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FePt thin films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 1914–1916 www.elsevier.com/locate/jmmm
Magnetoresistance and interlayer coupling studies in FePt/Os/FePt thin films S.Y. Chena,b, Y.D. Yaob,, J.M. Wua a
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC b Institute of Physics, Academia Sinica, Taipei 115, Taiwan, ROC Available online 27 November 2006
Abstract Effects of the Os layer on the magnetic properties of FePt/Os/FePt were investigated. The magnetic layer of FePt in the as-deposited Ta/FePt/Os/FePt/Ta thin films exhibited very strong (1 1 1) orientation in all the cases. As the thickness of Os spacer layer between 1 and 3 A˚, it appeared an antiferromagnetic exchange coupling (AFC) behavior. The exchange coupling transforms to ferromagnetic when the thickness of Os spacer layer was thicker than 4 A˚. Although the J value for FePt/Os/FePt thin films is 0.22 erg/cm2, which is relatively small than that of Co/Ru system, the GMR effects could be observed. In general, the AFC in multilayer system shows the first maximum of AFC peak occurring at spacer thickness between 8 and 11 A˚ for most spacer materials. In our investigation, this is the first time that a clear AFC behavior observed in FePt/Os/FePt system with Os spacer thickness at 2 A˚. r 2006 Published by Elsevier B.V. PACS: 71.25.Hc; 75.60.Ej; 75.70.Pa Keywords: FePt; Osmium; GMR; Interlayer exchange coupling; Magnetoresistance
In the last decade, considerable attention has been paid to the giant magnetoresistance effect in spin-valve systems which are composed of two ferromagnetic layers separated by a non-magnetic spacer layer, since the effect was first observed in Fe/Cr superlattices [1]. The antiferromagnetic coupling of the moments is due to the oscillatory exchange coupling phenomenon which reported by Parkin [2]. The ferromagnetic layers in the multilayer structure exhibit an exchange coupling that oscillates between ferromagnetic and antiferromagnetic which depends on the thickness of the spacer layer. Several non-magnetic transition metals spacer layer such as Cr, Cu and Ru have stimulated considerable interest [3–5]. Because Os has very high melting and boiling point, which is predicted to have good effect on preventing interdiffusion between layers, and the disordered FePt is a very soft ferromagnetic material. In
Corresponding author. Tel.: +886 2 2789 6717;fax: +886 2 2651 0530.
E-mail addresses: [email protected], [email protected] (Y.D. Yao). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.10.1105
this investigation, we attempt to study on the effect of the Os layer to the FePt layers. The FePt/Os/FePt thin films were deposited on natural oxidized Si(1 0 0) substrates with a 5 nm Ta buffer layer at room temperature by an ultra-high vacuum dc-magnetron sputtering system with a base pressure less than 1 108 Torr. The thickness of the ferromagnetic layers was 100 A˚ in all cases and a series of trilayers with Os spacer varied from 0 to 9 A˚. The FePt ferromagnetic layers were deposited using Fe50Pt50 alloying target and the composition of Pt and Fe checking by inductively coupled plasma spectra are 48.6% and 51.4 %, respectively. The crystal structures were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Magnetic properties of the films were measured by a vibrating sample magnetometer (VSM). The electric resistance and magnetoresistance (MR) were measured with a standard DC four-point probe technique. The magnetic layer of FePt in the Si//Ta/FePt/Os/FePt/ Ta thin films exhibited very strong (1 1 1) orientation in all the cases as indicated by X-ray data near 40.81 as shown in
ARTICLE IN PRESS S.Y. Chen et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1914–1916
Fig. 1. X-ray patterns of Si//Ta/FePt/Os/FePt/Ta thin films.
Fig. 2. The M–H loops of Si//Ta/FePt/Os/FePt/Ta thin films with various Os spacer thickness (a), The squareness (b) and saturation field (c) vs. various Os thickness are plotted, respectively.
