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IrMn thin film
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 303 (2006) e188–e191 www.elsevier.com/locate/jmmm
Angular and NiFe thickness dependence of exchange bias in IrMn/NiFe/IrMn thin film Yong-Goo Yooa,, Seong-Gi Minb, Ho-Jun Ryua, Nam-Seok Parkc, Seong-Cho Yub a
Electronics and Telecommunications Research Institute, Daejeon 305-350, South Korea Department of Physics, Chungbuk National University, Cheongju 360-763, South Korea c Department of Electrical and Electronic engineering, Chungju National University, Chungju 380-702, South Korea b
Available online 17 February 2006
Abstract Exchange biased IrMn/NiFe/IrMn thin films were studied as a function of NiFe thickness. In plane angular dependence of a resonance field distribution which is measured by FMR was analyzed as a combined effect of an unidirectional anisotropy and an uniaxial anisotropy. The unidirectional anisotropic field and the uniaxial anisotropic field were linearly varied with NiFe thickness while the films with a thicker NiFe layer do not follow the linear variation. Resonance field and linewidth variations were also analysed with NiFe thickness. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 68.55.a Keywords: FMR; Uniaxial anisotropy; Unidirectional anisotropy; Exchange bias
1. Introduction The exchange bias phenomena have been extensively studied due to their potential applications in spin valve structure and the physical interest to magnetic coupling mechanism in multilayer structure. Results indicate that the exchange coupling between ferromagnet (FM) and antiferromagnet (AFM) comes from their spin configuration at the interface [1]. The exchange coupling strength which is caused by interfacial spin configuration of FM/ AFM, depends on the thickness of FM and AFM and/or their domain structure [1,2]. Especially, the AFM domain structure plays an important role in determining the characteristic properties such as the unidirectional anisotropy and the coercivity enhancement [3,4]. Most theoretical models assumed that the FM layer has uniform magnetization along the thickness direction [2]. However, in the case of FM layer with enough thickness, it is also important to note that there is a uniform magnetization distribution throughout the thickness of the FM layer that Corresponding author. Tel.: +82 42 860 5222; fax: +82 42 860 5202.
E-mail address: [email protected] (Y.-G. Yoo). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.051
is coupled with the AFM layer. Ferromagnetic resonance (FMR) measurements represent a powerful tool for studying magnetic anisotropy [5]. The latter can be determined by the intrinsic property of the FM layer and/or an exchange coupling in the case of an exchange biased thin film [6]. In this study, we report on the variation in exchange bias as a function of FM layer thickness in IrMn/NiFe/IrMn trilayer structure. In particular, FM layers with enough thickness were examined. The top and bottom IrMn layer were located to give symmetric surface effect to NiFe layer for FMR measurement. The IrMn as was chosen AFM layer because it has the FCC structure preferred to NiFe layer and there is no need for the post annealing process in order to induce unidirectional anisotropy. The unidirectional exchange anisotropy and an uniaxial anisotropy were analysed by FMR measurements 2. Experimentals The exchange biased IrMn(10 nm)/NiFe(t nm)/IrMn(10 nm) thin films were fabricated by magnetron sputtering on Si(1 0 0) wafers with a seed layer of Ta(5 nm) at room
ARTICLE IN PRESS Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e188–e191
3. Results and discussions In plane angular dependence of the resonance field (Hr) for the exchange biased thin films was determined by FMR measurements. The angular variation in Hr shows a manifest unidirectional behavior for the exchange biased IrMn/NiFe/IrMn thin film, as shown in Fig. 1. As the NiFe thickness increases, the bell-shaped curve of Hr changes into the flat shaped curve near top of the bell for the NiFe thickness of 80 nm and then it changes into the two-peaked curve for the NiFe thickness of 160 nm which is comparable to that of the unbiased single NiFe thin film in the inset of Fig. 1. The resonance field distribution with angular measurement implies combined effect of the unidirectional anisotropy and the uniaxial anisotropy. The unidirectional anisotropic field (Hex) and the uniaxial anisotropic field (Hk) can be extracted by the phenomenological expression [7] of cosine series neglecting high-order
terms as follows: H r ðjH Þ ¼ H r0 H ex cos jH H k cos 2jH ,
(1)
