On the room-temperature ferromagnetism of Zn1−xCrxO thin films deposited by reactive co-sputtering

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Solar Energy Materials & Solar Cells 91 (2007) 1496–1502 www.elsevier.com/locate/solmat

On the room-temperature ferromagnetism of Zn1xCrxO thin films deposited by reactive co-sputtering K.M. Reddya, R. Bensonb, J. Haysa, A. Thurbera, M.H. Engelhardc, V. Shutthanandanc, R. Hansonb, W.B. Knowltonb,d, A. Punnoosea, a Department of Physics, Boise State University, Boise, ID 83725, USA Department of Electrical and Computer Engineering, Boise State University, Boise, ID 83725, USA c Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, WA 99352, USA d Department of Materials Science and Engineering, Boise State University, Boise, ID 83725, USA b

Available online 4 May 2007

Abstract We report on the preparation and detailed characterization of ferromagnetic (FM) Zn1xCrxO thin films deposited on Si substrates using reactive co-sputtering of Cr and Zn in controlled oxygen atmosphere. X-ray diffraction (XRD) data showed wurtzite ZnO peaks in the FM films prepared with lower Cr sputter powers, whose position and intensities are influenced by Cr doping. However, samples prepared with higher Cr powers did not show ferromagnetism but displayed evidence of Cr2O3 and ZnCr2O4 phases with no zinc oxide (ZnO) phase. The magnetization is higher (saturation magnetization Ms ¼ 18 emu/cm3) for lower Cr concentrations and decreases for higher Cr doping. The samples were investigated extensively to understand the film composition, dopant distribution, homogeneity and potential origin of the observed ferromagnetism. Particle-induced X-ray emission (PIXE) studies were employed to determine the chemical composition as well as the Cr/Zn ratio in the films. Film uniformity and homogeneity, investigated using Rutherford backscattering spectrometry, showed a relatively uniform ZnO layer in the as-prepared samples but, in a sample annealed at 800 1C, showed some diffusion of Si from the substrate. X-ray photoelectron spectroscopy (XPS) studies indicated that Cr ions are in the oxidized state, but showed changes in the binding energy and Cr concentration when measured after removing 10 nm from the surface using Ar ion sputtering. Possible origins of the observed FM behavior are discussed based on the comprehensive characterization results. r 2007 Elsevier B.V. All rights reserved. Keywords: ZnO thin films; Semiconductors; Spintronics; Ferromagnetism

Zinc oxide (ZnO) is an n-type semiconductor with a wide band gap of 3.4 eV and a large exciton binding energy of 60 meV [1] making it an attractive material for a wide variety of applications including opto-electronic devices [2], LEDs [3], thermoelectric devices [4], varistors [5,6], flat panel displays [6] and surface acoustic wave devices [7]. Recent theoretical predictions [8–11] proposed transition metal (TM)-doped ZnO as one of the most promising candidates for room-temperature ferromagnetism. Electronic structure calculations by Sato and Yoshida [9–11] indicates that V-, Cr-, Fe-, Co- or Ni-doped ZnO might

Corresponding author. Tel.: +1 208 426 2268; fax: +1 208 426 4330.

E-mail address: [email protected] (A. Punnoose). 0927-0248/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.solmat.2007.03.012

directly stabilize a ferromagnetic (FM) state while Mndoped ZnO favors a spin glass state. The extensive theoretical work and predictions of hightemperature ferromagnetism in TM-doped ZnO stimulated an unprecedented amount of experimental studies in this system [12–18]. Chromium was chosen as the preferred TM dopant by several research groups [19–22] because (i) it possesses a high magnetic moment, (ii) theoretical research on chromium-based FM semiconductor supports the prospect of producing ferromagnetism [9–11], (iii) Cr metal is antiferromagnetic and thus any possible presence in the form of segregated clusters would not result in ferromagnetism and (iv) the only FM oxide of chromium, CrO2, with a Tc of 395 K is very unlikely to form under the low oxygen pressure conditions usually employed in vacuum

