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Thin film growth and magnetic anisotropy of epitaxial Sr0.775Y0.225CoO3−δ
ARTICLE IN PRESS Journal of Crystal Growth 310 (2008) 3649– 3652
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Thin film growth and magnetic anisotropy of epitaxial Sr0.775Y0.225CoO3d J.Y. Son a, Y.-H. Shin a,, S.B. Park a, C.S. Park a, Hyungjun Kim a, J.H. Cho b, A.I. Ali c a b c
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea RCDAMP and Department of Physics, Pusan National University, Pusan 609-735, Republic of Korea Department of Physics, Helwan University, Ain Helwan, Cairo 11795, Egypt
a r t i c l e in fo
abstract
Article history: Received 17 April 2008 Accepted 11 May 2008 Communicated by D.P. Norton Available online 27 May 2008
Epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films were grown on single crystal (1 0 0) and (111) SrTiO3 substrates by the pulsed laser deposition method. We observed the relatively small strain and the ferromagnetism with the relatively large grains in the epitaxial (111) Sr0.775Y0.225CoO3d thin film at room temperature. However, the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film showed the relatively high strain along the in-plane orientation and the superparamagnetic property with the relative small grains. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.20.g 75.30.Gw 75.50.y 75.60.Ej Keywords: A1. X-ray diffraction A3. Laser epitaxy B1. Oxides B2. Ferromagnetic materials
Co oxides exhibit extraordinarily interesting physical properties, such as superconductivity [1,2], thermoelectricity [3], ferromagnetism [4–6], and giant magnetoresistance [7]. Electronic transport and magnetic properties in these transition metal oxides are principally determined by the electronic configurations of Co ions with three spin states: the low spin state, the intermediate spin state, and the high spin state [8,9]. In addition, when a strong electron–electron interaction is involved, the electronic transport and magnetic properties can be dramatically varied, and this is based on Mott–Hubbard metal–insulator transition [10–12]. For A-site ordered perovskite Sr1xRxCoO3 (R: rare earth element) with the Co oxide perovskite structure, the doping level can change the electronic transport and magnetic properties by changing the spin state of Co ions, and this can provide a theoretical understanding of transition metal oxides as well as an opportunity for the application of these magnetic materials. Room temperature ferromagnetism is a useful property for applying magnetic material in many fields such as hard disk read head and magnetic random-access memory [13,14]. Sr1xYxCoO3d which belongs to Sr1xRxCoO3 family is a room temperature ferromagnet with a Curie temperature of 335 K in a
Corresponding author. Tel.: +82 54 279 2795; fax: +82 54 279 2399.
E-mail address: [email protected] (Y.-H. Shin). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.05.030
narrow range of composition (0.2pxp0.25) [5]. From a practical point of view, the thin film study of Sr1xYxCoO3d can yield the opportunity of applications which utilize the variations of the physical properties depending on the strained structure of thin films under various stresses. So far, however, there has been no study on the thin film of Sr0.775Y0.225CoO3d. In this study, the epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films were grown on (1 0 0) and (111) single crystal SrTiO3 substrates by pulsed laser deposition (PLD). We show the structural and magnetic properties of the epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films on (1 0 0) and (111) single crystal SrTiO3 substrates. Epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films were simultaneously grown on (1 0 0) and (111) single crystal SrTiO3 substrates, respectively, by PLD. The stoichiometric transfer of material from target to substrate is one of the remarkable advantages of the PLD method [15]. The (1 0 0) and (111) single crystal SrTiO3 substrates with a pseudocubic lattice constant of 3.905 A˚ have a relatively small lattice misfit of 1.7% with Sr0.775Y0.225CoO3d [5]. We used the pseudocubic lattice constant of 3.840 A˚ for Sr0.775Y0.225CoO3d. The Sr0.775Y0.225CoO3d target was prepared by a conventional solid-state reaction method. High-purity stoichiometric Sr0.775Y0.225CoO3d was mixed in a molar ratio of Y:Sr:Co ¼ 0.225:0.775:1.000 with Y2O3 (99.99%, MTI), SrCO3 (99.999%, Cerac), and Co3O4 (99.99%, Aldrich). The mixed powder was pressed and calcined at 1000 1C