where fH indicates the azimuthal angle between the applied field direction and the easy axis of the FM layer. Hr0 is an average resonance field indicating an angular independent term. The second term represents the exchange coupling between the FM and AFM layer and the third term describes the uniaxial anisotropy of the FM layer as expressed by Hk ¼ 2Ku/MF. The angular variation of Hr for IrMn/NiFe/IrMn trilayer thin films were well fitted by Eq. (1) as can be seen by the solid lines in Fig. 1. Fig. 2 shows the variation of Hex and Hk, which were extracted from Eq. (1), and Hex and Hc measured by VSM with NiFe thickness. The Hex values exhibit a linear decrease with the NiFe thickness increase which can be expressed by Hex1/tF [1]. However, the films with NiFe layer above 80 nm do not follow the linearity. This implies that the magnetization distribution in FM layer can affect the exchange coupling strength. The discrepancy in Hex values obtained from the two measurements could arise from hysteresis loop asymmetry for the sweep field direction [8]. The asymmetry comes from the irreversible 1000 VSM FMR
Hex (Oe)
temperature. Ta(5 nm) was deposited on top of all the samples, to prevent oxidation. The base pressure of the chamber was below 5 108 torr and the Ar working pressure was 2 mTorr. During deposition, a magnetic field of 200 Oe was applied to form the uniaxial anisotropy in the NiFe layer. The thickness of the NiFe layer varied from 2 to 160 nm. A single layer of NiFe(t nm) was also fabricated for comparison. The hysteresis loop and the coercivity were obtained by vibrating sample magnetometer (VSM). The FMR measurements were performed at 9.4 GHz (X band) with a JEOL JES-TE300 ESR Spectrometer. The angular dependence of the resonance field was measured by rotating the sample to an in-plane direction with respect to the applied magnetic field.
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160 nm 80 nm 40 nm 20 nm
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940
VSMH (biased) c FMRH (biased) k FMRH (unbiased) k VSMH (unbiased) c VSMH (unbiased) k
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120 180 240 300 360 φH (degree)
180
240
1 (b)
300
360
φH (degree) Fig. 1. Angular dependence of the Hr for exchange biased IrMn(10 nm)/ NiFe(t nm)/ IrMn(10 nm) thin films as a function of NiFe layer thickness. Inset figure shows films with thick NiFe layer for magnification. The single NiFe indicates unbiased single NiFe(5 nm) thin film. The solid lines indicate data fitted to Eq. (1).
1
10 NiFe thickness (nm)
100
Fig. 2. Variation in Hex (a) and Hk and Hc (b) for an exchange-biased IrMn(10 nm)/NiFe(t nm)/IrMn(10 nm) thin films with NiFe layer thickness measured by VSM and FMR, respectively. The unbiased indicates single NiFe(5 nm) thin film. The solid lines indicate linear fitting line for guideing the eye.
ARTICLE IN PRESS Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e188–e191
switching of magnetization to the sweep field direction. Variations of Hk and Hc for exchange biased IrMn/NiFe/ IrMn thin films are similar to the Hex variation. Fig. 2(b) shows the linear decrease with increasing NiFe thickness up to 40 nm. It is worth noting that the unbiased single NiFe films show a small increase of Hk and Hc with increasing NiFe thickness. This result suggests that uniaxial anisotropy of the exchange-biased NiFe layer can be induced by an exchange interaction with the AFM layer. Actually, the decrease in the NiFe thickness corresponding to the Hex increase gives rise to a large difference in the Hk values of the exchange-biased and the unbiased NiFe thin films. However, the biased NiFe films above 80 nm exhibit a comparable Hk value to that of unbiased NiFe films. This result indicates that the Hk of the films with thick NiFe layer could be determined predominantly by the NiFe intrinsic property due to the rather weak exchange bias effect. In plane resonance field of exchange-biased FM thin film exhibits negative field shift from unbiased single FM thin film. According to McMichael et al., the negative field shift of the resonance field in FMR measurement was due to a rotatable anisotropy which comes from irreversible AFM domain behavior [9]. The size distribution of the AFM domain near the interface with the FM layer produces a pining field affecting magnetization reversal of the FM layer. The temperature and AFM thickness dependence of Hr were explained by the rotatable anisotropy [6,9]. These results indicate that temperature and AFM thickness affect the AFM domain behavior corresponding to Hex variation. In our case, the NiFe thickness dependence of Hr0 is shown in Fig. 3. The Hr0 steeply increased at small NiFe thickness while a small increase was shown at large NiFe thickness. The increase of NiFe thickness could cause a change in the interfacial microstructure so that the Hr0 variation occurs by different interfacial condition, since the resonance field
1100 1000
Hr0 (Oe)
900
800
100
10 1
10 NiFe thickness (nm)
100
Fig. 4. The mean peak-to-peak linewidth variation of exchange biased IrMn(10 nm)/NiFe(t nm)/IrMn(10 nm) thin films as a function of NiFe thickness. The triangle symbol indicates the linewidth of unbiased single NiFe(5 nm) thin film.
