2572 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009

Magneto-Transport Properties of La� �Ca� � �Sr�MnO� Thin FilmsDeposited by Pulsed Electron Deposition

Young-Min Kang, Nam-Hee Ka, Dae-Gil Yoo, Geo-Myung Shin, Kyoung-Pil Ko, and Sang-Im Yoo

Department of Materials Science and Engineering, Seoul National University, Seoul 151-744, Korea

We report the magneto-transport properties of ��� ���� � �������� ( , 0.075, 0.1, 0.125, 0.15, 0.3) thin films on (100)������ substrates prepared by pulsed electron-beam deposition (PED). With increasing from 0 to 0.3, both Curie temperature �and metal-insulator transition temperature � of the films were increased from 206 and 212 K to 339 and 366 K, respectively. Ex-cept ��� film, with increasing , all other LCSMO films showed a tendency of gradual decrease in the maximum MR ratios ���� at increased maximum MR temperature ��� � values. The film of ��� exhibited extraordinary high ���

value of 16% at 306 K. After post-annealing in a pure oxygen atmosphere at 900 �, this film showed more enhanced ��� of 18%at 325 K.

Index Terms—Colossal magnetoresistance (CMR), perovskite manganites films, pulsed electron-beam deposition (PED).

I. INTRODUCTION

T HE magneto-transport properties of perovskite-typemanganites, where is a rare

earth element and is an alkarine earth element, have beenwidely studied because of their potential applications andabundant physical properties [1]. Among the manganite family,Ca-doped lanthanum manganites, (LCMO)with doping level, , have been most extensively studiedafter the first observation of over than fold change inresistivity at low temperature under the high magnetic field [2].However, for a real application, the Curie temperature, ofthe manganite should be increased above room temperature,which is reported possible by the substitution of forion in (LCSMO) [3]. The reason for

enhancement is attributed to strengthening in the doubleexchange due to a decrease in the lattice distortion and an in-crease in the Mn-O-Mn bond angle as the substitutes the

-site of perovskite. Although is increased by dopingin the LCSMO system, the metal-insulator transition observedin the resistivity-temperature curve becomes more broadenedand the CMR effect gradually decreases with increasing .In addition, the structural transition from orthorhombic torhombohedral phases is reported to occur at the composi-tion between 0.125 and 0.15 [3]–[5].

While numerous reports are available for LCSMO polycrys-talline samples, a systematic study on LCSMO thin films withvarying is rarely found in the literature, which motivatedpresent study. In this work, we prepared the LCSMO thinfilms on (LAO) (001) single crystal substrates withchanging to in by usingpulsed electron-beam deposition (PED) method. We here reportthe variation of magneto-transport properties of LCSMO filmswith varying .

Manuscript received November 10, 2008. Current version published May 20,2009. Corresponding author: S. I. Yoo (e-mail: siyoo@snu.ac.kr).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TMAG.2009.2018891

II. EXPERIMENTAL SETUP

The (LCSMO) PED targets forand were prepared by the

solid state reaction. Precursors of , , , andpower of 99.9% purity are weighed and ball-milled.

Mixed powder was calcined twice at 1100 for 8 h with anintermediate ball milling. Calcined powder was pressed intopellets, and then the pellets were sintered at 1300 for 4 hin air. The films were deposited on the (LAO) (100)single crystal substrates by PED (PEBS-20, Neocera Inc.)The base pressure in the vacuum chamber was maintainedat . The energy density of electron beam is10 in maximum and pulsed power density equals to

. Total shots of 5000 electron pulses with5 Hz repetition rate and accelerating voltage of 16 kV wereused for the film deposition. We used a 17-cm-long aluminatube with respective inner and outer diameters of 1/8 inchand 1/4 inch to guide an e-beam. Systematic variations of thePED parameters, including target to substrate distancein the range of 8 11 cm and processing gas pressure of 15and 20 mTorr, where a stable plasma plume is maintained,were employed. The structures of bulk samples and films wereanalyzed by using XRD (M18XHF-SRA, Mac. Science Co.and D8-Advanced, Bruker Miller Co.) and microstructurewas observed by FE-SEM (JSM-6330F, JEOL) and atomicforce microscope (AFM) (NANOStationII, Surface ImagingSystems Co.). SEM-cross section views were used to definethe films thickness. RMS surface roughness of films was an-alyzed by Atomic Force Microscopy. Compositional analysisof the film was performed by Electron Probe X-ray MicroAnalyzer (EPMA) (JXA-8900R, JEOL). Temperature and fielddependences of magnetization were measured with a SQUIDmagnetometer (MPMS XL5, Quantum Design). of thefilms was determined by extrapolation of magnetization curvesto zero value. For the transport measurements, the films withdimensions 10.0 mm 2.0 mm were used. Measurements ofMR properties were performed with the four probe methodwithin the SQUID magnetometer.

