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Orientation control in L10 FePt films by using magnetic field annealing around Curie temperature
Applied Surface Science 258 (2012) 5770–5773
Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Orientation control in L10 FePt films by using magnetic field annealing around Curie temperature Liwang Liu a , Hua Lv a , Wei Sheng a , Yuanfu Lou a , Jianmin Bai a , Jiangwei Cao a,∗ , Bin Ma b , Fulin Wei a a b
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, PR China State Key Laboratory for Advanced Photonic Materials and Devices, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, PR China
a r t i c l e
i n f o
Article history: Received 17 August 2011 Received in revised form 16 February 2012 Accepted 18 February 2012 Available online 24 February 2012 Keywords: FePt Magnetic field annealing Exchange energy Curie temperature
a b s t r a c t A new method to improve (0 0 1) texture of L10 FePt films was reported. In this work, in order to obtain the perpendicular orientation of L10 FePt films, the FePt films were annealed at 700 ◦ C (TA1 ) for 30 min and then annealed at TA2 (430–600 ◦ C) for an hour either in a magnetic field of ∼3500 Oe along the normal direction of the films (Route A) or without magnetic field (Route B). Compared with the Route B, I(0 0 1) /I(1 1 1) showed a peak value of 7 when TA2 was around 478 ◦ C (around the Curie temperature of L10 FePt films) and the AFM image of FePt sample was annealed around 478 ◦ C showed an obvious change of grains through Route A. It is related to the following reason: when the temperature is near Curie temperature of L10 FePt films, the thermal disturbing term is close to the exchange energy of the magnetization, therefore, the magnetic field of ∼3500 Oe is effective for promoting the preferential growth direction of L10 structure. © 2012 Elsevier B.V. All rights reserved.
1. Introduction L10 FePt films with perpendicular orientation have been studied extensively as one of promising candidates for future ultrahigh density magnetic recording media [1]. As well known, as perpendicular magnetic recording media, c-axis of FePt films should be normal to the films and a strong (0 0 1) texture is desired. However, the FePt films deposited directly on amorphous substrates tend to possess (1 1 1) texture because (1 1 1) plane is the close-packed plane [2,3]. Therefore, one of the key challenges is how to align the magnetic easy axis (c-axis) to normal direction of FePt films. Usually, several approaches are being used to obtain/enhance the (0 0 1) orientation in FePt films: expitaxial growth on single crystal substrates [4–8]; hetero-expitaxial growth on textured underlayers/seedlayers [9–20]; alternate deposition of Fe and Pt layers [21–26]; FePt–X (X = B2 O3 , SiO2 , MgO, C, Ag, etc.) nanocomposite deposition [27–34]; magnetic field annealing the antiferromagnetic/FePt structure by using antiferromagnetic exchange energy [35]; aligning the magnetic particles in a magnetic field by using magnetostatic energy [36–39]. Previous studies have demonstrated that magnetic field annealing is helpful to preferred orientation of alloy or nanowires [40,41]. In this work, in order to obtain the perpendicular orientation of L10 FePt films, we proposed a method to control the c-axis of L10 FePt by annealing the films in a magnetic
∗ Corresponding author. E-mail address: [email protected] (J. Cao). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.090
field around the Curie temperature (Tc , the Tc of L10 FePt films is 477 ◦ C). As for a ferromagnetic material, the magnetic moments spontaneously align to the same direction inside a single magnetic domain at a temperature below the Tc , while, their directions disperse randomly when the temperature is above the Tc . If L10 FePt alloys were annealed around its Tc in a magnetic field, the preferential direction of magnetic easy axis of L10 structure should be enhanced by the external magnetic field during the rearrangement process of Fe and Pt atoms. 2. Experiment FePt films with structure of Si (substrate)/Fe (2 nm)/FePt (20 nm)/Pt (2 nm) were deposited by magnetron sputtering system. The base pressure in chamber is 2 × 10−6 Pa and the nominal composition of the FePt layer is Fe50 Pt50 . The substrate temperature was raised up to 550 ◦ C for 30 min after the deposition. Then, the as-prepared FePt films were heated up to 700 ◦ C (TA1 ) for 30 min to enhance the formation of L10 FePt phase. Then these L10 FePt films were annealed at different temperature (TA2 = 400–600 ◦ C) below 700 ◦ C for 60 min either in a magnetic field of ∼3500 Oe along the normal direction of the films (Route A in Fig. 1) or without magnetic field (Route B in Fig. 1). The morphologies of the films were obtained via atomic force microscopy (AFM). The crystal structure of the films was characterized by X-ray diffraction (XRD) with Cu target. The magnetic properties were measured at room temperature by using a Quantum Design MPMS XL SQUID with applied field up to 7 T.