shift arises from a variation of exchange anisotropy due to interfacial condition [10]. In addition to the resonance field distribution by FMR measurement, a line broadening gives useful information to study exchange coupling in FM/AFM thin films. The line broadening in FMR measurement comes from intrinsic properties such as anisotropy, structural defects, measuring frequency and inhomogeneous coupling in the case of multilayers [5]. Commonly, exchange-biased thin film exhibits larger line broadening than that of unbiased single layer thin film [6]. Speriosu et al. reported that the line broadening of exchange-biased film is caused from a local pinning of the FM layer due to an inhomogeneous surface anisotropy based on Malozemoff’s model [11]. So, the amount of line broadening varies in proportion to exchange bias strength in FM/AFM thin film. Fig. 4 shows the mean peak-to-peak linewidth variation of exchangebiased IrMn/NiFe/IrMn thin films as a function of NiFe thickness. With increasing NiFe thickness, the linewidth decreased and then it showed little change at thick NiFe layered thin films, which is comparable to that of the unbiased single NiFe(5 nm) thin film. From the results, the linewidth variation could be described by 1/tF line fitting up to 40 nm. It means that the linewidth variation arises from the interfacial nature of exchange coupling like the Hex variation. 4. Conclusion
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Line width (Oe)
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Fig. 3. Variation in Hr0 for exchange biased IrMn(10 nm)/NiFe(t nm)/ IrMn(10 nm) thin films as a function of NiFe thickness. The triangle symbol indicates the Hr0 of unbiased single NiFe(5 nm) thin film.
NiFe thickness dependence of IrMn/NiFe /IrMn thin films was studied. In-plane angular dependence of a resonance field measured by FMR showed unidirectional behavior due to exchange bias effect. The unidirectional anisotropic field and the uniaxial anisotropic field showed linear decrease with increasing NiFe thickness except at films with thick NiFe layer. The uniaxial anisotropy
ARTICLE IN PRESS Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e188–e191
variation can be induced by the unidirectional anisotropy. The resonance field shift increased with increasing NiFe thickness corresponding to Hex reduction. The linewidth of the resonance peak for exchange-biased thin films was larger than unbiased thin film and the variation showed a trend similar to that Hex variation. References [1] J. Nogue´s, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [2] M. Kiwi, J. Magn. Magn. Mater. 234 (2001) 584. [3] N.C. Koon, Phys. Rev. Lett. 78 (1997) 4865.
e191
[4] C. Mauri, H.C. Siegmann, P.S. Bagus, E. Kay, J. Appl. Phys. 62 (1987) 3047. [5] J. Lindner, K. Baberschke, J. Phys.: Condens. Matter 15 (2003) R193. [6] H. Xi, K.R. Mountfield, R.M. White, J. Appl. Phys. 87 (2000) 4367. [7] M. Rubinstein, P. Lubitz, S.F. Cheng, J. Magn. Magn. Mater. 195 (1999) 299. [8] H. Xi, R.M. White, Phys. Rev. B 60 (1999) 14837. [9] R.D. McMichael, M.D. Stiles, P.J. Chen, W.F. Egelhoff Jr., Phys. Rev. B 58 (1998) 8605. [10] Z. Qian, J.M. Sivertsen, J.H. Judy, J. Appl. Phys. 83 (1998) 6825. [11] V.S. Speriosu, S.S.P. Parkin, IEEE Trans. Magn. MAG-23 (1987) 2999.