0018-9464/$25.00 © 2009 IEEE

KANG et al.: MAGNETO-TRANSPORT PROPERTIES OF La Ca Sr MnO THIN FILMS 2573

TABLE IPROCESSING PARAMETERS, THICKNESS, SURFACE ROUGHNESS, AND

MAGNETIC PROPERTIES OF LCMO FILMS: OXYGEN PRESSURE OF CHAMBER

��� �, TARGET-TO-SUBSTRATE DISTANCE �� �, FILM THICKNESS ���,ROOT MEAN SQUARE (RMS) VALUES OF SURFACE ROUGHNESS, CURIE

TEMPERATURE �� � AND MAGNETIZATION ��� VALUE AT 100 K AND 1 kOe

Fig. 1. (a) Temperature dependence of resistivity of �� �� �� filmsand bulk sample at 0 T and 1 T, and (b) temperature dependence of MR ratio,defined as ��� ������ � ���� ������� �� for the films.

III. RESULTS AND DISCUSSION

In order to find optimum film deposition condition, we firstcarefully studied the effects of processing parameters on thesurface roughness and magnetic properties of the LCMO films

. The results are summarized in Table I. It is shown thatthe film morphology and magnetic properties are very sensitiveto the processing parameters of and . Since relativelyhigher and values with lower RMS roughness are de-sirable for high performance MR devices, we selected the pro-cessing parameters of 15 mtorr-10 cm and 20 mtorr-9 cm for thedeposition of other films.

Fig. 1(a) and (b) shows the resistivity and magnetoresistance(MR) ratio as a function of the temperature at zero-field and 1 Tfor the LCMO films deposited at two different con-ditions of 15 mtorr-10 cm and 20 mtorr-9 cm. For a comparison,the data for the polycrystalline LCMO bulk sample are also rep-resented. In comparison with the bulk sample, thin films showmuch reduced metal-insulator transition temperature inFig. 1(a), where the samples show their maximum resistivity,and also much lowered . Much lowered resistivity andvery small MR ratio at low temperature are attributed to absenceof high angle grain boundaries in the films unlike the polycrys-talline bulk sample. The 20 mTorr-9 cm processed film, havinglower resistivity than the 15 mTorr-10 cm processed film, ex-hibits much higher MR effect. The peak MR value of 70.8%is similar to that of PLD-processed LCMO films with 100 nmthickness on the same kind of substrate [6]. Sharper and higherMR peak in (b) and lower resistivity values in (a) are indirectevidences of highly epitaxial films. On the basis of the above re-sults, we determined of 20 mTorr-9 cm are optimumprocessing conditions for obtaining high functional LCMO thin

Fig. 2. (a) XRD patterns LCSMO films deposited on LAO(100) substrates, and(b) those of LCSMO bulk samples mixed with Si reference.

films, and thus these parameters were employed for the depo-sition of all LCSMO films with ,and in the present study.

In Fig. 2(a) and (b), the XRD patterns of the LCSMOfilms and bulk samples are represented, respectively. One cansee that LCSMO films are highly textured in Fig. 2(a). The mainpeak intensity of polycrystalline bulk is greatly sup-pressed, and only the peaks near (100) and (200) of LAO areobserved. LCSMO bulk samples for are or-thorhombic phase and the sample of and arerhombohedral phase as manifested in Fig. 2(b), which weremeasured with a high resolution XRD using the monochromaticbeam of . For the bulk sample with ,the peak splitting between (202) and (040) is foundand it slightly increases with . However, the peak splittingfound in the patterns of LCSMO films except the film of

is much higher than that of bulk samples and thedegree of splitting does not change significantly with . Thisdiscrepancy in peak splitting between films and bulks is mostprobably due to a strain effect in films since the LAO substratehaving pseudocubic structure with can induce acompressive stress on the epitaxially grown LCSMO films alongthe in-plane direction. The lattice mismatch of LCMO (202) and(040) planes to LAO (001) is 2.04 and 2.27%, respectively, andboth of the planes are aligned to LAO (001) direction with dif-ferent degree of cell elongation due to the different lattice mis-match. For the films with , the XRD pattern alsoshow two separated peaks at around 47 unlike the patternof bulk sample showing phase. These results imply that filmswith and also have phase or mixed phases ofand . Although this structural behavior of the LCSMO filmsaccording to variation is hard to understand, it is suggestedthat the structural transition from to phases, which occursin the LCSMO bulk samples by doping around 0.135,might be strongly suppressed in our LCSMO films. To confirmthe strain-induced large peak splitting and suppression ofstructural transition for and films, further structuralanalysis is under progress. The composition, of the LCSMOfilms obtained by EPMA quantitative analysis are presented inTable II. It is shown that the film compositions are reasonablyclose to the target compositions. Fig. 3(a)–(f) show the surfacemorphologies of the LCMSO films. We can see that all the film