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for 12 h. The calcined pellet was reground, pressed into a pellet, and sintered at 1170 1C for 48 h. The phase purity as well as the lattice constant was checked by using powder X-ray diffraction (XRD) with Cu Ka radiation (Philips). For the deposition, a frequency tripled (wavelength 355 nm, pulse repetition rate 10 Hz) Nd:YAG laser was used and the laser power of approximately 2 J/cm2 was used. The base pressure was 1 106 Torr, and the oxygen pressure of 200 mTorr was maintained prior to the deposition of Sr0.775Y0.225CoO3d thin films. For the optimal conditions for the thin film growth, the substrate temperature and the distance between target and substrate were set to 500 1C and 6 cm, respectively. The tentative compositions of Sr0.775Y0.225CoO3d thin films were obtained by energy dispersive X-ray spectrometer (EDS). For the structural determination of the thin films, XRD data were obtained by conventional laboratory X-ray (CuKa radiation). The thicknesses of both (111) and (1 0 0) Sr0.775Y0.225CoO3d thin films were 800 A˚ determined from the spectroscopic ellipsometry. The surface morphology and topography of the thin films were observed by atomic force microscope (AFM, Seiko SPA400) in contact mode using an Si3N4 cantilever (a force constant of 0.08 N/m and a resonance frequency of 34 kHz). To check the ferromagnetic property, magnetic hysteresis loops were measured by superconducting quantum interference device (SQUID) magnetometer (quantum design). Fig. 1(a) shows an XRD y2y scan of an Sr0.775Y0.225CoO3d thin film on a (1 0 0) SrTiO3 substrate, revealing a nearly phase pure (1 0 0)-oriented film. The Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate has (1 0 0) oriented growth along the out-
of-plane direction. The full width at half maximum (FWHM) of the (2 0 0) rocking curve is about 0.81 indicating that the film is well crystalline. The out-of-plane lattice constant of 3.808 A˚ was obtained from the (a 0 0) peaks, and this lattice constant has a slightly decreased value of about 0.8% from the bulk lattice constant of Sr0.775Y0.225CoO3d. To check the in-plane orientation, we performed X-ray scattering with f scan geometry. Fig. 1(b) shows f scans of the (2 0 4) peak of the (1 0 0) Sr0.775Y0.225CoO3d thin film and the (1 0 1) peak of the (1 0 0) SrTiO3 substrate. The fourfold symmetries of the (2 0 4) peak of (1 0 0) Sr0.775Y0.225CoO3d thin film and the (1 0 1) peak of the (1 0 0) SrTiO3 substrate are observed, and these (2 0 4) and (1 0 1) peaks have the same position for the f angle. This signifies that the b- or c-axis of the (1 0 0) Sr0.775Y0.225CoO3d thin film are oriented along those of the (1 0 0) SrTiO3 substrate. The FWHM of the in-plane (2 0 4) peak is about 0.71 indicating a good epitaxial growth. From the 2y value of the (2 0 4) peak, the b- and c-axes constants of 3.899 A˚ are obtained, and these lattice constants are increased by about 1.7% from the bulk lattice constant of Sr0.775Y0.225CoO3d. This elongation of the in-plane lattice constant under the tensile stress is due to the lattice constant of the (1 0 0) SrTiO3 substrate with the lattice misfit of 1.5%, causing the lattice constant to shrink along the out-of-plane direction of the (1 0 0) Sr0.775Y0.225CoO3d thin film. Fig. 2(a) shows an XRD y2y scan of an epitaxial (111) Sr0.775Y0.225CoO3d thin film on a (111) SrTiO3 substrate. Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate has (111) oriented growth along the out-of-plane direction. The FWHM of the (111) rocking curve is about 0.61, and this FWHM is
Fig. 1. (a) XRD pattern (y2y scan) of the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate. (b) The f scans of the (2 0 4) peak of the (1 0 0) Sr0.775Y0.225CoO3d thin film and the (1 0 1) peak of the (1 0 0) SrTiO3 substrate.
