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Ag composite films for perpendicular recording
Acta Metall. Sin.(Engl. Lett.)Vol.22 No.5 pp392-396 October 2009
(001)-oriented FePt/Ag composite films for perpendicular recording Wenfeng LIU
1)∗
, Fang WANG
2)
and Xiaohong XU
2)
1) College of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China 2) School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, China Manuscript received 3 November 2008; in revised form 4 May 2009
FePt/Ag thin films were deposited by magnetron sputtering onto 7059 glass substrates, then were annealed at 550 ◦ C for 30 min. Nanostructured FePt/Ag films were successfully obtained with the magnetic easy axis of L10 FePt perpendicular to the film plane. It was found that the development of (001) texture depended strongly on the thicknesses of FePt magnetic layer and Ag underlayer. The L10 ordered FePt(15 nm)/Ag(50 nm) with (001) orientation can be obtained. And the perpendicular coercivity of FePt(15 nm)/Ag(50 nm) film reached to 7.2×105 A/m, whereas the longitudinal one was only 3.2×104 A/m. The non-magnetic Ag underlayer can not only induce (001) orientation and ordering of FePt grains, but also reduce the intergrain interactions. KEY WORDS FePt/Ag thin films; (001) orientation; FePt magnetic layer; Ag underlayer; Intergrain interaction
1 Introduction In order to increase the magnetic recording density to 1Tb/in2 , perpendicular magnetic recording is believed to be one of the potential technologies to replace the current longitudinal magnetic recording in the near future[1] , because the volume of the recording bit in the perpendicular magnetic recording medium can be maintained well by increasing the medium thickness when the surface area of the recording bit is reduced[2] . L10 FePt alloy thin films have attracted significant attention as ultrahigh density magnetic recording media due to their high perpendicular magnetic anisotropy and high coercivity[3−5] . Generally, FePt films deposited at room temperature are face-centered cubic (FCC) and show soft-magnetic behaviour[6] . The FCC FePt phase has to be annealed at a temperature higher than 550 ◦ C in order to obtain a hard-magnetic face-centered tetragonal (FCT or L10 ordered phase[7] . However, it is essential to reduce the growth temperature of FePt for its practical use. ∗
Corresponding author. Assistant professor, Master; Tel.: +86 351 2812318 or 13754807782. E-mail address: [email protected] (Wenfeng LIU) DOI: 10.1016/S1006-7191(08)60113-1
· 393 · Introducing the non-magnetic Ag can reduce the ordering temperature of FePt films[8] . FePt films with the Ag additive deposited by magnetron sputtering usually tend to grow with random orientation or with (111) texture that places the c-axes of grains 37◦ out of the film plane. To obtain (001) texture, many methods have been attempted, such as annealing [FePt/Ag]n multilayers deposited onto MgO (100) single crystal substrates with laser ablation[9] or onto Ag/MgO layers by e-beam evaporation[10] , sputtering FePt/Ag onto Si (100) substrates[11] . However, it is more practical to adopt glass substrates. In this research, we have successfully deposited highly (001)-oriented FePt/Ag thin films on the glass substrates using magnetron sputtering, and further investigated the microstructure and magnetic properties of FePt/Ag thin films and dependence of FePt layer thickness on the Ag underlayer thickness. 2 Experimental FePt/Ag thin films were deposited on the 7059 glass substrates by RF and DC magnetron sputtering. Fe-Pt composite target and Ag target were used. The base pressure of the deposition chamber was 5×10−5 Pa and high purity Ar was introduced during sputtering. FePt magnetic layers were deposited at a RF sputtering power of 20 W and Ar pressure of 0.8 Pa. Ag underlayers were deposited at a DC sputtering power of 10 W and Ar pressure of 1.3 Pa. During the deposition, there are not any controls of the substrate temperature. After deposition, the samples were annealed at 550 ◦ C for 30 min with the pressure of 2×10−4 Pa. The structure of the films was analysed by a Bruker D8 X-ray diffraction (XRD) with CuKα radiation. The thicknesses of Ag underlayer and FePt layer were determined by a surface profile. Magnetic properties were measured using a Lakeshore 7407 vibrating sample magnetometer (VSM) with the magnetic field applied longitudinal and perpendicular to the film plane. 