L. Liu et al. / Applied Surface Science 258 (2012) 5770–5773
Fig. 1. Schematic diagram for two-step annealing process.
Fig. 2. Dependence of intensity ratio I(0 0 1) /I(1 1 1) on TA2 for FePt (20 nm) films annealed through Route A and Route B; and insets: XRD patterns of FePt (20 nm) films: (a) as prepared, (b) post-annealed at 700 ◦ C for 30 min, (c) annealed through Route B, and (d) annealed through Route A.
3. Results Fig. 2(insets) shows the XRD patterns of the FePt films. As for the as-prepared FePt films, only (1 1 1) peak was detected. After annealed at 700 ◦ C (TA1 ) for 30 min, (0 0 1) and (002) peaks of L10
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phase were observed. When the films were further annealed at 478 ◦ C (TA2 ) for 60 min, big difference can be seen in between those L10 FePt films annealed by Route A and Route B. Comparing with Route B (the integrated intensity of peak (0 0 1), I0 0 1 is comparable with that of peak (1 1 1), I1 1 1 ), FePt films annealed by Route A showed much higher value of intensity ratio I0 0 1 /I1 1 1 . This indicated that the magnetic field at 478 ◦ C did induce the preferential growth direction of L10 structure. In order to investigate the influence of the magnetic field on the orientation degree of the L10 structures during the annealing process, the L10 FePt films were annealed through either Route A or Route B. Fig. 2 shows the dependence of the intensity ratio of I(0 0 1) /I(1 1 1) from XRD patterns on the different annealing temperature of TA2 for both of Route A and Route B. As for the Route B, there is no visible influence of TA2 can be seen, while I(0 0 1) /I(1 1 1) showed a peak value of 7 when TA2 was around 478 ◦ C (around the Tc of L10 FePt films [1]) through Route A. In order to demonstrate the role of the magnetic field when annealing around Curie temperature, the XRD pattern of FePt film annealed at 700 ◦ C for 30 min and then annealed at 478 ◦ C for 60 min in a magnetic field of 3500 Oe along the plane of the film was also obtained. The results shows that the (2 0 0) peak is observed and (1 1 1) peak intensity increases greatly when the magnetic field parallel to the film plane of L10 FePt films and these results indicate that annealing around Curie temperature in magnetic field can induce orientation of the c-axis of FePt toward particular crystal orientation. Fig. 3 shows the typical hysteresis loops of FePt (20 nm) films. Comparing with the as prepared state, the in-plane and out-ofplane coercivities of FePt films were remarkably enhanced after heat treatment, which can be attributed to the formation of perfect L10 phase. It should be noted that the in-plane remanence and the squareness ratio of as prepared FePt films were both larger than the out-of-plane ones, while the situation reversed for FePt films annealed through either Route A or Route B. Moreover, as shown in Fig. 3(d), the out-of-plane squareness ratio increases drastically when the FePt films were annealed at 478 ◦ C (TA2 ) in a magnetic field applied along the normal direction of FePt films. This indicates that the (0 0 1) orientation of FePt films were much improved and perpendicular anisotropy were obtained, which is consistent with the XRD patterns shown in Fig. 2(insets).
Fig. 3. Typical hysteresis loops of FePt (20 nm) films: (a) as prepared, (b) post-annealed at 700 ◦ C for 30 min, (c) annealed through Route B, and (d) annealed through Route A.
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L. Liu et al. / Applied Surface Science 258 (2012) 5770–5773
Fig. 4. AFM images of the FePt films (20 nm): (a) as prepared, (b) post-annealed at 700 ◦ C for 30 min, (c) annealed through Route B, and (d) annealed through Route A.
