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High-frequency characteristics of CoFeVAlONb thin films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) e189–e191 www.elsevier.com/locate/jmmm
High-frequency characteristics of CoFeVAlONb thin films K.E. Leea, N.D. Hab, M.H. Phanc, C.O. Kimb, a
Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Korea Research Center for Advanced Magnetic Materials, Chungnam National University, Daejeon 305-764, Korea c Department of Aerospace Engineering, University of Bristol, Queen’s Building, Bristol BS8 1TR, England
b
Available online 2 March 2006
Abstract High-frequency characteristics of CoFeVAlONb thin films were studied. A thin film of Co43.47Fe35.30V1.54Al5.55O9.93Nb4.21 is observed to exhibit excellent magnetic properties; magnetic coercivity of 1.24 Oe, uniaxial in-plane anisotropy field of 66.99 Oe, and saturation magnetization of 19.8 kG. The effective permeability of the film is as high as 1089 and is stable up to 1.8 GHz, and with ferromagnetic resonance over 3 GHz. This film also has very high electrical resistivity of about 628 mO cm. These superior properties make it ideal for high-frequency magnetic applications. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.i Keywords: Nanogranular; Soft magnetic thin films; High frequency thin films; Microstructure
1. Introduction Nanocrystalline and nanogranular ferromagnetic materials have attracted great interest in the magnetic community from both fundamental and applied points of view. For instance, thin film microinductors/microtransformers and magnetic recording heads have shown their importance of keeping up with the strong demands of the wireless communication and the data storage industries [1–5]. In order to obtain soft magnetic films for MHz–GHz frequency range applications, there are two main limiting factors, natural ferromagnetic resonance frequency (FMR) and eddy current losses should be considered carefully. For thin films with uniaxial in-plane anisotropy, the Landau–Lifshitz equation can be simplified to f FMR ¼ ðg=2pÞðM S H K =m0 Þ1=2 with the gyro magnetic constant, g, saturation magnetization, Ms, and the uniaxial anisotropy field, Hk. Among ferromagnets investigated, CoFe-based materials with addition of Zr (B, Pd, Nb, O, N) are interesting because of their excellent soft magnetic properties [6–9]. In particular, CoFe-based nanogranular films have been found to have high resistivity as well as excellent Corresponding author. Tel.: +82 42 821 6235; fax: +82 42 822 6272.
E-mail address: [email protected] (C.O. Kim). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.157
soft magnetic properties, and to be attractive for highfrequency applications [10–13]. In this work, we report on the specific experimental methods of controlling the anisotropy, electrical resistivity, and saturation magnetization of nanogranular CoFeVAlONb films to improve the high-frequency characteristics of these typical films. 2. Experimental procedure Co43.47Fe35.30V1.54Al5.55O9.93Nb4.21 (CoFeVAlONb) films with thickness of about 200 nm were deposited onto a water-cooled silicon (1 0 0) substrate using radio magnetron sputtering in O2/Ar atmosphere of 2 mTorr at an input power of 300 W and a background pressure of less than 2.0 107 Torr. The composite target was composed of eight pure aluminum chip and four niobium chips of 5 5 1 mm fixed on the Co49Fe49V2 target of 100 mm diameter and 1.5 mm thickness. The compositions of CoFeVAlONb films were analyzed by Auger electron spectroscopy (AES). Thickness of the samples was determined using both surface profilometer alpha step 500 and high-resolution scanning electron microscopy (HRSEM). The thickness of the thinner films was determined by proper calibrating the deposition time and
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rate. Magnetic measurements were carried out with a vibrating sample magnetometer (VSM). The crystal structures of thin films were verified by X-ray diffraction (XRD). The grain size of CoFeV films were calculated using Scherrer formulas [14] L ¼ 0:9l=FWHMð2yÞ cos y (full width at haft-maximum—FWHM) and HRSEM cross-sectional images. The ac complex permeability m0 and m00 were measured with the network analyzer (HP8752) in the range of 1–3000 MHz. Electrical resistivity was measured at room temperature using a conventional fourpoint probe method. 3. Results and discussion Fig. 1 shows the hysteresis loops of as-deposited CoFeVAlONb thin film. The film is magnetically soft and has uniaxial magnetic anisotropy. The hard-axis anisotropy is estimated to be H k ¼ 66:99 Oe. Interestingly, the hard-axis coercivity shows a negative coercivity of about 0.42 Oe [15–20]. This suggests that the CoFeVAlONb film exhibits a magnetic characteristic similarly to nanogranular CoFeAlO thin films [21]. In addition, no degradation of the soft magnetic properties of the film was observed after aging at 250 1C for 24 h in high vacuum furnace (2.0 107 Torr) under a magnetic field of 120 Oe parallel to the film plane, indicating sufficient thermal stability to withstand magnetic microdevices fabrication process. It is known that the appearance of magnetic anisotropy for as-deposited films is closely related to the film microstructure (e.g., grain size, shape, and texture). In order to clarify the origin of the soft magnetic properties, resistivity, and thermal stability, the microstructural change of this alloy system has been examined by selecting a series of as-deposited samples, representing the different composition of CoFeVAlONb films fabricated with differ-
Fig. 1. Experimental in-plane hysteresis loops of Co43.47Fe35.30V1.54 Al5.55O9.93Nb4.21 films in hard and easy axes.