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deposition techniques [23,24]. However, the reported experimental results on the studies of Cr-doped ZnO have been very conflicting. Extensive searches carried out by Ueda et al. [13], Venkatesan et al. [14] and Jin et al. [15] did not provide any observation of ferromagnetism in Crdoped ZnO. However, Roberts et al. [20] and Satoh et al. [21] observed clear ferromagnetism in ZnO thin films doped with chromium. Lee et al. [22] did not observe ferromagnetism in sol–gel prepared Zn1xCrxO thin films, however, a clear FM behavior emerged when the same Zn1xCrxO thin films were co-doped with Li. Experimental observations of nanoscale metal clusters and other impurity phases causing extrinsic ferromagnetism have been reported in TM-doped semiconductor systems, especially when prepared by vacuum-based fabrication techniques [23–25]. It is therefore imperative to thoroughly investigate the microstructure, chemical composition and homogeneity, and dopant distribution in TM-doped FM ZnO films in order to understand the actual origin of the observed ferromagnetism. Motivated by these reasons, we carried out a detailed investigation of Zn1xCrxO films on silicon substrates prepared by a reactive co-sputter deposition technique. Our work demonstrates that low-doped Zn1xCrxO films are FM, nonetheless, detailed and thorough characterization of the TM-doped semiconductor samples using high sensitivity techniques is critical in determining the actual origin of the FM behavior. Cr-doped ZnO films were deposited on (1 0 0) p-type Si substrates (7.5 cm  7.5 cm) using a home-built DC reactive co-sputtering system and using 7.5 cm diameter zinc (99.99%) and cobalt (99.99%) metal targets. High pressure N2 was used to remove substrate surface particulates prior to deposition. A high purity (99.999%) gas mixture of Ar (90%) and O2 (10%) at a total pressure of 1.0 Pa were used as the sputtering and reactive gases, respectively. The base pressure was approximately 1 mPa. Pre-sputtering was conducted for all samples under the following conditions: 5 min, 3.3 Pa, 50 sccm Ar and 100 W/ 100 W (Zn/Cr targets). Deposition rates and final film thicknesses were measured by a quartz crystal thin film monitoring system. Final film thicknesses of 5000 A˚ were targeted for each sample. The samples were investigated using particle-induced X-ray emission (PIXE), X-ray diffraction (XRD), Rutherford backscattering spectrophotometry (RBS), X-ray photoelectron spectroscopy (XPS), and magnetometry. Detailed experimental procedures for PIXE, XRD, XPS (collected using monochromatic Al Ka X-rays) and magnetometry are given elsewhere [26–28]. For RBS measurements, a 2.0 MeV He ion beam was used and the backscattered He ions from the targets were collected at the scattering angle of 165o. RBS experimental data were then fitted with theoretical model using SIMNRA simulation code. The XRD patterns of the as-prepared Zn1xCrxO thin films showed weak signatures of ZnO. However, when the samples were annealed in air at 800 1C for 3 h to improve

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the crystallinity, well-defined peaks of the hexagonal wurtzite structure of ZnO were observed, as shown in Fig. 1. For comparison, reference data patterns for ZnO, Cr2O3 and ZnCr2O4 are provided. The undoped ZnO sample showed polycrystalline growth along (1 0 0), (0 0 2) and (1 0 1) directions. However, with increasing Cr power, growth along the (0 0 2) direction is strongly favored at the expense of the other peaks indicating the influence of Cr incorporation. Also, the peak intensities, positions and widths of the (0 0 2) XRD line, shown in the inset of Fig. 1, indicate noticeable changes with Cr power further supporting Cr incorporation into the ZnO lattice. The (0 0 2) peak is representative of the basal ZnO planes in grains oriented with c-axes perpendicular to the substrate [7]. Up to 50 W Cr power, the peak positions showed shifts to the higher angles suggesting a decrease in the d-spacing and in the c parameter. Since all Cr ions, except high-spin Cr2+, have lower ionic radii than 0.74 A˚ tetrahedral Zn2+ ions irrespective of their oxidation states and coordination numbers [29,30], such a lattice contraction is supportive of substitutional Cr ion incorporation into the ZnO host lattice. When the Cr power was increased to 100 W, the (0 0 2) peak showed an opposite shift to lower angle indicating an expansion of the lattice. Such drastic changes are not very uncommon in DMS systems and similar effects were observed in Co-doped ZnO in thin film [31] and bulk [32] forms, as well as in Co-doped SnO2 powders

Fig. 1. XRD patterns of Zn1xCrxO thin films deposited on Si substrates. Reference patterns for ZnO, Cr2O3 and ZnCr2O4 are given for comparison. The inset illustrates the changes in the (0 0 2) XRD peak of ZnO with different Cr concentrations. * indicates unidentified peaks in the films.