Fig. 4 shows the AFM images of FePt films prepared at different conditions. Note that the as-prepared sample (a) shows a smooth surface with low surface roughness around 1.1 nm, and the roughness increases to 3.0 nm for the annealed sample at 700 ◦ C (b). By further annealing at 478 ◦ C without (c) or with magnetic field (d) for an hour, the roughness of the films increases to 4.4 nm and 4.7 nm, respectively. These results suggest that a re-growing process happens during re-annealing at 478 ◦ C, even though the films were already annealed at higher temperature. The image (d) shows that the grain size and its distribution are larger and homogeneous compared with the case of the image (c). One possibility of these changes is due to acceleration orientation of the c-axis of FePt toward normal direction of the film plane. Now, we discuss the reason why the external magnetic field of ∼3500 Oe is effective only when the films were annealed around Tc . When the temperature is above Tc , the energy of the magnetic field of ∼3500 Oe (J H ≈ 7 × 10−24 J, J = 2.2B is the magnetization of the Fe atom, where B is the Bohr magneton) is far less than the thermal disturbing term (through Route B, TA2 = 400–600 ◦ C, kB T ≈ 10−20 J); when the temperature is near Tc , the thermal disturbing term is close to the exchange energy (near Tc , Eex ≈ kB T ≈ 10−20 J) of the magnetization, therefore, the magnetic field of ∼3500 Oe is effective for promoting the preferential growth direction of L10 structure; but when temperature is below Tc , the exchange energy becomes dominate and a spontaneous magnetization comes into being. Because of the larger demagnetization factor along the normal direction of the film, the exerted magnetic field of ∼3500 Oe is almost no effective to the preferential growth direction of L10 structure.
4. Conclusion In conclusion, FePt (20 nm) films were annealed around the Tc of L10 FePt in a magnetic field of ∼3500 Oe along the normal direction of FePt films. The influence of magnetic field on (0 0 1) orientation of FePt films during annealing were investigated. It was shown that magnetic easy axis alignment and excellent (0 0 1) orientation could be achieved in FePt films by using magnetic field annealing around the Tc of L10 FePt. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 6080 3035). References [1] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.-U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [2] Y.-N. Hsu, S. Jeong, D.E. Laughlin, D.N. Lambeth, J. Appl. Phys. 89 (2001) 7068. [3] S.C. Chen, P.C. Kuo, C.Y. Chou, A.C. Sun, J. Appl. Phys. 97 (2005) 10N107. [4] T. Shima, K. Takanashi, Y.K. Takahashi, K. Hono, Appl. Phys. Lett. 81 (2002) 1050. [5] K. Barmak, J. Kim, L.H. Lewis, K.R. Coffey, M.F. Toney, A.J. Kellock, J.-U. Thiele, J. Appl. Phys. 98 (2005) 033904. [6] B. Laenens, F.M. Almeida, N. Planckaert, K. Temst, J. Meersschaut, A. Vantomme, C. Rentenberger, M. Rennhofer, B. Sepiol, J. Appl. Phys. 105 (2009) 073913. [7] Y.F. Ding, J.S. Chen, E. Liu, J. Cryst. Growth 276 (2005) 111. [8] E.B. Svedberg, J.J. Mallett, S. Sayan, A.J. Shapiro, W.F. Egelhoff Jr., T. Moffat, Appl. Phys. Lett. 85 (2004) 1353. [9] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, A. Cebollada, G.R. Harp, J. Appl. Phys. 79 (1996) 5967. [10] J.-U. Thiele, L. Folks, M.F. Toney, D.K. Weller, J. Appl. Phys. 84 (1998) 5686. [11] Y.F. Ding, J.S. Chen, E. Liu, C.J. Sun, G.M. Chow, J. Appl. Phys. 97 (2005) 10H303. [12] T. Seki, T. Shima, K. Takanashi, Y. Takahashi, E. Matsubara, Y.K. Takahashi, K. Hono, J. Appl. Phys. 96 (2004) 1127.