3 Results and Discussion The XRD patterns of FePt(15 nm)/Ag films annealed at 550 ◦ C for 30 min are shown in Fig.1. The superlattice (001), (110), (002) and (003) peaks of FePt appearing in the XRD patterns indicate that the FCC phase has been partially transformed into FCT phase. Meanwhile, Ag (111) and (200) peaks are observed, which indicates that Ag underlayer has a (200) texture mixed with (111) texture. It is interesting to find that the orientation of FePt films strongly depends on Ag underlayer thickness. There are (111), (110) and (001) peaks of FePt in XRD specFig.1 XRD patterns of FePt(15 nm)/Ag films annealed at 550 ◦ C for 30 min. trum as Ag underlayer thickness is less than 50 nm, which means that the films consist of L10 FePt grains with random orientation. When the thickness of Ag underlayer is 50 nm, the intensity of FePt (001) peak rapidly increases and FePt (002) peak as well as FePt (003)
· 394 · peak emerges, while FePt (111) peak presents a relatively small increase. This indicates that ordering degree of FePt is promoted and (001) texture is preferred. If we further increase Ag underlayer thickness to 100 nm, the (001) orientation is deteriorated. The ratio of the intensity of FePt (001) peak to that of FePt (111) peak (I001 /I111 ) can be used as a figure-of-merit of the degree of (001) texturing and it is plotted as a function of Ag underlayer thickness in Fig.2. It notes that the intensity ratio of I001 /I111 is up to the maximum at the Ag underlayer of 50 nm, it indicates that c-axis prefers to orient perpendicularly to the film plane. As mentioned above, it is demonstrated that Ag underlayer plays a very important role in the improvement of (001) orientation and ordering degree of FePt films. This may be Fig.2 Intensity ratio of I001 /I111 as a function of Ag underlayer thickness of due to that Ag has a 5.5% larger unit cell FePt(15 nm)/Ag thin films annealed at than FePt, by which the strain can expand 550 ◦ C. the in-plane a-axis of FePt and shrink the c-axis normal to the film plane. Therefore this effect can improve the ordering degree and (001) oriented growth. And the average grain size d was estimated by the Scherrer formula from the (001) peak width of XRD patterns. The average grain size is only 10.2 nm in FePt(15 nm)/Ag(50 nm) film, which can satisfy the requirement of highdensity magnetic recording media well. From above discussion, we know that the thickness of Ag underlayer can affect the FePt oriented growth. Fig.3 shows the reFig.3 Relationship between coercivities and FePt layer thickness of FePt/ lationship between coercivities (longitudinal Ag(50 nm) films annealed at 550 ◦ C. and perpendicular) and FePt magnetic layer The inset shows the variations of the thickness of FePt/Ag(50 nm) films. On the coercivity ratio of HC⊥ /HC// . one hand, the longitudinal coercivity nearly keeps a constant with increasing FePt layer thickness from 5 nm to 15 nm, and drastically increases when FePt layer thickness reaches 20 nm, then slowly decreases as the FePt layer thickness reaches 30 nm. On the other hand, the perpendicular coercivity rapidly increases with increasing FePt layer thickness from 5 nm to 20 nm, and slowly decreases as the FePt layer thickness reaches 30 nm. It can also be seen from Fig.3 that the perpendicular coercivities are always higher than the longitudinal ones. Meanwhile the variations of the coercivity ratio of HC⊥ /HC// with FePt layer thickness are shown in the inset of Fig.3. It is noticed that the maximum value of HC⊥ /HC// is obtained when FePt layer thickness is 15 nm. The above result can be explained by that the strain energy arises from the misfit between Ag layer and FePt layer, which is known to drive the preferential growth of FePt