Fig. 2. XRD patterns of CoFeVAlONb thin films at different partial pressures of oxygen. These films were fabricated at 300 W, working pressure of 2 mTorr, and the film thickness of 200 nm and background pressure of less than 2.0 107 Torr.
ent partial pressure of oxygen (see Fig. 2). As shown in Fig. 2, the XRD patterns of CoFeVAlONb films show a halo pattern, depending on the partial pressure of oxygen, indicating an amorphous state. The amorphous phases change from mixture of glassy metallic and oxides of Al2O3, CoFe-oxides and very fine CoX2O4 (X ¼ Fe, Al, Nb) particles to glassy oxides at 4% and 10% of partial pressure of oxygen, respectively. The peaks at 2y (43–461) may be indexed using the aCoFe (1 1 0), a-Co(Fe) (1 1 0), FeV (1 1 0), CoX2O4 (4 0 0) or CoFe2O4 (3 1 1), CoFeV (2 0 2) and mixture of amorphous oxide phases of Al2O3, and oxides of Co(Fe) structure. The peaks at 2y (63–661), (81–831) can be indexed using the a-CoFe (2 0 0), and a-CoFe (2 1 1), respectively. In addition, the peaks at 2y (63–661), (81–831) can be indexed using amorphous or very small oxide particles of CoFe-oxide, alumina or minor phase. It is noted that such peaks disappeared at 4% or 10% of partial pressure of oxygen. The microstructures and phase compounds of the films strongly affect the magnetic properties with magnetic nanogranular phases with high magnetic moment [e.g., a-CoFe (1 1 0)] surrounded by the amorphous phases with high electrical resistivity. The magnetic thermal stability can be explained considering the role of V and Nb in the films. Herein, the atoms are dissolved in the BCC phases a-CoFe (1 1 0) or a-CoFe (2 0 0) or a-CoFe (2 1 1), and a small amount of these elements combined with CoFe-major phases to form (CoFe)3V(Nb) carbide phases, that is, one of four interpenetrating simple cubic sublattices of the LI2 structure. The grain size remains nearly unchanged after aging process. It is reasonable to claim that the stability of grain size with aging time is partially due to the uniform distribution of very fine (CoFe)3V(Nb) carbide phases in the nanogranular. The presence of these phases in the CoFeVAlONb film impedes grain boundary migration and nanogranular growth. The averaged grain size of the film, calculated using the Scherrer’s formula, is about 10 nm.
ARTICLE IN PRESS K.E. Lee et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e189–e191
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Fig. 3. (a) Bright field plan-view TEM image of Co43.47Fe35.30V1.54Al5.55O9.93Nb4.21 film. Fabricated conditions: Input power of 300 W, partial pressure of oxygen of 8%, working pressure of 2 mTorr, and thickness of 200 nm; (b) frequency dependence of permeability of the film.