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Contents lists available at SciVerse ScienceDirect
Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
Orientation control in L10 FePt films by using magnetic field annealing around Curie temperature Liwang Liu a , Hua Lv a , Wei Sheng a , Yuanfu Lou a , Jianmin Bai a , Jiangwei Cao a,∗ , Bin Ma b , Fulin Wei a a b
Key Laboratory for Magnetism and Magnetic Materials of the Ministry of Education, Lanzhou University, Lanzhou 730000, PR China State Key Laboratory for Advanced Photonic Materials and Devices, Department of Optical Science and Engineering, Fudan University, Shanghai 200433, PR China
a r t i c l e
i n f o
Article history: Received 17 August 2011 Received in revised form 16 February 2012 Accepted 18 February 2012 Available online 24 February 2012 Keywords: FePt Magnetic field annealing Exchange energy Curie temperature
a b s t r a c t A new method to improve (0 0 1) texture of L10 FePt films was reported. In this work, in order to obtain the perpendicular orientation of L10 FePt films, the FePt films were annealed at 700 ◦ C (TA1 ) for 30 min and then annealed at TA2 (430–600 ◦ C) for an hour either in a magnetic field of ∼3500 Oe along the normal direction of the films (Route A) or without magnetic field (Route B). Compared with the Route B, I(0 0 1) /I(1 1 1) showed a peak value of 7 when TA2 was around 478 ◦ C (around the Curie temperature of L10 FePt films) and the AFM image of FePt sample was annealed around 478 ◦ C showed an obvious change of grains through Route A. It is related to the following reason: when the temperature is near Curie temperature of L10 FePt films, the thermal disturbing term is close to the exchange energy of the magnetization, therefore, the magnetic field of ∼3500 Oe is effective for promoting the preferential growth direction of L10 structure. © 2012 Elsevier B.V. All rights reserved.
1. Introduction L10 FePt films with perpendicular orientation have been studied extensively as one of promising candidates for future ultrahigh density magnetic recording media [1]. As well known, as perpendicular magnetic recording media, c-axis of FePt films should be normal to the films and a strong (0 0 1) texture is desired. However, the FePt films deposited directly on amorphous substrates tend to possess (1 1 1) texture because (1 1 1) plane is the close-packed plane [2,3]. Therefore, one of the key challenges is how to align the magnetic easy axis (c-axis) to normal direction of FePt films. Usually, several approaches are being used to obtain/enhance the (0 0 1) orientation in FePt films: expitaxial growth on single crystal substrates [4–8]; hetero-expitaxial growth on textured underlayers/seedlayers [9–20]; alternate deposition of Fe and Pt layers [21–26]; FePt–X (X = B2 O3 , SiO2 , MgO, C, Ag, etc.) nanocomposite deposition [27–34]; magnetic field annealing the antiferromagnetic/FePt structure by using antiferromagnetic exchange energy [35]; aligning the magnetic particles in a magnetic field by using magnetostatic energy [36–39]. Previous studies have demonstrated that magnetic field annealing is helpful to preferred orientation of alloy or nanowires [40,41]. In this work, in order to obtain the perpendicular orientation of L10 FePt films, we proposed a method to control the c-axis of L10 FePt by annealing the films in a magnetic
∗ Corresponding author. E-mail address: [email protected] (J. Cao). 0169-4332/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2012.02.090
field around the Curie temperature (Tc , the Tc of L10 FePt films is 477 ◦ C). As for a ferromagnetic material, the magnetic moments spontaneously align to the same direction inside a single magnetic domain at a temperature below the Tc , while, their directions disperse randomly when the temperature is above the Tc . If L10 FePt alloys were annealed around its Tc in a magnetic field, the preferential direction of magnetic easy axis of L10 structure should be enhanced by the external magnetic field during the rearrangement process of Fe and Pt atoms. 2. Experiment FePt films with structure of Si (substrate)/Fe (2 nm)/FePt (20 nm)/Pt (2 nm) were deposited by magnetron sputtering system. The base pressure in chamber is 2 × 10−6 Pa and the nominal composition of the FePt layer is Fe50 Pt50 . The substrate temperature was raised up to 550 ◦ C for 30 min after the deposition. Then, the as-prepared FePt films were heated up to 700 ◦ C (TA1 ) for 30 min to enhance the formation of L10 FePt phase. Then these L10 FePt films were annealed at different temperature (TA2 = 400–600 ◦ C) below 700 ◦ C for 60 min either in a magnetic field of ∼3500 Oe along the normal direction of the films (Route A in Fig. 1) or without magnetic field (Route B in Fig. 1). The morphologies of the films were obtained via atomic force microscopy (AFM). The crystal structure of the films was characterized by X-ray diffraction (XRD) with Cu target. The magnetic properties were measured at room temperature by using a Quantum Design MPMS XL SQUID with applied field up to 7 T.