Fig. 2. (a) XRD pattern (y2y scan) of the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate. (b) The f scans of the (4 0 0) peak of the Sr0.775Y0.225CoO3d thin film and the (2 0 0) peak of the (111) SrTiO3 substrate.
ARTICLE IN PRESS J.Y. Son et al. / Journal of Crystal Growth 310 (2008) 3649–3652
slightly smaller than that of the (1 0 0) Sr0.775Y0.225CoO3d thin film. Fig. 2(b) shows the f scans of the (4 0 0) peak of the (111) Sr0.775Y0.225CoO3d thin film and the (2 0 0) peak of the (111) SrTiO3 substrate. The FWHM of the in-plane (4 0 0) peak is about 0.61, and this FWHM is also smaller than that of the (1 0 0) Sr0.775Y0.225CoO3d thin film, indicating that the (111) Sr0.775Y0.225CoO3d thin film has a better crystal texture along the in-plane direction. From the (4 0 0) and (2 0 0) peaks, the a-axis constant of 3.840 A˚ is obtained, and b- and c-axes constants of 3.837 A˚ are obtained. These lattice constants are about the same as the bulk lattice constants of Sr0.775Y0.225CoO3d, and this result means that the (111) Sr0.775Y0.225CoO3d thin film has a smaller strain than the (1 0 0) Sr0.775Y0.225CoO3d thin film. From the comparison of the strain of the (1 0 0) Sr0.775Y0.225CoO3d thin film with that of the (111) Sr0.775Y0.225CoO3d thin film, it is inferred that the magnetic property of bulk Sr0.775Y0.225CoO3d is expected for the (111) Sr0.775Y0.225CoO3d thin film due to the relatively low strain. Fig. 3 shows AFM images of the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate and the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate. The root mean square (rms) surface roughnesses of (1 0 0) Sr0.775Y0.225CoO3d and (111) Sr0.775Y0.225CoO3d thin films for 1 mm 1 mm area are 130 and 39 A˚, respectively. Relatively small grains of average grain size of about 60 nm are observed on the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film, but the epitaxial (111) Sr0.775Y0.225CoO3d thin film has relatively large grains of average grain size of about 1 mm. This means that the epitaxial (111) Sr0.775Y0.225CoO3d thin film has a longer diffusion length of an adatom along the surface than the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film during the deposition. From the XRD and AFM data, it is expected that the (1 0 0) Sr0.775Y0.225CoO3d thin film will exhibit more highly strained physical properties than the Sr0.775Y0.225CoO3d thin film. Fig. 4 shows magnetization versus in-plane applied field loops of the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate and the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate at room temperature. When the magnetic field is applied along the out-of-plane direction, the paramagnetic properties of both thin films were obtained due to the magnetic anisotropy caused by the insufficient thickness of thin film (not shown) [16,17]. For the in-plane magnetic field, the (111) Sr0.775Y0.225CoO3d thin film shows ferromagnetic hysteresis with the remanent magnetization of 7 105 emu and the
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saturation magnetization of 2 104 emu. In the (1 0 0) Sr0.775Y0.225CoO3d thin film, however, the superparamagnetic hysteresis is observed. On the basis of the different misfits of (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films on (1 0 0) and (111) SrTiO3 substrates, it is inferred that this superparamagnetism results from the relatively highly strained structural property of the (1 0 0) Sr0.775Y0.225CoO3d thin film [18,19]. In conclusion, epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films were grown on single crystal (1 0 0) and (111) SrTiO3 substrates, respectively, by PLD. The elongation of the in-plane lattice constants under the tensile stress causes the lattice constant to shrink along the out-of-plane direction of the (1 0 0) Sr0.775Y0.225CoO3d thin film. The lattice constants of the (111) epitaxial Sr0.775Y0.225CoO3d thin film were nearly the same as the bulk lattice constants of Sr0.775 Y0.225CoO3d, and the ferromagnetic property was observed with the relatively large grains at room temperature. However, the (1 0 0) epitaxial Sr0.775Y0.225CoO3d thin film showed the relatively high strain along the out-of-plane direction and the superparamagnetic property with the relatively small grains.