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by the authors [28]. The expansion of these lattices when the doping concentration exceeds certain limits has been attributed to interstitial incorporation of the dopants with higher coordination numbers [28,31,32]. Interestingly, when the Cr power was further raised above 100 W, the films did not show any ZnO phase formation and instead resulted in peaks of ZnCr2O4 (along with Cr2O3 at still higher Cr powers). There are no signs of metallic zinc or chromium in the fresh samples indicating that the 0.1 Pa oxygen partial pressure was high enough to prevent metal cluster formation. Also, the samples prepared with intentionally high Cr powers help establish that any impurity Cr clusters, if formed under the preparation conditions employed in this work, will most likely be nonFM ZnCr2O4 and Cr2O3. Considering the fact that the XRD system employed can only detect phases that are 41.5%, it cannot be confirmed from XRD alone if any nanoscale impurities are present in the films with concentrations below this detection limit. Moreover, the XRD data also showed some weak unidentified impurity peaks. Therefore, it is imperative to employ other characterization techniques to determine the exact composition of the films. A cursory search for ferromagnetism was conducted in a series of Zn1xCrxO film samples prepared under different Cr sputter powers in the as-prepared form as well as after annealing at 800 1C for 3 h by measuring the sample magnetization M as a function of magnetic field H at room temperature. Samples that showed clear Cr2O3 and/or ZnCr2O4 in XRD, prepared with higher Cr powers (4100 W), did not show any FM behavior, thus ruling out any role of these oxide phases in the observed magnetic behavior. It also indicates that the deposition conditions employed do not contribute to any unknown impurity phases that are FM. In general, samples prepared with lower Cr sputter powers were more FM, while those prepared with higher sputter powers were paramagnetic in nature presumably due to the reduced presence or complete absence of the ZnO phase. To investigate the dopant uniformity and film homogeneity in addition to understand the actual origin of the observed weak FM behavior, four FM (strongest and weakest FM samples) Zn1xCrxO thin film samples, prepared with p100 W chromium power, were selected for detailed investigation using magnetometry, PIXE, XPS and Rutherford backscattering spectrometry. Included in these were two as-prepared samples A and B and two annealed samples C and D. The M vs. H data of these four selected samples are shown in Fig. 2a. The samples B and C showed highest saturation magnetization among the entire set of FM samples investigated while A and D were the weakest. Fig. 2b shows the hysteresis loop with a saturation magnetization Ms ¼ 16 emu/cm3 and coercive field Hc ¼ 52 Oe for sample C at room temperature. In Fig. 3a, the variation of the magnetic susceptibility w vs. temperature T of sample C is shown indicating a dominant paramagnetic behavior at low temperature. This might result from paramagnetic Cr ions or superparamagnetic Cr oxide clusters which might

Fig. 2. (a) Room-temperature M vs. H plots obtained from Zn1xCrxO thin films, (b) room-temperature hysteresis loop from sample C showing a coercivity of 52 Oe (inset).

be present in the samples in addition to the FM component. The lack of any anomalies at the Neel transition temperature (307 K) of Cr2O3 rules out the presence of this phase, at least in the bulk form, in the sample. Fig. 3b shows the normalized high-temperature M vs. T data measured from a piece of sample C with an applied field of 8 kOe. The plot did not show any clear anomaly near the Curie temperature (395 K) of the CrO2 phase suggesting that presence of this phase, as an impurity in the film, producing the observed FM behavior is unlikely. The data show that the sample magnetization gradually decreases in the 450–900 K range indicating a gradual transition from the FM phase at such high temperatures. The inset of the Fig. 3b shows a hysteresis loop measured at 500 K displaying a saturating behavior with a coercivity that is unclear due to the relatively high noise level. Thus, the observed FM behavior exists at temperatures higher than the Curie temperature (395 K) reported for the only known FM Cr oxide, CrO2. The Cr concentrations of the four Zn1xCrxO thin film samples were determined from high-sensitivity PIXE