L. Liu et al. / Applied Surface Science 258 (2012) 5770–5773
Fig. 1. Schematic diagram for two-step annealing process.
Fig. 2. Dependence of intensity ratio I(0 0 1) /I(1 1 1) on TA2 for FePt (20 nm) films annealed through Route A and Route B; and insets: XRD patterns of FePt (20 nm) films: (a) as prepared, (b) post-annealed at 700 ◦ C for 30 min, (c) annealed through Route B, and (d) annealed through Route A.
3. Results Fig. 2(insets) shows the XRD patterns of the FePt films. As for the as-prepared FePt films, only (1 1 1) peak was detected. After annealed at 700 ◦ C (TA1 ) for 30 min, (0 0 1) and (002) peaks of L10
5771
phase were observed. When the films were further annealed at 478 ◦ C (TA2 ) for 60 min, big difference can be seen in between those L10 FePt films annealed by Route A and Route B. Comparing with Route B (the integrated intensity of peak (0 0 1), I0 0 1 is comparable with that of peak (1 1 1), I1 1 1 ), FePt films annealed by Route A showed much higher value of intensity ratio I0 0 1 /I1 1 1 . This indicated that the magnetic field at 478 ◦ C did induce the preferential growth direction of L10 structure. In order to investigate the influence of the magnetic field on the orientation degree of the L10 structures during the annealing process, the L10 FePt films were annealed through either Route A or Route B. Fig. 2 shows the dependence of the intensity ratio of I(0 0 1) /I(1 1 1) from XRD patterns on the different annealing temperature of TA2 for both of Route A and Route B. As for the Route B, there is no visible influence of TA2 can be seen, while I(0 0 1) /I(1 1 1) showed a peak value of 7 when TA2 was around 478 ◦ C (around the Tc of L10 FePt films [1]) through Route A. In order to demonstrate the role of the magnetic field when annealing around Curie temperature, the XRD pattern of FePt film annealed at 700 ◦ C for 30 min and then annealed at 478 ◦ C for 60 min in a magnetic field of 3500 Oe along the plane of the film was also obtained. The results shows that the (2 0 0) peak is observed and (1 1 1) peak intensity increases greatly when the magnetic field parallel to the film plane of L10 FePt films and these results indicate that annealing around Curie temperature in magnetic field can induce orientation of the c-axis of FePt toward particular crystal orientation. Fig. 3 shows the typical hysteresis loops of FePt (20 nm) films. Comparing with the as prepared state, the in-plane and out-ofplane coercivities of FePt films were remarkably enhanced after heat treatment, which can be attributed to the formation of perfect L10 phase. It should be noted that the in-plane remanence and the squareness ratio of as prepared FePt films were both larger than the out-of-plane ones, while the situation reversed for FePt films annealed through either Route A or Route B. Moreover, as shown in Fig. 3(d), the out-of-plane squareness ratio increases drastically when the FePt films were annealed at 478 ◦ C (TA2 ) in a magnetic field applied along the normal direction of FePt films. This indicates that the (0 0 1) orientation of FePt films were much improved and perpendicular anisotropy were obtained, which is consistent with the XRD patterns shown in Fig. 2(insets).
Fig. 3. Typical hysteresis loops of FePt (20 nm) films: (a) as prepared, (b) post-annealed at 700 ◦ C for 30 min, (c) annealed through Route B, and (d) annealed through Route A.
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Fig. 4. AFM images of the FePt films (20 nm): (a) as prepared, (b) post-annealed at 700 ◦ C for 30 min, (c) annealed through Route B, and (d) annealed through Route A.