Fig. 4. Magnetization–magnetic field curves of the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate and the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate at room temperature.
Fig. 3. AFM topographic images of (a) the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate and (b) the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate.
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Acknowledgements This work was supported by the Brain Korea 21 Project 2008 and JHC was supported by the Korea Research Foundation Grant KRF-2006-005-J02801. References [1] T. Shimojima, K. Ishizaka, S. Tsuda, T. Kiss, T. Yokoya, A. Chainani, S. Shin, P. Badica, K. Yamada, K. Togano, Phys. Rev. Lett. 97 (2006) 267003. [2] M. Karppinen, I. Asako, T. Motohashi, H. Yamauchi, Chem. Mater. 16 (2004) 1693. [3] K. Sugiura, H. Ohta, K. Nomura, M. Hirano, H. Hosono, K. Koumoto, Appl. Phys. Lett. 89 (2006) 032111. [4] J. Matsuno, Y. Okimoto, Z. Fang, X.Z. Yu, Y. Matsui, N. Nagaosa, M. Kawasaki, Y. Tokura, Phys. Rev. Lett. 93 (2004) 167202. [5] W. Kobayashi, S. Ishiwata, I. Terasaki, M. Takano, I. Grigoraviciute, H. Yamauchi, M. Karppinen, Phys. Rev. B 72 (2005) 104408. [6] R. Caciuffo, D. Rinaldi, G. Barucca, J. Mira, J. Rivas, M.A. Senaris-Rodriguez, P.G. Radaelli, D. Fiorani, J.B. Goodenough, Phys. Rev. B 59 (1999) 1068.
[7] A.C. Masset, C. Michel, A. Maignan, M. Hervieu, O. Toulemonde, F. Studer, B. Raveau, J. Hejtmanek, Phys. Rev. B 62 (2000) 166. [8] C. Zobel, M. Kriener, D. Bruns, J. Baier, M. Gru¨ninger, T. Lorenz, P. Reutler, A. Revcolevschi, Phys. Rev. B 66 (2002) 020402. [9] P.G. Radaelli, S.-W. Cheong, Phys. Rev. B 66 (2002) 094408. [10] S. Yamaguchi, Y. Okimoto, Y. Tokura, Phys. Rev. B 54 (1996) R11022. [11] J.-Q. Yan, J.-S. Zhou, J.B. Goodenough, Phys. Rev. B 69 (2004) 134409. [12] G. Khaliullin, W. Koshibae, S. Maekawa, Phys. Rev. Lett. 93 (2004) 176401. [13] Y. Nagamine, H. Maehara, K. Tsunekawa, D.D. Djayaprawira, N. Watanabe, S. Yuasa, K. Ando, Appl. Phys. Lett. 89 (2006) 162507. [14] S. Tehrani, B. Engel, J.M. Slaughter, E. Chen, M. DeHerrera, M. Durlam, P. Naji, R. Whig, J. Janesky, J. Calder, IEEE Trans. Magn. 36 (2000) 2752. [15] D.B. Chrisey, G.K. Hubler, Pulsed Laser Deposition of Thin Films, Wiley, New York, 1994. [16] B. Heinrich, J.F. Cochran, Adv. Phys. 42 (1993) 523. [17] D. Sander, Rep. Prog. Phys. 62 (1999) 809. [18] J. Xu, M.A. Howson, B.J. Hickey, D. Greig, E. Kolb, P. Veillet, N. Wiser, Phys. Rev. B 55 (1997) 416. [19] D.H. Kim, J.S. Yang, Y.S. Kim, D.W. Kim, T.W. Noh, S.D. Bu, Y.W. Kim, Y.D. Park, S.J. Pearton, Y. Jo, J.G. Park, Appl. Phys. Lett. 83 (2003) 4574.