· 395 · grains at the specific crystallographic orientation as the thickness of FePt is suitable. It is hypothesized that the strain is minimized at the FePt magnetic layer of 15 nm, which should be investigated in detail in the further study. The hysteresis loops of FePt(15, 30 nm)/Ag(50 nm) films annealed at 550 ◦ C are shown in Fig.4a and Fig.4b, respectively. As seen in Fig.4a, the perpendicular coercivity of FePt(15 nm)/Ag(50 nm) film reaches to 7.2×105 A/m, whereas the longitudinal one is only 3.2×104 A/m. For FePt(30 nm)/Ag(50 nm) film, the longitudinal and perpendicular coercivities are similar, about 5.81×105 A/m. It is clear that the suitable thicknesses of Ag underlayer and FePt layer are chosen, which can induce the Fig.4 In-plane and perpendicular hysteresis (001) oriented growth and further achieve loops of films annealed at 550 ◦ C: the high perpendicular magnetic anisotropy. (a) FePt(15 nm)/Ag(50 nm); (b) FePt(30 nm)/Ag(50 nm). In addition, the kink of the hysteresis loops reflects the co-existence of the two phases, disordered fcc phase and ordered fct phase[12] , which is in agreement with the results shown in Fig.1. Except for possessing the high magnetic anisotropy, the high-density magnetic recording media also require magnetic grains to be isolated to reduce intergrain interactions, which consequently reduces media noise. Henkel plot[13] has been proposed to estimate the interactions between magnetic FePt grains. Fig.5a and 5b show the Henkel plots of FePt(15 nm) and FePt(15 nm)/Ag(50 nm) films, respectively. In the Henkel plot, the isothermal remanence curve (M r(H)) was taken from an ac-demagnetized initial state, and the dc-demagnetization (M d(H)) was taken after dc-saturation. The Wohlfarth relation M d(H) = 1 − 2M r(H) is denoted by the straight line and valid for single-domain particles with no intergrain interactions[14] . Deviations from the Wohlfarth relation manifest the existence of the interaction effects. The deviations are indicated by nonlinear curves representing the magnetizing-like or demagnetizing-like effects[15] . As is evident, there is a magnetizing-like deviation from the Wohlfarth relation in FePt(15 nm) film, which means
Fig.5 Henkel plots of films annealed at 550 ◦ C: (a) FePt(15 nm); (b) FePt(15 nm)/Ag(50 nm).
· 396 · that there exist intergrain interactions to a certain degree. In contrast, the linear curve is close to the Wohlfarth relation for the FePt(15 nm)/Ag(50 nm) film. This suggests that there are almost no intergrain interactions in FePt(15 nm)/Ag(50 nm) film. Its reason may be that the Ag atoms diffuse into the grain boundaries of FePt films and separate the FePt grains[16] , which results in the reduction of the intergrain interactions of FePt grains. 4 Conclusions In summary, L10 FePt (001) orientation is strongly dependent on the thicknesses of Ag underlayer and FePt magnetic layer. FePt(15 nm)/Ag(50 nm) film with perfect (001) texture as well as high perpendicular anisotropy was successfully deposited onto the glass substrate by magnetron sputtering. The non-magnetic Ag underlayer can not only induce the (001) orientation and the ordering of FePt grains, but also reduce the intergrain interactions. Altogether the results suggest that the system of FePt/Ag thin films is a promising media candidate for extremely high-density magnetic recording. REFERENCES [1] A.C. Sun, J.H. Hsu, P.C. Kuo, H.L. Huang, H.C. Lu and S.F. Wang, J Magn Magn Mater 310 (2007) 2650. [2] S. Suzuki, IEEE Trans Magn 20 (1984) 675. [3] C.P. Luo and D.J. Sellmyer, IEEE Trans Magn 31 (1995) 2764. [4] K.R. Coffey, M.A. Parker and J.K. Howard, IEEE Trans Magn 31 (1995) 2737. [5] N. Li and B.M. Lairson, IEEE Trans Magn 35 (1999) 1077. [6] S.H. Whang, Q. Feng and Y.Q. Gao, Acta Mater 46 (1998) 6485. [7] M. Daniil, P.A. Farber, H. Okumura, G.C. Hadjipanayis and D. Weller,J Magn Magn Mater 246 (2002) 297. [8] X.H. Xu, H.S. Wu, F. Wang and X.L. Li, Thin Solid Films 472 (2005) 222. [9] K. Kang, T. Yang and T. Suzuki, IEEE Trans Magn 38 (2002) 2039. [10] K. Kang, Z.G. Zhang, C. Papusoi and T. Suzuki, Appl Phys Lett 82 (2003) 3284. [11] Y.N. Hsu, S. Jeong, D.N. Lambeth and D.E. Laughlin, IEEE Trans Magn 36 (2000) 2945. [12] Y.F. Ding, J.S. Chen and E. Liu, Thin Solid Films 474 (2005) 141. [13] J.J. Delaunay, T. Hayashi, M. Tomita and S. Hirono, IEEE Trans Magn 34 (1998) 1627. [14] E.P. Wolfforth, J Appl Phys 29 (1958) 595. [15] V. Basso and G. Bertotti, IEEE Trans Magn 30 (1994) 64. [16] Z.L. Zhao, J.S. Chen, J. Ding, K. Inaba and J.P. Wang, J Magn Magn Mater 282 (2004) 105.