2574 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009

TABLE IIPROPERTIES OF �� �� �� ��� : CURIE TEMPERATURE � OF THE

BULK AND THE FILMS, FILM THICKNESS �, METAL-INSULATOR TRANSITION

TEMPERATURE � , MAXIMUM MR TEMPERATURE � , RESISTIVITY

AT � � OF THE FILMS

Sr composition of thin films are calculated from atomic ratio of Sr: Caobtained by EPMA quantitative measurement

Fig. 3. Surface images of �� �� �� ��� films (a) � � � (b) � ���� �, (c) � � ��� (d) � � ����� (e) � � ���� (f) � � ���.

surfaces are covered with outgrowths with the size ranging froma few nanometers to microns.

Temperature dependence on magnetization of the LCSMOfilms is shown in Fig. 4 and the parameters and properties of thefilms are presented in Table II. With increasing from 0 to 0.3,

values of films are increased from 206 to 339 K. The Curietemperature, exhibits relatively lower values compared tothose of bulk samples except the sample with . The

film shows relatively large magnetization value thanthe other samples at the magnetic field of 1 kOe. Temperaturedependence of resistivity of the films shows a metal-insulatortransition for all samples [Fig. 5(a)]. The LCSMO films show atendency that increases and the resistivity at decreaseswith increasing . However, resistivity at low temperature doesnot follow this tendency.

According to the report on the LCSMO single crystal [3],the resistivity of the films show a clear tendency of decreasingresistivity with increasing because lattice distortion in theperovksite is decreased and Mn-O-Mn bond angle is increased(thus band width, is increased) with increasing sub-stitution for the -site [3]. However, for thin films, in additionto doping level and average size of dopant, the induced latticestrain, caused by the lattice mismatch between the film andsubstrates, also plays an important role in the transport prop-erties [1], [6]–[9]. It is reported that the compressive strainusually reduces the resistivity and shifts towards higher

Fig. 4. Temperature dependence of magnetization of �� �� ����� � � � � ��� films.

Fig. 5. (a) Temperature dependence of resistivity of �� �� �� ���� � � � ��� films at 0 T and 1 T, and (b) temperature dependence ofmagneto-resistance (MR) ratio defined as ��� � � � �� � � �� ������� � � for the films.

temperature for LCMO and LSMO films grown on varioussubstrates [6]–[9]. The defects in the matrix, such as grainboundaries (GBs) or dislocations, however, can cause increasein the resisvity. Therefore, the anormalities forfilm, showing higher and lower resistivity than the valuesexpected from the tendency can be explained as the following;This film shows better epitaxy than the other samples in theXRD pattern [Fig. 2(a)]. The films with showsome amount of polycrystalline peak intensity at around33 , while it is negligible in films. Thus, it canbe understood that higher resistivity at low temperature of thefilms with and 0.3 compared with film isattributed to GBs in the films. Similarly, as is increased, theLCSMO films show a tendency of a gradual decrease in themaximum MR ratios with a gradual increase in themaximum MR temperature , except film asshown in Fig. 5(b). The MR (%) values of the LCSMO filmsshow their peak values at the temperature close to theirvalues. The film of shows the of 16% at

of 306 K. These values are larger than expected valuesin the tendency. Since this anomalous mag-neto-transport property of LCSMO with has neverbeen observed for a single crystal [3], it might be related to thefilm epitaxy and strain-related effects. Further investigation is,however, required to unveil its origin. On the other hand, thefilms with showing poorer epitaxy than the other films[Fig. 2(a)] exhibit constant MR of 4% in the wide range of

KANG et al.: MAGNETO-TRANSPORT PROPERTIES OF La Ca Sr MnO THIN FILMS 2575

TABLE IIIPROPERTIES OF POST-ANNEALED FILMS (��-DEPOSITED FILMS): CURIE

TEMPERATURE �� �, METAL-INSULATOR TRANSITION TEMPERATURE �� �,MAXIMUM MR TEMPERATURE �� �, RESISTIVITY AT � �� �

Fig. 6. (a) Temperature dependence of magneto-resistance (MR), ���� ��� � ��� � � � � ������� � �� of ��-deposited (solid symbols) andpost-annealed (open symbols) �� � �� ��� films (for � � ���,0.125, 0.15), and (b) magnetic field dependence of ����� � ���� � ������� � �������� for � � ����� sample.

temperature, which is due to spin-dependent scattering of spincurrent at the residual GBs.

