AlC filmdeposited on an Al2O3(0001) substrate.

from thermal energy and momentum-partitioning from theenergetic deposition flux, characteristic of arc depositions.

The island nature of the film is also evident in theSTEM cross-sectional image shown in Fig. 3a. EDX ele-mental mapping as shown in Fig. 3b was performed on aMAX phase grain, as indicated in Fig. 3a, linking the ele-ments Cr, Mn, Al, and C (not shown) to the particle. Theelemental ratio of (Cr + Mn):Al could be quantified toapproximately 2:1, with a total Mn content of ~10 at%( x = 0.2). This is slightly higher than previously synthe-sized magnetron sputtered (Cr,Mn)2AlC, where 8 at% ofMn was reported [8]. The atomically resolved STEM im-age in Fig. 3c reveals high structural quality. The slightly bent STEM specimen provides the characteristic alternat-ing bright/dark-layered contrast of the M2X and A layers intwo corners of the image. In the center of the STEM imagethe sample is exactly on-axis such that the sample, giventhe strong diffracting contrast conditions, does not exhibita strong Z-contrast component.

XRD phase identification indicated a phase-pure epi-taxial (000n)-oriented MAX-phase film, as no other peakswere observed in the 2 scan. However, it has recently

1 m

200 nm

Figure 2 SEM image of an epitaxial (Cr,Mn)2AlC film depositedon an Al2O3(0001) substrate.


3/4

422 A. Mockute et al.: Synthesis and characterization of arc deposited magnetic (Cr,Mn) 2AlC MAX phase films

2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.pss-rapid.com

s t a t u s

s o

l i d i

p h y s

i c a rrl

Figure 3 (a) Cross-sectional STEM overview image revealing

flat islands of uniform height. (b) EDX elemental mapping on aMAX phase grain as indicated in (a), and (c) atomically resolvedSTEM image demonstrating the characteristic nanolaminated na-ture and high structural quality.

been shown that phase identification of (Cr,Mn)2AlCcannot be based on XRD 2 measurements alone, dueto the possible formation of a competing (Cr 1 yMn y)5Al8 phase with (ll 0) peaks precisely overlapping with the(Cr,Mn)2AlC (000n) peaks [10]. Although only in minoramounts, the presence of (Cr 1 yMn y)5Al8 particles with x = 0.72 was observed in the films by complementarySTEM and EDX analysis. For further details, see Ref. [10].

Figure 4 shows the in-plane magnetization of the(Cr,Mn)2AlC thin film as a function of magnetic field. AnS-shaped magnetic response is detected throughout thewhole investigated temperature range of 5280 K. No hys-teresis is observed, showing that in the absence of anapplied field the magnetic moments are either randomlyoriented or cancel each other out. A random orientationcan be obtained due to the non-uniform island structure ofthe film, where single domain islands interact weaklythrough dipole fields. Ferromagnetic domains could there-fore cancel each other out. The cancelling of moments canalso imply an antiferromagnetic configuration at low fields.The small field, which is required to rotate the moments, isconsistent with the near degeneracy of the ferromagneticand antiferromagnetic configurations predicted by theory[6]. The saturation magnetization dependence on tempera-ture is weak in the temperature range studied, as shown inthe bottom-right inset (at 4 T). This implies that the order-ing temperature is significantly higher than room tempera-ture.

In order to evaluate the possible contribution to themagnetic signal from the observed (Cr 0.28Mn0.72)5Al8 competing phase, the magnetization of a magnetron sputtered(Cr 0.28Mn0.72)5Al8 film deposited on an Al2O3 substratehas been measured for Ref. [10]. It is known that pureCr 5Al8 is paramagnetic [11], while Mn5Al8 has been meas-ured to exhibit a weak magnetic signal with magnetic mo-ment per Mn atom of only 0.03 B at 10 K [10]. In Fig. 4,

Figure 4 Magnetic response of the (Cr,Mn)2AlC thin film meas-ured by VSM with the magnetic field applied parallel to the film plane. The diamagnetic contribution from the Al2O3 substrate has been subtracted. A magnetic signal is observed up to the maxi-mum measurement temperature of 280 K. Bottom-right inset: Thetemperature dependence of the saturation magnetization in(Cr,Mn)2AlC at 4 T. Top-left inset: Low field in-plane magneti-zation of (Cr,Mn)2AlC and (Cr 0.28Mn0.72)5Al8 films at 280 K.