(001)-oriented FePt/Ag composite films for perpendicular recording Wenfeng LIU
1)∗
, Fang WANG
2)
and Xiaohong XU
2)
1) College of Materials Science and Engineering, Taiyuan University of Science and Technology, Taiyuan 030024, China 2) School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, China Manuscript received 3 November 2008; in revised form 4 May 2009
FePt/Ag thin films were deposited by magnetron sputtering onto 7059 glass substrates, then were annealed at 550 ◦ C for 30 min. Nanostructured FePt/Ag films were successfully obtained with the magnetic easy axis of L10 FePt perpendicular to the film plane. It was found that the development of (001) texture depended strongly on the thicknesses of FePt magnetic layer and Ag underlayer. The L10 ordered FePt(15 nm)/Ag(50 nm) with (001) orientation can be obtained. And the perpendicular coercivity of FePt(15 nm)/Ag(50 nm) film reached to 7.2×105 A/m, whereas the longitudinal one was only 3.2×104 A/m. The non-magnetic Ag underlayer can not only induce (001) orientation and ordering of FePt grains, but also reduce the intergrain interactions. KEY WORDS FePt/Ag thin films; (001) orientation; FePt magnetic layer; Ag underlayer; Intergrain interaction
1 Introduction In order to increase the magnetic recording density to 1Tb/in2 , perpendicular magnetic recording is believed to be one of the potential technologies to replace the current longitudinal magnetic recording in the near future[1] , because the volume of the recording bit in the perpendicular magnetic recording medium can be maintained well by increasing the medium thickness when the surface area of the recording bit is reduced[2] . L10 FePt alloy thin films have attracted significant attention as ultrahigh density magnetic recording media due to their high perpendicular magnetic anisotropy and high coercivity[3−5] . Generally, FePt films deposited at room temperature are face-centered cubic (FCC) and show soft-magnetic behaviour[6] . The FCC FePt phase has to be annealed at a temperature higher than 550 ◦ C in order to obtain a hard-magnetic face-centered tetragonal (FCT or L10 ordered phase[7] . However, it is essential to reduce the growth temperature of FePt for its practical use. ∗
Corresponding author. Assistant professor, Master; Tel.: +86 351 2812318 or 13754807782. E-mail address: [email protected] (Wenfeng LIU) DOI: 10.1016/S1006-7191(08)60113-1
· 393 · Introducing the non-magnetic Ag can reduce the ordering temperature of FePt films[8] . FePt films with the Ag additive deposited by magnetron sputtering usually tend to grow with random orientation or with (111) texture that places the c-axes of grains 37◦ out of the film plane. To obtain (001) texture, many methods have been attempted, such as annealing [FePt/Ag]n multilayers deposited onto MgO (100) single crystal substrates with laser ablation[9] or onto Ag/MgO layers by e-beam evaporation[10] , sputtering FePt/Ag onto Si (100) substrates[11] . However, it is more practical to adopt glass substrates. In this research, we have successfully deposited highly (001)-oriented FePt/Ag thin films on the glass substrates using magnetron sputtering, and further investigated the microstructure and magnetic properties of FePt/Ag thin films and dependence of FePt layer thickness on the Ag underlayer thickness. 2 Experimental FePt/Ag thin films were deposited on the 7059 glass substrates by RF and DC magnetron sputtering. Fe-Pt composite target and Ag target were used. The base pressure of the deposition chamber was 5×10−5 Pa and high purity Ar was introduced during sputtering. FePt magnetic layers were deposited at a RF sputtering power of 20 W and Ar pressure of 0.8 Pa. Ag underlayers were deposited at a DC sputtering power of 10 W and Ar pressure of 1.3 Pa. During the deposition, there are not any controls of the substrate temperature. After deposition, the samples were annealed at 550 ◦ C for 30 min with the pressure of 2×10−4 Pa. The structure of the films was analysed by a Bruker D8 X-ray diffraction (XRD) with CuKα radiation. The thicknesses of Ag underlayer and FePt layer were determined by a surface profile. Magnetic properties were measured using a Lakeshore 7407 vibrating sample magnetometer (VSM) with the magnetic field applied longitudinal and perpendicular to the film plane. 