Fig. 3(a) shows a TEM image of the as-deposited CoFeVAlONb film. It can be seen that the grains have an anisotropic shape. The major and minor axes of the grains are about 3.5–5 nm and about 12–20 nm, respectively. This result is consistent with that calculated from the XRD patterns. The nanogranules were separated by an amorphous matrix with space of about 5–8 A˚. This suggests that the uniaxial in-plane magnetic anisotropy of the nanogranular CoFeVAlONb film results from magnetic interactions between the nanograins due to the net anisotropic structure of intergrain oxides and dipole interaction between nanogranules. In this case, the film microstructure is considered to be the critical factor for controlling the uniaxial magnetic anisotropy of the CoFeVAlONb film. Fig. 3(b) displays the frequency dependence of permeability measured along the hard axis, for the as-deposited CoFeVAlONb film. It is interesting to mention that the effective permeability of the CoFeVAlONb film with a thickness of 200 nm has a high value of more than 1089 and this value remains nearly constant up to 1.8 GHz. This indicates that the CoFeVAlONb film is suitable for the applications of high-frequency micromagnetic devices. 4. Conclusion The excellent soft magnetic properties of CoFeVAlONb films such as high saturation induction of 19.8 kG, uniaxial in-plane magnetic anisotropy field of 66.99 Oe, and low coercivity of 1.24 Oe have been achieved. The effective permeability of the film with a thickness of 200 nm has a high value of more than 1089 and this value remains nearly constant up to 1.8 GHz together with ferromagnetic resonance over 3 GHz. These superior magnetic properties, in addition to the high electrical resistivity of 628 mO cm, make these films ideal for high-frequency applications. It is proposed that the microstructure of the film contains anisotropic grains and this is the critical factor for controlling the uniaxial magnetic anisotropy of the film.
Acknowledgments This work is supported by the Korean Science and Engineering Foundation through the Research center for Advanced Magnetic Materials at Chungnam National University and the BK 21 program. References [1] W.D. Li, O. Kitakami, Y. Shimada, J. Appl. Phys. 83 (1998) 6661. [2] L.H. Chen, H.K. Chen, C.T. Hsieh, Y.H. Shih, I.G. Chen, S.Y. Chen, H.P. Liu, J. Appl. Phys. 91 (2002) 8450. [3] M.E. McHenry, D.E. Laughlin, Acta Mater. 48 (2000) 223. [4] P.C. Kuo, S.S. Chang, C.M. Kuo, Y.D. Yao, H.L. Huang, J. Appl. Phys. 83 (1998) 6643. [5] N.D. Ha, T.S. Yoon, H.B. Kim, J.J. Lim, C.G. Kim, C.O. Kim, J. Magn. Magn. Mater. 290–291 (2005) 1571. [6] X.Y. Xiong, T.R. Finlayson, B.C. Muddle, J. Phys. D: Appl. Phys. 34 (2001) 2845. [7] J.M. Shin, Y.M. Kim, J. Kim, S.H. Han, H.J. Kim, J. Appl. Phys. 93 (2003) 6677. [8] S. Ohnuma, N. Kobayashi, H. Fujimori, T. Masumoto, X.Y. Xiong, K. Hono, Scripta Mater. 48 (2003) 903. [9] M. Munakata, M. Yagi, M. Motoyama, Y. Shimada, M. Baba, M. Yamaguchi, K.I. Arai, IEEE Trans. Magn. 37 (2003) 2258. [10] S. Ohnuma, H. Fujimori, T. Masumoto, X.Y. Xiong, D.H. Ping, K. Hono, Appl. Phys. Lett. 82 (2003) 946. [11] N.D. Ha, B.C. Park, C.G. Kim, B.K. Min, C.O. Kim, IEEE Trans. Magn. 40 (2004) 2727. [12] N.D. Ha, T.S. Yoon, H.B. Kim, J.J. Lim, C.G. Kim, C.O. Kim, J. Magn. Magn. Mater. 290–291 (2005) 1469. [13] L.H. Chien, T.J. Klemmer, K.A. Ellis, R.B. van Dover, S. Jin, J. Appl. Phys. 87 (2000) 5858. [14] B.E. Warren, in: X-ray Diffraction, Addison-Wesley, Reading, MA, 1969. [15] M. Cougo dos Santos, J. Geshev, J.E. Schmidt, S.R. Teixeira, L.G. Pereira, Phys. Rev. B. 61 (2000) 1311. [16] J. Chen, J.L. Erskine, Phys. Rev. Lett. 68 (1992) 1212. [17] X. Yan, Y. Xu, J. Appl. Phys. 79 (1996) 6013. [18] A. Aharoni, J. Appl. Phys. 76 (1994) 6977. [19] Y.Z. Wu, G.S. Dong, X.F. Jin, Phys. Rev. B. 64 (2001) 214406. [20] J. Geshev, A.D.C. Viegas, J.E. Schmidt, J. Appl. Phys. 84 (1998) 1488. [21] N.D. Ha, T.S. Yoon, E. Gan’shina, M.H. Phan, C.G. Kim, C.O. Kim, J. Magn. Magn. Mater. 295 (2005) 126.