the magnetic response per unit volume of both the(Cr,Mn)2AlC and (Cr 0.28Mn0.72)5Al8 at 280 K is shown inthe top-left inset. The magnetization of (Cr 0.28Mn0.72)5Al8is only 1/6 that of the (Cr,Mn)2AlC sample. Clearly,

the magnetic contribution from (Cr 0.28Mn0.72)5Al8 cannot account for the observed magnetic response ofthe (Cr,Mn)2AlC sample, which shows that the(Cr 0.8Mn0.2)2AlC MAX phase is indeed magnetic, in agree-ment with theoretical predictions [6]. The higher magneticsignal observed for (Cr,Mn)2AlC indicates larger magneticmoment per M atom than in (Cr 0.28Mn0.72)5Al8, which has previously been determined to be 0.16 B at 10 K [10].

The presented VSM measurements reveal (Cr,Mn)2AlCas a new magnetic MAX phase. It is worth noting that themagnetization dependence on temperature is small, indicat-ing magnetic transition temperature is considerably higherthan 280 K. For previously reported magnetic MAX phases,

(Cr,Mn)2GeC and Mn2GaC, magnetic ordering disappearsat ~230 K for the latter [5], while the magnetic response of(Cr,Mn)2GeC, although still present at 300 K, exhibits aconsiderable decrease compared to lower temperatures[3, 4]. A high magnetic transition temperature is a gener-ally required property for functionalization of magnetic phases, which makes (Cr,Mn)2AlC a highly promising ma-terial for further investigations.

4 Conclusions Epitaxial (000n)-oriented(Cr 0.8Mn0.2)2AlC MAX phase thin films can be synthesized by pulsed cathodic arc. The films are solid solutions withthe highest Mn concentration to date (10 at%). A magneticresponse significantly above room temperature can be con-cluded for the as-deposited films. The recently discovered


4/4

Phys. Status Solidi RRL 8, No. 5 (2014) 423

www.pss-rapid.com 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Rapid

Research Letter

group of magnetic MAX phases has thus gained a newmember in (Cr,Mn)2AlC.

Acknowledgements The research leading to these resultshas received funding from the European Research Council underthe European Communities Seventh Framework Programme(FP7/20072013)/ERC Grant agreement No. [258509]. J.R. ac-knowledges funding from the Swedish Research Council (VR)grant No. 642-2013-8020 and from the KAW Fellowship pro-gram. P.O..P. and L.H. acknowledge the Swedish ResearchCouncil (VR) and the Knut and Alice Wallenberg Foundation.J.R. and P.O..P acknowledge support from the SSF synergygrant FUNCASE Functional Carbides and Advanced SurfaceEngineering. F.M. acknowledges funding from the University ofIceland Research fund and the Carl Trygger Foundation.

References

[1] H. Nowotny, Prog. Solid State Chem.2, 27 (1970).[2] M. W. Barsoum, Prog. Solid State Chem.28 , 201 (2000).

[3] A. S. Ingason, A. Mockute, M. Dahlqvist et al., Phys. Rev.Lett. 110 , 195502 (2013).

[4] Z. Liu, T. Waki, Y. Tabata et al., Phys. Rev. B89 , 054435(2014).

[5] A. S. Ingason, A. Petruhins, M. Dahlqvist et al., Mater. Res.Lett. 2, 89 (2013).

[6] M. Dahlqvist, B. Alling, I. A. Abrikosov et al., Phys. Rev. B84 , 220403 (2011).

[7] A. Mockute, J. Lu, E. J. Moon et al., submitted for publication.

[8] A. Mockute, M. Dahlqvist, J. Emmerlich et al., Phys. Rev.B 87 , 094113 (2013).

[9] J. Emmerlich, H. Hgberg, S. Sasvari et al., J. Appl. Phys.96 , 4817 (2004).

[10] A. Mockute, J. Lu, A. S. Ingason et al., submitted for publication.

[11] V. I. Cherchernikov, V. I. Nedelko, and A. V. Vedyayev,

Phys. Met. Metallogr.29 , 223 (1976).

2014-Phys.Stat.Solidi-Synthesis&characterization arc depo mag .pdf

Documents

Transcript of 2014-Phys.Stat.Solidi-Synthesis&characterization arc depo mag .pdf