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Fig. 3. (a) Magnetic susceptibility w of sample C measured as a function of temperature with an applied field H ¼ 8 kOe. Panel (b) shows the average sample magnetization after normalization as a function of temperature measured with H ¼ 8 kOe. The inset shows a the hysteresis loop collected at 500 K from the sample.

measurements and the spectra for samples C and D are plotted in Fig. 4a. The samples were irradiated with 2.0 MeV He ions and the resultant peaks were indexed to Zn, Cr and Si (from the substrate), which are well expected. Estimation of the Cr concentrations was done by simulating the experimental PIXE spectra after removing the background due to bremsstrahlung. These estimates showed 0.3, 0.4 and 4.2% Cr in samples B, C and D, respectively, while the Cr content in sample A was below the detection limit of 0.1% of the PIXE technique. However, the data also showed some impurity Fe peaks, albeit very weak in intensity, in all the samples. This impurity Fe inclusion might have come from some unknown sources in the deposition chamber. The RBS ion-channeling technique is used to reveal the distribution of dopants and chemical composition of the host films. Figs. 4b and 4c show the RBS simulated spectra of all four samples. First the substrate (Si) portion of the RBS spectrum was fit to obtain the composition of the substrate. Then the film portion of the RBS spectrum was fit assuming that the film contains Zn, Cr and O. The solid

Fig. 4. Panel (a) shows the PIXE spectra of samples C (solid line) and D (dashed line). (b) RBS spectra of samples C and D along with their simulations. Panel (c) shows the experimental and simulated RBS spectra for samples A and B.

lines shown on these figures are from the RBS spectra simulated using the SIMNRA program. As can be seen from these figures, the fits agree well with the experimental data. The RBS spectra of the A and B have broad Zn peaks indicating that these samples have thicker Zn rich regions (thicker films) compared to the other two samples. The oxygen peak fits for C and D samples suggest that all these films are well-oxidized ZnO without any metallic zinc layers. The data simulation clearly indicates that in the

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annealed samples, oxygen is present up to layers far below those containing Zn, presumably indicating oxidation of the Si substrate below the ZnO layers during the annealing process. For the as-prepared films A and B, the broad oxygen peaks indicate that the entire Zn-rich regions of the films are oxidized. The simulation did not indicate any oxidation of the underlying Si substrate layers in the asprepared films. Thus, the samples A and B have thicker oxygen-rich ZnO films deposited on Si substrates. However, the annealed samples C and D have thinner ZnO layers and are separated from the Si substrates by a silicon oxide layer. The Si peak edge is relatively abrupt for samples A, B and D as expected. However, for C, the scattering yield increases slowly and the data simulation results indicate that some Si ions from the substrate diffused into the ZnO film region in this sample. Samples C and D show weak Cr peaks also and their estimates from the RBS data simulation indicates 0.2% and 2.5% Cr, respectively, in broad agreement with PIXE results. The difference between the RBS and PIXE results are related to the lower sensitivity of RBS. The XPS survey spectra of the C and D samples were recorded after removing 2 nm (to remove any adsorbed surface contaminants) and 10 nm (to investigate compositional differences with depth) using Ar ion sputtering to investigate the oxidation state and distribution of Cr ions. Displayed in Fig. 5(a)–(b) are panels showing the Cr 2p and Cr 3p core level spectra for both samples. The core level binding energies observed (see Table 1) for the Cr 2p3/ 2 primary peak at 577.570.1 eV recorded from the surface (2 nm depth) of the two samples are clearly different from 574.02 eV of Cr metal and 576.0 eV of Cr2+ [33–35]. These peaks can be indexed to Cr3+ ions as they match well with the reported binding energy (577.270.2 eV) of Cr3+ states [33–35]. Also, the Cr 3p peak at 44.470.2 eV matches well with that reported for Cr3+. Based on the XRD results suggesting that with increasing Cr power, the sample composition changes from the Zn1xCrxO phase (for p100 W) to ZnCr2O4 (200 W) and then to a mix of both ZnCr2O4 and Cr2O3, and the absence of any anomalies at 307 K in the M vs. T data shown in Fig. 3, it is likely that Cr exists as Cr3+ in the Zn1xCrxO phase (instead of any Cr2O3 phase) in films prepared with Cr power p100 W. When a 10 nm layer was removed from the surface of the samples using Ar ion sputtering followed by charge neutralization, the binding energies showed a slight decrease as shown in the Table 1. This might indicate a change in the chemical environment/oxygen stoichiometry and/or lower oxidation state, but still does not suggest any Cr metal inclusions. However, the Cr peaks in both samples (C and D) showed an increase in intensity when a 10 nm layer was removed suggesting a non-uniform distribution of Cr concentration in the films. The Zn LMM Auger lines interfere with the more intense Cr 2p lines in the XPS data. Therefore, the Cr 3p lines were used for the quantification of Cr concentration in samples C and D, and these estimates are shown in Table 1. The XPS-based