Journal of Magnetism and Magnetic Materials 304 (2006) e189–e191 www.elsevier.com/locate/jmmm
High-frequency characteristics of CoFeVAlONb thin films K.E. Leea, N.D. Hab, M.H. Phanc, C.O. Kimb, a
Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, Korea Research Center for Advanced Magnetic Materials, Chungnam National University, Daejeon 305-764, Korea c Department of Aerospace Engineering, University of Bristol, Queen’s Building, Bristol BS8 1TR, England
b
Available online 2 March 2006
Abstract High-frequency characteristics of CoFeVAlONb thin films were studied. A thin film of Co43.47Fe35.30V1.54Al5.55O9.93Nb4.21 is observed to exhibit excellent magnetic properties; magnetic coercivity of 1.24 Oe, uniaxial in-plane anisotropy field of 66.99 Oe, and saturation magnetization of 19.8 kG. The effective permeability of the film is as high as 1089 and is stable up to 1.8 GHz, and with ferromagnetic resonance over 3 GHz. This film also has very high electrical resistivity of about 628 mO cm. These superior properties make it ideal for high-frequency magnetic applications. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.i Keywords: Nanogranular; Soft magnetic thin films; High frequency thin films; Microstructure
1. Introduction Nanocrystalline and nanogranular ferromagnetic materials have attracted great interest in the magnetic community from both fundamental and applied points of view. For instance, thin film microinductors/microtransformers and magnetic recording heads have shown their importance of keeping up with the strong demands of the wireless communication and the data storage industries [1–5]. In order to obtain soft magnetic films for MHz–GHz frequency range applications, there are two main limiting factors, natural ferromagnetic resonance frequency (FMR) and eddy current losses should be considered carefully. For thin films with uniaxial in-plane anisotropy, the Landau–Lifshitz equation can be simplified to f FMR ¼ ðg=2pÞðM S H K =m0 Þ1=2 with the gyro magnetic constant, g, saturation magnetization, Ms, and the uniaxial anisotropy field, Hk. Among ferromagnets investigated, CoFe-based materials with addition of Zr (B, Pd, Nb, O, N) are interesting because of their excellent soft magnetic properties [6–9]. In particular, CoFe-based nanogranular films have been found to have high resistivity as well as excellent Corresponding author. Tel.: +82 42 821 6235; fax: +82 42 822 6272.
E-mail address: [email protected] (C.O. Kim). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.157
soft magnetic properties, and to be attractive for highfrequency applications [10–13]. In this work, we report on the specific experimental methods of controlling the anisotropy, electrical resistivity, and saturation magnetization of nanogranular CoFeVAlONb films to improve the high-frequency characteristics of these typical films. 2. Experimental procedure Co43.47Fe35.30V1.54Al5.55O9.93Nb4.21 (CoFeVAlONb) films with thickness of about 200 nm were deposited onto a water-cooled silicon (1 0 0) substrate using radio magnetron sputtering in O2/Ar atmosphere of 2 mTorr at an input power of 300 W and a background pressure of less than 2.0 107 Torr. The composite target was composed of eight pure aluminum chip and four niobium chips of 5 5 1 mm fixed on the Co49Fe49V2 target of 100 mm diameter and 1.5 mm thickness. The compositions of CoFeVAlONb films were analyzed by Auger electron spectroscopy (AES). Thickness of the samples was determined using both surface profilometer alpha step 500 and high-resolution scanning electron microscopy (HRSEM). The thickness of the thinner films was determined by proper calibrating the deposition time and
ARTICLE IN PRESS e190
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rate. Magnetic measurements were carried out with a vibrating sample magnetometer (VSM). The crystal structures of thin films were verified by X-ray diffraction (XRD). The grain size of CoFeV films were calculated using Scherrer formulas [14] L ¼ 0:9l=FWHMð2yÞ cos y (full width at haft-maximum—FWHM) and HRSEM cross-sectional images. The ac complex permeability m0 and m00 were measured with the network analyzer (HP8752) in the range of 1–3000 MHz. Electrical resistivity was measured at room temperature using a conventional fourpoint probe method. 3. Results and discussion Fig. 1 shows the hysteresis loops of as-deposited CoFeVAlONb thin film. The film is magnetically soft and has uniaxial magnetic anisotropy. The hard-axis anisotropy is estimated to be H k ¼ 66:99 Oe. Interestingly, the hard-axis coercivity shows a negative coercivity of about 0.42 Oe [15–20]. This suggests that the CoFeVAlONb film exhibits a magnetic characteristic similarly to nanogranular CoFeAlO thin films [21]. In addition, no degradation of the soft magnetic properties of the film was observed after aging at 250 1C for 24 h in high vacuum furnace (2.0 107 Torr) under a magnetic field of 120 Oe parallel to the film plane, indicating sufficient thermal stability to withstand magnetic microdevices fabrication process. It is known that the appearance of magnetic anisotropy for as-deposited films is closely related to the film microstructure (e.g., grain size, shape, and texture). In order to clarify the origin of the soft magnetic properties, resistivity, and thermal stability, the microstructural change of this alloy system has been examined by selecting a series of as-deposited samples, representing the different composition of CoFeVAlONb films fabricated with differ-
Fig. 1. Experimental in-plane hysteresis loops of Co43.47Fe35.30V1.54 Al5.55O9.93Nb4.21 films in hard and easy axes.