3 Results and Discussion The XRD patterns of FePt(15 nm)/Ag films annealed at 550 ◦ C for 30 min are shown in Fig.1. The superlattice (001), (110), (002) and (003) peaks of FePt appearing in the XRD patterns indicate that the FCC phase has been partially transformed into FCT phase. Meanwhile, Ag (111) and (200) peaks are observed, which indicates that Ag underlayer has a (200) texture mixed with (111) texture. It is interesting to find that the orientation of FePt films strongly depends on Ag underlayer thickness. There are (111), (110) and (001) peaks of FePt in XRD specFig.1 XRD patterns of FePt(15 nm)/Ag films annealed at 550 ◦ C for 30 min. trum as Ag underlayer thickness is less than 50 nm, which means that the films consist of L10 FePt grains with random orientation. When the thickness of Ag underlayer is 50 nm, the intensity of FePt (001) peak rapidly increases and FePt (002) peak as well as FePt (003)
· 394 · peak emerges, while FePt (111) peak presents a relatively small increase. This indicates that ordering degree of FePt is promoted and (001) texture is preferred. If we further increase Ag underlayer thickness to 100 nm, the (001) orientation is deteriorated. The ratio of the intensity of FePt (001) peak to that of FePt (111) peak (I001 /I111 ) can be used as a figure-of-merit of the degree of (001) texturing and it is plotted as a function of Ag underlayer thickness in Fig.2. It notes that the intensity ratio of I001 /I111 is up to the maximum at the Ag underlayer of 50 nm, it indicates that c-axis prefers to orient perpendicularly to the film plane. As mentioned above, it is demonstrated that Ag underlayer plays a very important role in the improvement of (001) orientation and ordering degree of FePt films. This may be Fig.2 Intensity ratio of I001 /I111 as a function of Ag underlayer thickness of due to that Ag has a 5.5% larger unit cell FePt(15 nm)/Ag thin films annealed at than FePt, by which the strain can expand 550 ◦ C. the in-plane a-axis of FePt and shrink the c-axis normal to the film plane. Therefore this effect can improve the ordering degree and (001) oriented growth. And the average grain size d was estimated by the Scherrer formula from the (001) peak width of XRD patterns. The average grain size is only 10.2 nm in FePt(15 nm)/Ag(50 nm) film, which can satisfy the requirement of highdensity magnetic recording media well. From above discussion, we know that the thickness of Ag underlayer can affect the FePt oriented growth. Fig.3 shows the reFig.3 Relationship between coercivities and FePt layer thickness of FePt/ lationship between coercivities (longitudinal Ag(50 nm) films annealed at 550 ◦ C. and perpendicular) and FePt magnetic layer The inset shows the variations of the thickness of FePt/Ag(50 nm) films. On the coercivity ratio of HC⊥ /HC// . one hand, the longitudinal coercivity nearly keeps a constant with increasing FePt layer thickness from 5 nm to 15 nm, and drastically increases when FePt layer thickness reaches 20 nm, then slowly decreases as the FePt layer thickness reaches 30 nm. On the other hand, the perpendicular coercivity rapidly increases with increasing FePt layer thickness from 5 nm to 20 nm, and slowly decreases as the FePt layer thickness reaches 30 nm. It can also be seen from Fig.3 that the perpendicular coercivities are always higher than the longitudinal ones. Meanwhile the variations of the coercivity ratio of HC⊥ /HC// with FePt layer thickness are shown in the inset of Fig.3. It is noticed that the maximum value of HC⊥ /HC// is obtained when FePt layer thickness is 15 nm. The above result can be explained by that the strain energy arises from the misfit between Ag layer and FePt layer, which is known to drive the preferential growth of FePt