For some selected films with 0.1, 0.125, and 0.3, post-an-nealing process was performed in flowing a pure oxygen gas at900 for 1 h. The properties of post-annealed films are pre-sented in Table III. After the post-annealing, the resistivity ofthe films was decreased and the and values were in-creased. The decrease in resistivity is probably attributable to asignificant reduction in the defects, such as dislocations, oxygenvacancies and atomic disorder in the lattices of the -depositedduring the annealing process. Temperature dependence of MRof the post-annealed films is shown in Fig. 6(a) and field de-pendence of MR for films, measured at 300, 325 and350 K are presented in Fig. 6(b). The values were alsoincreased except the film with . The film ofexhibited the highest performance among the films with the

values of 18% at [Fig. 6(a)] and 73% at[Fig. 6(b)] at 325 K. These values are higher than

those of the PLD films, showing maximumof 65% at 325 K [9].

IV. CONCLUSION

We studied the magneto-transport properties of LCSMO thinfilms deposited on LAO (001) substrates by the PED method,which is commercially more applicable than PLD for multi-component oxide film fabrication due to its relatively low main-taining cost. The strain effect and processing parameters, suchas and were very sensitive to structure and mag-neto-transport properties of films. Among the LCSMO filmsprepared with the same deposition condition, the optimum struc-tural and magneto-transport properties were obtained from the

film, where better epitaxy and higher MR effectwere achieved. For the films of , which is close tothe phase transition boundary, further investigation isrequired to clarify the peculiarities observed in structure, mag-netic and magneto-transport properties.

ACKNOWLEDGMENT

This research was supported by BK21 Materials Educationand Research Division.

REFERENCES

[1] A.-M. Haghiri-Gosnet and J.-P. Renard, “CMR manganites: Physics,thin films and devices,” J. Phys. D: Appl. Phys., vol. 20, pp.R127–R150, Apr. 2003.

[2] S. Jin, T. H. Tiefel, M. McCormack, R. A. Fastnacht, R. Ramesh, andL. H. Chen, “Thousandfold change in resistivity in magnetoresistiveLa-Ca-Mn-O films,” Science, vol. 264, pp. 413–415, Apr. 1994.

[3] Y. Tomioka, A. Asamitsu, and Y. Tokura, “Magnetotransportproperties and magnetostructural phenomen in single crystals of�� � � �� � ��� ,” Phys. Rev. B., vol. 63, p. 024421, Dec.2000.

[4] P. G. Radaelli, M. Marezio, H. Y. Hwang, S.-W. Cheong, and B. Bat-logg, “Charge localization by static and dynamic distortions of the��� octahedara in perovskite manganites,” Phys. Rev. B., vol. 54,pp. 8992–8995, 1996.

[5] A. N. Ulaynov, G. V. Gusakov, V. A. Bordin, N. Y. Starostyuk, and A.B. Mukhin, “Ferromagnetic ordering temperature and internal pressurein manganites �� � �� ��� ,” Solid State Commun., vol.118, pp. 103–105, 2001.

[6] T. Y. Koo, S. H. Park, K. B. Lee, and Y. H. Jeong, “Anisotropic strainsand magnetoresistance of �� � ��� ,” Appl. Phys. Lett., vol.71, pp. 977–979, Aug. 1997.

[7] C. Kwon, M. C. Robson, K.-C. Kim, J. Y. Gu, S. E. Lofland, S. M.Bhagat, Z. Trajanovic, M. Rajeswari, T. Venkatesan, A. R. Kratz,R. D. Gomez, and R. Ramesh, “Stress-induced effects in epitaxial��� �� ���� films,” J. Magn. Magn. Mater., vol. 172, pp.229–236, Aug. 1997.

[8] F. S. Razavi, G. Gross, H. U. Habermeier, O. Lebedev, S. Amelinckx,G. Van Tendeloo, and A. Vigliante, “Epitaxial strain induced metal in-sulator transition in �� �� ��� and �� �� ��� thinfilms,” Appl. Phys. Lett., vol. 76, pp. 155–157, Jan. 2000.

[9] J. Dho, N. H. Hur, I. S. Kim, and Y. K. Park, “Oxygen pressure andthickness dependent lattice strain in �� �� ��� films,” J. Appl.Phys., vol. 94, pp. 7670–7674, Dec. 2003.