Contents lists available at ScienceDirect
Journal of Crystal Growth journal homepage: www.elsevier.com/locate/jcrysgro
Thin film growth and magnetic anisotropy of epitaxial Sr0.775Y0.225CoO3d J.Y. Son a, Y.-H. Shin a,, S.B. Park a, C.S. Park a, Hyungjun Kim a, J.H. Cho b, A.I. Ali c a b c
Department of Materials Science and Engineering, Pohang University of Science and Technology (POSTECH), Pohang 790-784, Republic of Korea RCDAMP and Department of Physics, Pusan National University, Pusan 609-735, Republic of Korea Department of Physics, Helwan University, Ain Helwan, Cairo 11795, Egypt
a r t i c l e in fo
abstract
Article history: Received 17 April 2008 Accepted 11 May 2008 Communicated by D.P. Norton Available online 27 May 2008
Epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films were grown on single crystal (1 0 0) and (111) SrTiO3 substrates by the pulsed laser deposition method. We observed the relatively small strain and the ferromagnetism with the relatively large grains in the epitaxial (111) Sr0.775Y0.225CoO3d thin film at room temperature. However, the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film showed the relatively high strain along the in-plane orientation and the superparamagnetic property with the relative small grains. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.20.g 75.30.Gw 75.50.y 75.60.Ej Keywords: A1. X-ray diffraction A3. Laser epitaxy B1. Oxides B2. Ferromagnetic materials
Co oxides exhibit extraordinarily interesting physical properties, such as superconductivity [1,2], thermoelectricity [3], ferromagnetism [4–6], and giant magnetoresistance [7]. Electronic transport and magnetic properties in these transition metal oxides are principally determined by the electronic configurations of Co ions with three spin states: the low spin state, the intermediate spin state, and the high spin state [8,9]. In addition, when a strong electron–electron interaction is involved, the electronic transport and magnetic properties can be dramatically varied, and this is based on Mott–Hubbard metal–insulator transition [10–12]. For A-site ordered perovskite Sr1xRxCoO3 (R: rare earth element) with the Co oxide perovskite structure, the doping level can change the electronic transport and magnetic properties by changing the spin state of Co ions, and this can provide a theoretical understanding of transition metal oxides as well as an opportunity for the application of these magnetic materials. Room temperature ferromagnetism is a useful property for applying magnetic material in many fields such as hard disk read head and magnetic random-access memory [13,14]. Sr1xYxCoO3d which belongs to Sr1xRxCoO3 family is a room temperature ferromagnet with a Curie temperature of 335 K in a
Corresponding author. Tel.: +82 54 279 2795; fax: +82 54 279 2399.
E-mail address: [email protected] (Y.-H. Shin). 0022-0248/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2008.05.030
narrow range of composition (0.2pxp0.25) [5]. From a practical point of view, the thin film study of Sr1xYxCoO3d can yield the opportunity of applications which utilize the variations of the physical properties depending on the strained structure of thin films under various stresses. So far, however, there has been no study on the thin film of Sr0.775Y0.225CoO3d. In this study, the epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films were grown on (1 0 0) and (111) single crystal SrTiO3 substrates by pulsed laser deposition (PLD). We show the structural and magnetic properties of the epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films on (1 0 0) and (111) single crystal SrTiO3 substrates. Epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films were simultaneously grown on (1 0 0) and (111) single crystal SrTiO3 substrates, respectively, by PLD. The stoichiometric transfer of material from target to substrate is one of the remarkable advantages of the PLD method [15]. The (1 0 0) and (111) single crystal SrTiO3 substrates with a pseudocubic lattice constant of 3.905 A˚ have a relatively small lattice misfit of 1.7% with Sr0.775Y0.225CoO3d [5]. We used the pseudocubic lattice constant of 3.840 A˚ for Sr0.775Y0.225CoO3d. The Sr0.775Y0.225CoO3d target was prepared by a conventional solid-state reaction method. High-purity stoichiometric Sr0.775Y0.225CoO3d was mixed in a molar ratio of Y:Sr:Co ¼ 0.225:0.775:1.000 with Y2O3 (99.99%, MTI), SrCO3 (99.999%, Cerac), and Co3O4 (99.99%, Aldrich). The mixed powder was pressed and calcined at 1000 1C