Fig. 5. XPS spectra showing (a) Cr 2p, and (b) Cr 3p core level regions of Zn1xCrxO samples C and D collected after removing 2 and 10 nm of surface layers using Ar ion sputtering.

elemental compositions shown in the Table 1 indicate that the surface region of both samples have less oxygen and higher Zn concentrations compared to a more stoichiometric composition at 10 nm below the surface. Also, the Cr concentration in both films varies between the surface region and 10 nm below the surface. Similar changes in the dopant concentration between the surface and core regions have been reported in other DMS thin film systems also [31,36]. Several of the above-mentioned experimental results including (i) the observed formation of the ZnCr2O4 phase initially followed by a mix of ZnCr2O4 and Cr2O3 when the Cr concentration exceeds the solubility limit, (ii) the shift and systematic changes in intensities of the ZnO XRD peaks with increasing Cr power, suggesting Cr

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Table 1 XPS positions and concentrations of Cr-doped ZnO films Sample details

Sample Sample Sample Sample

C C D D

Experimental conditions

Surface region (after 2 nm sputtering) After 10 nm total sputtering Surface region (after 2 nm sputtering) After 10 nm total sputtering

BE (eV) Cr 2p3/2

577.6 576.7 577.4 576.0

incorporation into the host lattice, (iii) the lack of any evidence of the CrO2 phase in the FM Zn1xCrxO films from XRD and XPS studies and more importantly, continuation of the FM behavior above 395 K in the magnetic measurements without any anomalies and (iv) the complete absence of any FM behavior in samples prepared at higher Cr powers (with no ZnO phase in the XRD data) suggest that the observed FM behavior may be from the Zn1xCrxO phase. The stronger FM behavior in samples with lower concentrations of Cr and its apparent weakening and subsequent disappearance at higher doping concentrations are in agreement with the model proposed by Coey et al. [37] for dilute magnetic semiconductors (DMS). It was shown that higher doping concentrations might lead to increasing antiferromagnetic interaction, causing the FM behavior to disappear [37]. On the other hand, several factors including (i) the observed presence of trace amounts of Fe in PIXE studies and (ii) the observed inhomogeniety in the chemical composition and dopant concentration observed from the XPS and RBS studies are obviously very disturbing. It should be noted that no Fe presence was observed in techniques such as XRD, XPS, RBS and magnetometry, which are the most commonly used techniques to characterize DMS materials. The absence of ferromagnetism in samples prepared in the same sample chamber using identical targets and under identical synthesis and processing conditions, but with higher Cr power indicates that the weak trace of Fe observed in the FM samples most likely is not the source of the observed FM behavior, although to draw a categorical conclusion requires this ferromagnetism to be produced in similar samples prepared with no Fe trace and dopant inhomogeniety. This study, however, highlights the complex nature of the deposition process as well as underlines the importance of employing high sensitivity microstructural techniques to carry out thorough characterization of the materials before assigning any FM behavior to dilute magnetic state. Acknowledgments The research work carried out at Boise State University was supported by grants from Petroleum Research Fund (PRF#41870-AC10), NSF-CAREER program (DMR0449639) and the DoE-EPSCoR program (DE-FG0204ER46142). A portion of the research described in this