· 395 · grains at the specific crystallographic orientation as the thickness of FePt is suitable. It is hypothesized that the strain is minimized at the FePt magnetic layer of 15 nm, which should be investigated in detail in the further study. The hysteresis loops of FePt(15, 30 nm)/Ag(50 nm) films annealed at 550 ◦ C are shown in Fig.4a and Fig.4b, respectively. As seen in Fig.4a, the perpendicular coercivity of FePt(15 nm)/Ag(50 nm) film reaches to 7.2×105 A/m, whereas the longitudinal one is only 3.2×104 A/m. For FePt(30 nm)/Ag(50 nm) film, the longitudinal and perpendicular coercivities are similar, about 5.81×105 A/m. It is clear that the suitable thicknesses of Ag underlayer and FePt layer are chosen, which can induce the Fig.4 In-plane and perpendicular hysteresis (001) oriented growth and further achieve loops of films annealed at 550 ◦ C: the high perpendicular magnetic anisotropy. (a) FePt(15 nm)/Ag(50 nm); (b) FePt(30 nm)/Ag(50 nm). In addition, the kink of the hysteresis loops reflects the co-existence of the two phases, disordered fcc phase and ordered fct phase[12] , which is in agreement with the results shown in Fig.1. Except for possessing the high magnetic anisotropy, the high-density magnetic recording media also require magnetic grains to be isolated to reduce intergrain interactions, which consequently reduces media noise. Henkel plot[13] has been proposed to estimate the interactions between magnetic FePt grains. Fig.5a and 5b show the Henkel plots of FePt(15 nm) and FePt(15 nm)/Ag(50 nm) films, respectively. In the Henkel plot, the isothermal remanence curve (M r(H)) was taken from an ac-demagnetized initial state, and the dc-demagnetization (M d(H)) was taken after dc-saturation. The Wohlfarth relation M d(H) = 1 − 2M r(H) is denoted by the straight line and valid for single-domain particles with no intergrain interactions[14] . Deviations from the Wohlfarth relation manifest the existence of the interaction effects. The deviations are indicated by nonlinear curves representing the magnetizing-like or demagnetizing-like effects[15] . As is evident, there is a magnetizing-like deviation from the Wohlfarth relation in FePt(15 nm) film, which means
Fig.5 Henkel plots of films annealed at 550 ◦ C: (a) FePt(15 nm); (b) FePt(15 nm)/Ag(50 nm).
· 396 · that there exist intergrain interactions to a certain degree. In contrast, the linear curve is close to the Wohlfarth relation for the FePt(15 nm)/Ag(50 nm) film. This suggests that there are almost no intergrain interactions in FePt(15 nm)/Ag(50 nm) film. Its reason may be that the Ag atoms diffuse into the grain boundaries of FePt films and separate the FePt grains[16] , which results in the reduction of the intergrain interactions of FePt grains. 4 Conclusions In summary, L10 FePt (001) orientation is strongly dependent on the thicknesses of Ag underlayer and FePt magnetic layer. FePt(15 nm)/Ag(50 nm) film with perfect (001) texture as well as high perpendicular anisotropy was successfully deposited onto the glass substrate by magnetron sputtering. The non-magnetic Ag underlayer can not only induce the (001) orientation and the ordering of FePt grains, but also reduce the intergrain interactions. Altogether the results suggest that the system of FePt/Ag thin films is a promising media candidate for extremely high-density magnetic recording. REFERENCES [1] A.C. Sun, J.H. Hsu, P.C. Kuo, H.L. Huang, H.C. Lu and S.F. Wang, J Magn Magn Mater 310 (2007) 2650. [2] S. Suzuki, IEEE Trans Magn 20 (1984) 675. [3] C.P. Luo and D.J. Sellmyer, IEEE Trans Magn 31 (1995) 2764. [4] K.R. Coffey, M.A. Parker and J.K. Howard, IEEE Trans Magn 31 (1995) 2737. [5] N. Li and B.M. Lairson, IEEE Trans Magn 35 (1999) 1077. [6] S.H. Whang, Q. Feng and Y.Q. Gao, Acta Mater 46 (1998) 6485. [7] M. Daniil, P.A. Farber, H. Okumura, G.C. Hadjipanayis and D. Weller,J Magn Magn Mater 246 (2002) 297. [8] X.H. Xu, H.S. Wu, F. Wang and X.L. Li, Thin Solid Films 472 (2005) 222. [9] K. Kang, T. Yang and T. Suzuki, IEEE Trans Magn 38 (2002) 2039. [10] K. Kang, Z.G. Zhang, C. Papusoi and T. Suzuki, Appl Phys Lett 82 (2003) 3284. [11] Y.N. Hsu, S. Jeong, D.N. Lambeth and D.E. Laughlin, IEEE Trans Magn 36 (2000) 2945. [12] Y.F. Ding, J.S. Chen and E. Liu, Thin Solid Films 474 (2005) 141. [13] J.J. Delaunay, T. Hayashi, M. Tomita and S. Hirono, IEEE Trans Magn 34 (1998) 1627. [14] E.P. Wolfforth, J Appl Phys 29 (1958) 595. [15] V. Basso and G. Bertotti, IEEE Trans Magn 30 (1994) 64. [16] Z.L. Zhao, J.S. Chen, J. Ding, K. Inaba and J.P. Wang, J Magn Magn Mater 282 (2004) 105.