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for 12 h. The calcined pellet was reground, pressed into a pellet, and sintered at 1170 1C for 48 h. The phase purity as well as the lattice constant was checked by using powder X-ray diffraction (XRD) with Cu Ka radiation (Philips). For the deposition, a frequency tripled (wavelength 355 nm, pulse repetition rate 10 Hz) Nd:YAG laser was used and the laser power of approximately 2 J/cm2 was used. The base pressure was 1 106 Torr, and the oxygen pressure of 200 mTorr was maintained prior to the deposition of Sr0.775Y0.225CoO3d thin films. For the optimal conditions for the thin film growth, the substrate temperature and the distance between target and substrate were set to 500 1C and 6 cm, respectively. The tentative compositions of Sr0.775Y0.225CoO3d thin films were obtained by energy dispersive X-ray spectrometer (EDS). For the structural determination of the thin films, XRD data were obtained by conventional laboratory X-ray (CuKa radiation). The thicknesses of both (111) and (1 0 0) Sr0.775Y0.225CoO3d thin films were 800 A˚ determined from the spectroscopic ellipsometry. The surface morphology and topography of the thin films were observed by atomic force microscope (AFM, Seiko SPA400) in contact mode using an Si3N4 cantilever (a force constant of 0.08 N/m and a resonance frequency of 34 kHz). To check the ferromagnetic property, magnetic hysteresis loops were measured by superconducting quantum interference device (SQUID) magnetometer (quantum design). Fig. 1(a) shows an XRD y2y scan of an Sr0.775Y0.225CoO3d thin film on a (1 0 0) SrTiO3 substrate, revealing a nearly phase pure (1 0 0)-oriented film. The Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate has (1 0 0) oriented growth along the out-
of-plane direction. The full width at half maximum (FWHM) of the (2 0 0) rocking curve is about 0.81 indicating that the film is well crystalline. The out-of-plane lattice constant of 3.808 A˚ was obtained from the (a 0 0) peaks, and this lattice constant has a slightly decreased value of about 0.8% from the bulk lattice constant of Sr0.775Y0.225CoO3d. To check the in-plane orientation, we performed X-ray scattering with f scan geometry. Fig. 1(b) shows f scans of the (2 0 4) peak of the (1 0 0) Sr0.775Y0.225CoO3d thin film and the (1 0 1) peak of the (1 0 0) SrTiO3 substrate. The fourfold symmetries of the (2 0 4) peak of (1 0 0) Sr0.775Y0.225CoO3d thin film and the (1 0 1) peak of the (1 0 0) SrTiO3 substrate are observed, and these (2 0 4) and (1 0 1) peaks have the same position for the f angle. This signifies that the b- or c-axis of the (1 0 0) Sr0.775Y0.225CoO3d thin film are oriented along those of the (1 0 0) SrTiO3 substrate. The FWHM of the in-plane (2 0 4) peak is about 0.71 indicating a good epitaxial growth. From the 2y value of the (2 0 4) peak, the b- and c-axes constants of 3.899 A˚ are obtained, and these lattice constants are increased by about 1.7% from the bulk lattice constant of Sr0.775Y0.225CoO3d. This elongation of the in-plane lattice constant under the tensile stress is due to the lattice constant of the (1 0 0) SrTiO3 substrate with the lattice misfit of 1.5%, causing the lattice constant to shrink along the out-of-plane direction of the (1 0 0) Sr0.775Y0.225CoO3d thin film. Fig. 2(a) shows an XRD y2y scan of an epitaxial (111) Sr0.775Y0.225CoO3d thin film on a (111) SrTiO3 substrate. Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate has (111) oriented growth along the out-of-plane direction. The FWHM of the (111) rocking curve is about 0.61, and this FWHM is
Fig. 1. (a) XRD pattern (y2y scan) of the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate. (b) The f scans of the (2 0 4) peak of the (1 0 0) Sr0.775Y0.225CoO3d thin film and the (1 0 1) peak of the (1 0 0) SrTiO3 substrate.
Fig. 2. (a) XRD pattern (y2y scan) of the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate. (b) The f scans of the (4 0 0) peak of the Sr0.775Y0.225CoO3d thin film and the (2 0 0) peak of the (111) SrTiO3 substrate.