BE (eV) Cr 3p

44.6 43.2 44.2 42.8

Elemental concentration (%) C 1s

O 1s

Cr 3p

Zn 2p

2.0 0.2 1.9 0.3

24.5 31.8 24.3 31.6

1.2 0.6 2.1 3.9

72.3 67.4 71.8 64.2

paper was performed in the Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Partial financial support via DARPA Contract N66001-01-C-80345, NSFMRI Award 0216312, and NIH INBRE P20RR16454 is acknowledged. References [1] U¨. O¨zgu¨r, A. Teke, C. Lui, S.-J. Cho, H. Morkoc- , Appl. Phys. Lett. 84 (2004) 3223. [2] A.N. Gruzintsev, V.T. Volkov, E.E. Yakimov, Semiconductors 37 (2003) 259. [3] H. Kim, J.S. Horwitz, W.H. Kim, A.J. Makinen, Z.H. Kafafi, D.B. Chrisey, Thin Solid Films 420–421 (2002) 539. [4] M. Ohtaki, T. Tsubota, K. Eguchi, H. Arai, J. Appl. Phys. 79 (1996) 1816. [5] T.R.N. Kutty, N. Raghu, Appl. Phys. Lett. 54 (1989) 1796. [6] M. Chen, Z.L. Pei, C. Sun, J. Gong, R.F. Huang, L.S. Wen, Mater. Sci. Eng. B 85 (2001) 212. [7] J. Lee, H. Lee, S. Seo, J. Park, Thin Solid Films 398–399 (2001) 641. [8] T. Dietl, H. Ohno, F. Matsukura, Phys. Rev. B 63 (2001) 195205. [9] K. Sato, H. Katayama-Yoshida, Physica B 308–310 (2001) 904. [10] K. Sato, H. Katayama-Yoshida, Phys. Stat. Sol. (b) 229 (2002) 673. [11] K. Sato, H. Katayama-Yoshida, Semicon. Sci. Technol. 17 (2002) 367. [12] K. Ando, H. Saito, Zhengwu Jin, T. Fukumura, M. Kawasaki, Y. Matsumoto, H. Koinuma, J. Appl. Phys. 89 (2001) 7284. [13] K. Ueda, H. Tabata, T. Kawai, Appl. Phys. Lett. 79 (2001) 988. [14] M. Venkatesan, C.B. Fitzgerald, J.G. Lunney, J.M.D. Coey, Phys. Rev. Lett. 93 (2004) 177206. [15] Z. Jin, T. Fukumura, M. Kawasaki, K. Ando, H. Saito, T. Sekiguchi, Y.Z. Yoo, M. Murakami, Y. Matsumoto, T. Hasegawa, H. Koinuma, Appl. Phys. Lett. 78 (2001) 3824. [16] See a recent review article, S. J. Pearton et al, J. Appl. Phys. (2003) 93 (1) and references therein. [17] D.C. Kundaliya, S.B. Ogale, S.E. Lofland, S. Dhar, C.J. Metting, S.R. Shinde, Z. Ma, B. Varughese, K.V. Ramanujachary, L. Salamanca-Riba, T. Venkatesan, Nat. Mater. 3 (2004) 709. [18] S.J. Pearton, D.P. Norton, K. Ip, Y.W. Heo, T. Steiner, Superlattices Microstruct (2003) 3 see a recent review article and references therein. [19] J. Philip, A. Punnoose, B.I. Kim, K.M. Reddy, S. Layne, J.O. Holmes, B. Satpati, P.R. LeClair, T.S. Santos, J.S. Moodera, Nat. Mater. 5 (2006) 298. [20] B.K. Roberts, A.B. Pakhomov, V.S. Shutthanandan, K.M. Krishnan, J. Appl. Phys. 97 (2005) 10D310. [21] I. Satoh, T. Kobayashi, Appl. Surf. Sci. 216 (2003) 603. [22] H.J. Lee, S.Y. Jeong, J.Y. Hwang, C.R. Cho, Europhys. Lett. 64 (2003) 797. [23] A. Punnoose, M.S. Seehra, W.K. Park, J.S. Moodera, J. Appl. Phys. 93 (2003) 7867.

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