Fig. 4 shows the AFM images of FePt films prepared at different conditions. Note that the as-prepared sample (a) shows a smooth surface with low surface roughness around 1.1 nm, and the roughness increases to 3.0 nm for the annealed sample at 700 ◦ C (b). By further annealing at 478 ◦ C without (c) or with magnetic field (d) for an hour, the roughness of the films increases to 4.4 nm and 4.7 nm, respectively. These results suggest that a re-growing process happens during re-annealing at 478 ◦ C, even though the films were already annealed at higher temperature. The image (d) shows that the grain size and its distribution are larger and homogeneous compared with the case of the image (c). One possibility of these changes is due to acceleration orientation of the c-axis of FePt toward normal direction of the film plane. Now, we discuss the reason why the external magnetic field of ∼3500 Oe is effective only when the films were annealed around Tc . When the temperature is above Tc , the energy of the magnetic field of ∼3500 Oe (J H ≈ 7 × 10−24 J, J = 2.2B is the magnetization of the Fe atom, where B is the Bohr magneton) is far less than the thermal disturbing term (through Route B, TA2 = 400–600 ◦ C, kB T ≈ 10−20 J); when the temperature is near Tc , the thermal disturbing term is close to the exchange energy (near Tc , Eex ≈ kB T ≈ 10−20 J) of the magnetization, therefore, the magnetic field of ∼3500 Oe is effective for promoting the preferential growth direction of L10 structure; but when temperature is below Tc , the exchange energy becomes dominate and a spontaneous magnetization comes into being. Because of the larger demagnetization factor along the normal direction of the film, the exerted magnetic field of ∼3500 Oe is almost no effective to the preferential growth direction of L10 structure.
4. Conclusion In conclusion, FePt (20 nm) films were annealed around the Tc of L10 FePt in a magnetic field of ∼3500 Oe along the normal direction of FePt films. The influence of magnetic field on (0 0 1) orientation of FePt films during annealing were investigated. It was shown that magnetic easy axis alignment and excellent (0 0 1) orientation could be achieved in FePt films by using magnetic field annealing around the Tc of L10 FePt. Acknowledgement This work was supported by the National Natural Science Foundation of China (Grant No. 6080 3035). References [1] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J.-U. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10. [2] Y.-N. Hsu, S. Jeong, D.E. Laughlin, D.N. Lambeth, J. Appl. Phys. 89 (2001) 7068. [3] S.C. Chen, P.C. Kuo, C.Y. Chou, A.C. Sun, J. Appl. Phys. 97 (2005) 10N107. [4] T. Shima, K. Takanashi, Y.K. Takahashi, K. Hono, Appl. Phys. Lett. 81 (2002) 1050. [5] K. Barmak, J. Kim, L.H. Lewis, K.R. Coffey, M.F. Toney, A.J. Kellock, J.-U. Thiele, J. Appl. Phys. 98 (2005) 033904. [6] B. Laenens, F.M. Almeida, N. Planckaert, K. Temst, J. Meersschaut, A. Vantomme, C. Rentenberger, M. Rennhofer, B. Sepiol, J. Appl. Phys. 105 (2009) 073913. [7] Y.F. Ding, J.S. Chen, E. Liu, J. Cryst. Growth 276 (2005) 111. [8] E.B. Svedberg, J.J. Mallett, S. Sayan, A.J. Shapiro, W.F. Egelhoff Jr., T. Moffat, Appl. Phys. Lett. 85 (2004) 1353. [9] R.F.C. Farrow, D. Weller, R.F. Marks, M.F. Toney, A. Cebollada, G.R. Harp, J. Appl. Phys. 79 (1996) 5967. [10] J.-U. Thiele, L. Folks, M.F. Toney, D.K. Weller, J. Appl. Phys. 84 (1998) 5686. [11] Y.F. Ding, J.S. Chen, E. Liu, C.J. Sun, G.M. Chow, J. Appl. Phys. 97 (2005) 10H303. [12] T. Seki, T. Shima, K. Takanashi, Y. Takahashi, E. Matsubara, Y.K. Takahashi, K. Hono, J. Appl. Phys. 96 (2004) 1127.
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