Journal of Magnetism and Magnetic Materials 303 (2006) e188–e191 www.elsevier.com/locate/jmmm
Angular and NiFe thickness dependence of exchange bias in IrMn/NiFe/IrMn thin film Yong-Goo Yooa,, Seong-Gi Minb, Ho-Jun Ryua, Nam-Seok Parkc, Seong-Cho Yub a
Electronics and Telecommunications Research Institute, Daejeon 305-350, South Korea Department of Physics, Chungbuk National University, Cheongju 360-763, South Korea c Department of Electrical and Electronic engineering, Chungju National University, Chungju 380-702, South Korea b
Available online 17 February 2006
Abstract Exchange biased IrMn/NiFe/IrMn thin films were studied as a function of NiFe thickness. In plane angular dependence of a resonance field distribution which is measured by FMR was analyzed as a combined effect of an unidirectional anisotropy and an uniaxial anisotropy. The unidirectional anisotropic field and the uniaxial anisotropic field were linearly varied with NiFe thickness while the films with a thicker NiFe layer do not follow the linear variation. Resonance field and linewidth variations were also analysed with NiFe thickness. r 2006 Elsevier B.V. All rights reserved. PACS: 75.70.Cn; 68.55.a Keywords: FMR; Uniaxial anisotropy; Unidirectional anisotropy; Exchange bias
1. Introduction The exchange bias phenomena have been extensively studied due to their potential applications in spin valve structure and the physical interest to magnetic coupling mechanism in multilayer structure. Results indicate that the exchange coupling between ferromagnet (FM) and antiferromagnet (AFM) comes from their spin configuration at the interface [1]. The exchange coupling strength which is caused by interfacial spin configuration of FM/ AFM, depends on the thickness of FM and AFM and/or their domain structure [1,2]. Especially, the AFM domain structure plays an important role in determining the characteristic properties such as the unidirectional anisotropy and the coercivity enhancement [3,4]. Most theoretical models assumed that the FM layer has uniform magnetization along the thickness direction [2]. However, in the case of FM layer with enough thickness, it is also important to note that there is a uniform magnetization distribution throughout the thickness of the FM layer that Corresponding author. Tel.: +82 42 860 5222; fax: +82 42 860 5202.
E-mail address: [email protected] (Y.-G. Yoo). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.01.051
is coupled with the AFM layer. Ferromagnetic resonance (FMR) measurements represent a powerful tool for studying magnetic anisotropy [5]. The latter can be determined by the intrinsic property of the FM layer and/or an exchange coupling in the case of an exchange biased thin film [6]. In this study, we report on the variation in exchange bias as a function of FM layer thickness in IrMn/NiFe/IrMn trilayer structure. In particular, FM layers with enough thickness were examined. The top and bottom IrMn layer were located to give symmetric surface effect to NiFe layer for FMR measurement. The IrMn as was chosen AFM layer because it has the FCC structure preferred to NiFe layer and there is no need for the post annealing process in order to induce unidirectional anisotropy. The unidirectional exchange anisotropy and an uniaxial anisotropy were analysed by FMR measurements 2. Experimentals The exchange biased IrMn(10 nm)/NiFe(t nm)/IrMn(10 nm) thin films were fabricated by magnetron sputtering on Si(1 0 0) wafers with a seed layer of Ta(5 nm) at room
ARTICLE IN PRESS Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e188–e191
3. Results and discussions In plane angular dependence of the resonance field (Hr) for the exchange biased thin films was determined by FMR measurements. The angular variation in Hr shows a manifest unidirectional behavior for the exchange biased IrMn/NiFe/IrMn thin film, as shown in Fig. 1. As the NiFe thickness increases, the bell-shaped curve of Hr changes into the flat shaped curve near top of the bell for the NiFe thickness of 80 nm and then it changes into the two-peaked curve for the NiFe thickness of 160 nm which is comparable to that of the unbiased single NiFe thin film in the inset of Fig. 1. The resonance field distribution with angular measurement implies combined effect of the unidirectional anisotropy and the uniaxial anisotropy. The unidirectional anisotropic field (Hex) and the uniaxial anisotropic field (Hk) can be extracted by the phenomenological expression [7] of cosine series neglecting high-order
terms as follows: H r ðjH Þ ¼ H r0 H ex cos jH H k cos 2jH ,
(1)