Fig. 2. XRD patterns of CoFeVAlONb thin films at different partial pressures of oxygen. These films were fabricated at 300 W, working pressure of 2 mTorr, and the film thickness of 200 nm and background pressure of less than 2.0 107 Torr.
ent partial pressure of oxygen (see Fig. 2). As shown in Fig. 2, the XRD patterns of CoFeVAlONb films show a halo pattern, depending on the partial pressure of oxygen, indicating an amorphous state. The amorphous phases change from mixture of glassy metallic and oxides of Al2O3, CoFe-oxides and very fine CoX2O4 (X ¼ Fe, Al, Nb) particles to glassy oxides at 4% and 10% of partial pressure of oxygen, respectively. The peaks at 2y (43–461) may be indexed using the aCoFe (1 1 0), a-Co(Fe) (1 1 0), FeV (1 1 0), CoX2O4 (4 0 0) or CoFe2O4 (3 1 1), CoFeV (2 0 2) and mixture of amorphous oxide phases of Al2O3, and oxides of Co(Fe) structure. The peaks at 2y (63–661), (81–831) can be indexed using the a-CoFe (2 0 0), and a-CoFe (2 1 1), respectively. In addition, the peaks at 2y (63–661), (81–831) can be indexed using amorphous or very small oxide particles of CoFe-oxide, alumina or minor phase. It is noted that such peaks disappeared at 4% or 10% of partial pressure of oxygen. The microstructures and phase compounds of the films strongly affect the magnetic properties with magnetic nanogranular phases with high magnetic moment [e.g., a-CoFe (1 1 0)] surrounded by the amorphous phases with high electrical resistivity. The magnetic thermal stability can be explained considering the role of V and Nb in the films. Herein, the atoms are dissolved in the BCC phases a-CoFe (1 1 0) or a-CoFe (2 0 0) or a-CoFe (2 1 1), and a small amount of these elements combined with CoFe-major phases to form (CoFe)3V(Nb) carbide phases, that is, one of four interpenetrating simple cubic sublattices of the LI2 structure. The grain size remains nearly unchanged after aging process. It is reasonable to claim that the stability of grain size with aging time is partially due to the uniform distribution of very fine (CoFe)3V(Nb) carbide phases in the nanogranular. The presence of these phases in the CoFeVAlONb film impedes grain boundary migration and nanogranular growth. The averaged grain size of the film, calculated using the Scherrer’s formula, is about 10 nm.
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Fig. 3. (a) Bright field plan-view TEM image of Co43.47Fe35.30V1.54Al5.55O9.93Nb4.21 film. Fabricated conditions: Input power of 300 W, partial pressure of oxygen of 8%, working pressure of 2 mTorr, and thickness of 200 nm; (b) frequency dependence of permeability of the film.