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slightly smaller than that of the (1 0 0) Sr0.775Y0.225CoO3d thin film. Fig. 2(b) shows the f scans of the (4 0 0) peak of the (111) Sr0.775Y0.225CoO3d thin film and the (2 0 0) peak of the (111) SrTiO3 substrate. The FWHM of the in-plane (4 0 0) peak is about 0.61, and this FWHM is also smaller than that of the (1 0 0) Sr0.775Y0.225CoO3d thin film, indicating that the (111) Sr0.775Y0.225CoO3d thin film has a better crystal texture along the in-plane direction. From the (4 0 0) and (2 0 0) peaks, the a-axis constant of 3.840 A˚ is obtained, and b- and c-axes constants of 3.837 A˚ are obtained. These lattice constants are about the same as the bulk lattice constants of Sr0.775Y0.225CoO3d, and this result means that the (111) Sr0.775Y0.225CoO3d thin film has a smaller strain than the (1 0 0) Sr0.775Y0.225CoO3d thin film. From the comparison of the strain of the (1 0 0) Sr0.775Y0.225CoO3d thin film with that of the (111) Sr0.775Y0.225CoO3d thin film, it is inferred that the magnetic property of bulk Sr0.775Y0.225CoO3d is expected for the (111) Sr0.775Y0.225CoO3d thin film due to the relatively low strain. Fig. 3 shows AFM images of the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate and the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate. The root mean square (rms) surface roughnesses of (1 0 0) Sr0.775Y0.225CoO3d and (111) Sr0.775Y0.225CoO3d thin films for 1 mm 1 mm area are 130 and 39 A˚, respectively. Relatively small grains of average grain size of about 60 nm are observed on the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film, but the epitaxial (111) Sr0.775Y0.225CoO3d thin film has relatively large grains of average grain size of about 1 mm. This means that the epitaxial (111) Sr0.775Y0.225CoO3d thin film has a longer diffusion length of an adatom along the surface than the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film during the deposition. From the XRD and AFM data, it is expected that the (1 0 0) Sr0.775Y0.225CoO3d thin film will exhibit more highly strained physical properties than the Sr0.775Y0.225CoO3d thin film. Fig. 4 shows magnetization versus in-plane applied field loops of the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate and the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate at room temperature. When the magnetic field is applied along the out-of-plane direction, the paramagnetic properties of both thin films were obtained due to the magnetic anisotropy caused by the insufficient thickness of thin film (not shown) [16,17]. For the in-plane magnetic field, the (111) Sr0.775Y0.225CoO3d thin film shows ferromagnetic hysteresis with the remanent magnetization of 7 105 emu and the
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saturation magnetization of 2 104 emu. In the (1 0 0) Sr0.775Y0.225CoO3d thin film, however, the superparamagnetic hysteresis is observed. On the basis of the different misfits of (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films on (1 0 0) and (111) SrTiO3 substrates, it is inferred that this superparamagnetism results from the relatively highly strained structural property of the (1 0 0) Sr0.775Y0.225CoO3d thin film [18,19]. In conclusion, epitaxially oriented (1 0 0) and (111) Sr0.775Y0.225CoO3d thin films were grown on single crystal (1 0 0) and (111) SrTiO3 substrates, respectively, by PLD. The elongation of the in-plane lattice constants under the tensile stress causes the lattice constant to shrink along the out-of-plane direction of the (1 0 0) Sr0.775Y0.225CoO3d thin film. The lattice constants of the (111) epitaxial Sr0.775Y0.225CoO3d thin film were nearly the same as the bulk lattice constants of Sr0.775 Y0.225CoO3d, and the ferromagnetic property was observed with the relatively large grains at room temperature. However, the (1 0 0) epitaxial Sr0.775Y0.225CoO3d thin film showed the relatively high strain along the out-of-plane direction and the superparamagnetic property with the relatively small grains.
Fig. 4. Magnetization–magnetic field curves of the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate and the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate at room temperature.
Fig. 3. AFM topographic images of (a) the epitaxial (1 0 0) Sr0.775Y0.225CoO3d thin film on the (1 0 0) SrTiO3 substrate and (b) the epitaxial (111) Sr0.775Y0.225CoO3d thin film on the (111) SrTiO3 substrate.
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