where fH indicates the azimuthal angle between the applied field direction and the easy axis of the FM layer. Hr0 is an average resonance field indicating an angular independent term. The second term represents the exchange coupling between the FM and AFM layer and the third term describes the uniaxial anisotropy of the FM layer as expressed by Hk ¼ 2Ku/MF. The angular variation of Hr for IrMn/NiFe/IrMn trilayer thin films were well fitted by Eq. (1) as can be seen by the solid lines in Fig. 1. Fig. 2 shows the variation of Hex and Hk, which were extracted from Eq. (1), and Hex and Hc measured by VSM with NiFe thickness. The Hex values exhibit a linear decrease with the NiFe thickness increase which can be expressed by Hex1/tF [1]. However, the films with NiFe layer above 80 nm do not follow the linearity. This implies that the magnetization distribution in FM layer can affect the exchange coupling strength. The discrepancy in Hex values obtained from the two measurements could arise from hysteresis loop asymmetry for the sweep field direction [8]. The asymmetry comes from the irreversible 1000 VSM FMR
Hex (Oe)
temperature. Ta(5 nm) was deposited on top of all the samples, to prevent oxidation. The base pressure of the chamber was below 5 108 torr and the Ar working pressure was 2 mTorr. During deposition, a magnetic field of 200 Oe was applied to form the uniaxial anisotropy in the NiFe layer. The thickness of the NiFe layer varied from 2 to 160 nm. A single layer of NiFe(t nm) was also fabricated for comparison. The hysteresis loop and the coercivity were obtained by vibrating sample magnetometer (VSM). The FMR measurements were performed at 9.4 GHz (X band) with a JEOL JES-TE300 ESR Spectrometer. The angular dependence of the resonance field was measured by rotating the sample to an in-plane direction with respect to the applied magnetic field.
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VSMH (biased) c FMRH (biased) k FMRH (unbiased) k VSMH (unbiased) c VSMH (unbiased) k
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120 180 240 300 360 φH (degree)
180
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1 (b)
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φH (degree) Fig. 1. Angular dependence of the Hr for exchange biased IrMn(10 nm)/ NiFe(t nm)/ IrMn(10 nm) thin films as a function of NiFe layer thickness. Inset figure shows films with thick NiFe layer for magnification. The single NiFe indicates unbiased single NiFe(5 nm) thin film. The solid lines indicate data fitted to Eq. (1).
1
10 NiFe thickness (nm)
100
Fig. 2. Variation in Hex (a) and Hk and Hc (b) for an exchange-biased IrMn(10 nm)/NiFe(t nm)/IrMn(10 nm) thin films with NiFe layer thickness measured by VSM and FMR, respectively. The unbiased indicates single NiFe(5 nm) thin film. The solid lines indicate linear fitting line for guideing the eye.
ARTICLE IN PRESS Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e188–e191
switching of magnetization to the sweep field direction. Variations of Hk and Hc for exchange biased IrMn/NiFe/ IrMn thin films are similar to the Hex variation. Fig. 2(b) shows the linear decrease with increasing NiFe thickness up to 40 nm. It is worth noting that the unbiased single NiFe films show a small increase of Hk and Hc with increasing NiFe thickness. This result suggests that uniaxial anisotropy of the exchange-biased NiFe layer can be induced by an exchange interaction with the AFM layer. Actually, the decrease in the NiFe thickness corresponding to the Hex increase gives rise to a large difference in the Hk values of the exchange-biased and the unbiased NiFe thin films. However, the biased NiFe films above 80 nm exhibit a comparable Hk value to that of unbiased NiFe films. This result indicates that the Hk of the films with thick NiFe layer could be determined predominantly by the NiFe intrinsic property due to the rather weak exchange bias effect. In plane resonance field of exchange-biased FM thin film exhibits negative field shift from unbiased single FM thin film. According to McMichael et al., the negative field shift of the resonance field in FMR measurement was due to a rotatable anisotropy which comes from irreversible AFM domain behavior [9]. The size distribution of the AFM domain near the interface with the FM layer produces a pining field affecting magnetization reversal of the FM layer. The temperature and AFM thickness dependence of Hr were explained by the rotatable anisotropy [6,9]. These results indicate that temperature and AFM thickness affect the AFM domain behavior corresponding to Hex variation. In our case, the NiFe thickness dependence of Hr0 is shown in Fig. 3. The Hr0 steeply increased at small NiFe thickness while a small increase was shown at large NiFe thickness. The increase of NiFe thickness could cause a change in the interfacial microstructure so that the Hr0 variation occurs by different interfacial condition, since the resonance field
1100 1000
Hr0 (Oe)
900
800
100
10 1
10 NiFe thickness (nm)
100
Fig. 4. The mean peak-to-peak linewidth variation of exchange biased IrMn(10 nm)/NiFe(t nm)/IrMn(10 nm) thin films as a function of NiFe thickness. The triangle symbol indicates the linewidth of unbiased single NiFe(5 nm) thin film.