Fig. 3(a) shows a TEM image of the as-deposited CoFeVAlONb film. It can be seen that the grains have an anisotropic shape. The major and minor axes of the grains are about 3.5–5 nm and about 12–20 nm, respectively. This result is consistent with that calculated from the XRD patterns. The nanogranules were separated by an amorphous matrix with space of about 5–8 A˚. This suggests that the uniaxial in-plane magnetic anisotropy of the nanogranular CoFeVAlONb film results from magnetic interactions between the nanograins due to the net anisotropic structure of intergrain oxides and dipole interaction between nanogranules. In this case, the film microstructure is considered to be the critical factor for controlling the uniaxial magnetic anisotropy of the CoFeVAlONb film. Fig. 3(b) displays the frequency dependence of permeability measured along the hard axis, for the as-deposited CoFeVAlONb film. It is interesting to mention that the effective permeability of the CoFeVAlONb film with a thickness of 200 nm has a high value of more than 1089 and this value remains nearly constant up to 1.8 GHz. This indicates that the CoFeVAlONb film is suitable for the applications of high-frequency micromagnetic devices. 4. Conclusion The excellent soft magnetic properties of CoFeVAlONb films such as high saturation induction of 19.8 kG, uniaxial in-plane magnetic anisotropy field of 66.99 Oe, and low coercivity of 1.24 Oe have been achieved. The effective permeability of the film with a thickness of 200 nm has a high value of more than 1089 and this value remains nearly constant up to 1.8 GHz together with ferromagnetic resonance over 3 GHz. These superior magnetic properties, in addition to the high electrical resistivity of 628 mO cm, make these films ideal for high-frequency applications. It is proposed that the microstructure of the film contains anisotropic grains and this is the critical factor for controlling the uniaxial magnetic anisotropy of the film.
Acknowledgments This work is supported by the Korean Science and Engineering Foundation through the Research center for Advanced Magnetic Materials at Chungnam National University and the BK 21 program. References [1] W.D. Li, O. Kitakami, Y. Shimada, J. Appl. Phys. 83 (1998) 6661. [2] L.H. Chen, H.K. Chen, C.T. Hsieh, Y.H. Shih, I.G. Chen, S.Y. Chen, H.P. Liu, J. Appl. Phys. 91 (2002) 8450. [3] M.E. McHenry, D.E. Laughlin, Acta Mater. 48 (2000) 223. [4] P.C. Kuo, S.S. Chang, C.M. Kuo, Y.D. Yao, H.L. Huang, J. Appl. Phys. 83 (1998) 6643. [5] N.D. Ha, T.S. Yoon, H.B. Kim, J.J. Lim, C.G. Kim, C.O. Kim, J. Magn. Magn. Mater. 290–291 (2005) 1571. [6] X.Y. Xiong, T.R. Finlayson, B.C. Muddle, J. Phys. D: Appl. Phys. 34 (2001) 2845. [7] J.M. Shin, Y.M. Kim, J. Kim, S.H. Han, H.J. Kim, J. Appl. Phys. 93 (2003) 6677. [8] S. Ohnuma, N. Kobayashi, H. Fujimori, T. Masumoto, X.Y. Xiong, K. Hono, Scripta Mater. 48 (2003) 903. [9] M. Munakata, M. Yagi, M. Motoyama, Y. Shimada, M. Baba, M. Yamaguchi, K.I. Arai, IEEE Trans. Magn. 37 (2003) 2258. [10] S. Ohnuma, H. Fujimori, T. Masumoto, X.Y. Xiong, D.H. Ping, K. Hono, Appl. Phys. Lett. 82 (2003) 946. [11] N.D. Ha, B.C. Park, C.G. Kim, B.K. Min, C.O. Kim, IEEE Trans. Magn. 40 (2004) 2727. [12] N.D. Ha, T.S. Yoon, H.B. Kim, J.J. Lim, C.G. Kim, C.O. Kim, J. Magn. Magn. Mater. 290–291 (2005) 1469. [13] L.H. Chien, T.J. Klemmer, K.A. Ellis, R.B. van Dover, S. Jin, J. Appl. Phys. 87 (2000) 5858. [14] B.E. Warren, in: X-ray Diffraction, Addison-Wesley, Reading, MA, 1969. [15] M. Cougo dos Santos, J. Geshev, J.E. Schmidt, S.R. Teixeira, L.G. Pereira, Phys. Rev. B. 61 (2000) 1311. [16] J. Chen, J.L. Erskine, Phys. Rev. Lett. 68 (1992) 1212. [17] X. Yan, Y. Xu, J. Appl. Phys. 79 (1996) 6013. [18] A. Aharoni, J. Appl. Phys. 76 (1994) 6977. [19] Y.Z. Wu, G.S. Dong, X.F. Jin, Phys. Rev. B. 64 (2001) 214406. [20] J. Geshev, A.D.C. Viegas, J.E. Schmidt, J. Appl. Phys. 84 (1998) 1488. [21] N.D. Ha, T.S. Yoon, E. Gan’shina, M.H. Phan, C.G. Kim, C.O. Kim, J. Magn. Magn. Mater. 295 (2005) 126.