shift arises from a variation of exchange anisotropy due to interfacial condition [10]. In addition to the resonance field distribution by FMR measurement, a line broadening gives useful information to study exchange coupling in FM/AFM thin films. The line broadening in FMR measurement comes from intrinsic properties such as anisotropy, structural defects, measuring frequency and inhomogeneous coupling in the case of multilayers [5]. Commonly, exchange-biased thin film exhibits larger line broadening than that of unbiased single layer thin film [6]. Speriosu et al. reported that the line broadening of exchange-biased film is caused from a local pinning of the FM layer due to an inhomogeneous surface anisotropy based on Malozemoff’s model [11]. So, the amount of line broadening varies in proportion to exchange bias strength in FM/AFM thin film. Fig. 4 shows the mean peak-to-peak linewidth variation of exchangebiased IrMn/NiFe/IrMn thin films as a function of NiFe thickness. With increasing NiFe thickness, the linewidth decreased and then it showed little change at thick NiFe layered thin films, which is comparable to that of the unbiased single NiFe(5 nm) thin film. From the results, the linewidth variation could be described by 1/tF line fitting up to 40 nm. It means that the linewidth variation arises from the interfacial nature of exchange coupling like the Hex variation. 4. Conclusion
700
600
1000
Line width (Oe)
e190
1
10 NiFe thickness (nm)
100
Fig. 3. Variation in Hr0 for exchange biased IrMn(10 nm)/NiFe(t nm)/ IrMn(10 nm) thin films as a function of NiFe thickness. The triangle symbol indicates the Hr0 of unbiased single NiFe(5 nm) thin film.
NiFe thickness dependence of IrMn/NiFe /IrMn thin films was studied. In-plane angular dependence of a resonance field measured by FMR showed unidirectional behavior due to exchange bias effect. The unidirectional anisotropic field and the uniaxial anisotropic field showed linear decrease with increasing NiFe thickness except at films with thick NiFe layer. The uniaxial anisotropy
ARTICLE IN PRESS Y.-G. Yoo et al. / Journal of Magnetism and Magnetic Materials 303 (2006) e188–e191
variation can be induced by the unidirectional anisotropy. The resonance field shift increased with increasing NiFe thickness corresponding to Hex reduction. The linewidth of the resonance peak for exchange-biased thin films was larger than unbiased thin film and the variation showed a trend similar to that Hex variation. References [1] J. Nogue´s, I.K. Schuller, J. Magn. Magn. Mater. 192 (1999) 203. [2] M. Kiwi, J. Magn. Magn. Mater. 234 (2001) 584. [3] N.C. Koon, Phys. Rev. Lett. 78 (1997) 4865.
e191
[4] C. Mauri, H.C. Siegmann, P.S. Bagus, E. Kay, J. Appl. Phys. 62 (1987) 3047. [5] J. Lindner, K. Baberschke, J. Phys.: Condens. Matter 15 (2003) R193. [6] H. Xi, K.R. Mountfield, R.M. White, J. Appl. Phys. 87 (2000) 4367. [7] M. Rubinstein, P. Lubitz, S.F. Cheng, J. Magn. Magn. Mater. 195 (1999) 299. [8] H. Xi, R.M. White, Phys. Rev. B 60 (1999) 14837. [9] R.D. McMichael, M.D. Stiles, P.J. Chen, W.F. Egelhoff Jr., Phys. Rev. B 58 (1998) 8605. [10] Z. Qian, J.M. Sivertsen, J.H. Judy, J. Appl. Phys. 83 (1998) 6825. [11] V.S. Speriosu, S.S.P. Parkin, IEEE Trans. Magn. MAG-23 (1987) 2999.