Fig. 1. Based on the lattice image, we learned that the FePt layer is polycrystalline and well orientated in (1 1 1) plane. We dealt with the FePt multilayer thin films without post annealing on purpose to achieve that the magnetism of FePt/Os/FePt thin films could tend to appear as antiferromagnetic exchange coupling (AFC) behavior. Fig. 2(a) shows the hysteresis loops of multilayer with Os spacer thickness varying from 0 to 9 A˚. As the thickness of Os spacer layer increased from 1 to 3 A˚, it appeared as an AFC behavior. The ferromagnetic behavior observed in samples with Os spacer layer of 0, 4, 5, 7 and 9 A˚. In the previous work of (Co,CoFe)/Os/(Co,CoFe) films [6,7], AFC appeared at spacer layer is 9 A˚ in thickness; however,
1915
in FePt/Os/FePt films, it appeared at 2 A˚. In general, the AFC in multilayer system shows the first maximum of AFC peak occurring at spacer thickness between 8 and 11 A˚ for most spacer materials [8]. In our investigation, a clear AFC behavior of the FePt/Os/FePt system could be observed at 2 A˚; this is the first time to be reported. The saturation magnetizations (Ms) of all FePt multilayer films were roughly 980 emu/cm3, which were relatively small in comparison with Ms ¼ 1900 emu/cm3 in CoFe/Os/CoFe films. The small Ms may be related to the AFC with a few mono layers of Os in FePt/Os/FePt system. From the M–H loops, the coercive force (Hc) of about several 10s of oersteds was found which indicated that the FePt has a disordered structure. The results showed the squareness value decreased from 0.77 to the minimum value of 0.04 at 2 A˚, and then increasing and dependence of saturation field on Os spacer layer thickness showed a maximum value of 2614 Oe at 2 A˚ as shown in Fig. 2(b) and 2(c), respectively. The interlayer coupling constant J can be derived from the in-plane saturation field (Hs) by using the simple relation: J ¼ DmMsHs/L, where Dm and Ms are the thickness and saturation magnetization of the magnetic layer, respectively. The factor L should be 2 when magnetic bilayers with two identical magnetic layers are used. The J value for FePt/Os/FePt thin films is 0.22 erg/cm2, which is relatively small than that of the Co/Ru system [8,9]. The anisotropic magnetoresistance is always existed in FePt multilayer system, even for samples with the Os thickness of 2 A˚, i.e. for sample with strongest antiferromagnetic coupling. Fig. 3(a) shows the variations of the DMR ratio with Os layer thickness. The measurements have been made with the magnetic field in the film plane and parallel to the current direction. The DMR ratio was defined as shown in the inset figure (DMR ratio ¼ 100(RpRo)/Ro), that Rp is the peak value of resistance due to interlayer exchange coupling. From the MR study, DMR ratio increased from 0.01% to the maximum value 0.14% at 2 A˚ and then decreased. The peak field of MR figures increased from 10 Oe to the maximum value 250 Oe at 2 A˚ and then decreasing as shown in Fig. 3(b). In summary, the FePt/Os/FePt thin films with Os layer varied between 1 and 9 A˚ were investigated. This is the first
Fig. 3. Relationship between DMR ratio and Os thickness (a) and the peak field strength vs. various Os thickness of Si//Ta/FePt/Os/FePt/Ta thin films (b).
ARTICLE IN PRESS 1916
S.Y. Chen et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1914–1916
time that a clear AFC behavior of the FePt/Os/FePt system could be observed for samples with Os at 2 A˚. The authors are grateful for financial support by the Acadmia Sinica and the National Science Council under Grant No. NSC 95-2112-M-001-059, Taiwan, ROC. References [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472.
[2] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [3] S.S.P. Parkin, Phys. Rev. Lett. 67 (1991) 3598. [4] S.S.P. Parkin, R. Bhadra, K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [5] Y. Wang, P.M. Levy, J.L. Fry, Phys. Rev. Lett. 65 (1990) 2732. [6] S.Y. Chen, Y.D. Yao, J.M. Wu, J. Magn. Magn. Mater. 304 (2006) e37. [7] C.Y. Chou, Y.D. Yao, P.C. Kuo, S.F. Lee, J.J. Chou, J. Magn. Magn. Mater. 304 (2006) e349. [8] U. Hartmann, Magnetic Multilayers and Giant Magnetoresistance, Springer, Berlin, 2000. [9] S.V. Pietambaram, J. Janesky, R.W. Dave, J.J. Sun, G. Steiner, J.M. Slaughter, IEEE Trans. Magn. 40 (2004) 619.
Journal of Magnetism and Magnetic Materials 310 (2007) 1914–1916 www.elsevier.com/locate/jmmm
Magnetoresistance and interlayer coupling studies in FePt/Os/FePt thin films S.Y. Chena,b, Y.D. Yaob,, J.M. Wua a
Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300, Taiwan, ROC b Institute of Physics, Academia Sinica, Taipei 115, Taiwan, ROC Available online 27 November 2006
Abstract Effects of the Os layer on the magnetic properties of FePt/Os/FePt were investigated. The magnetic layer of FePt in the as-deposited Ta/FePt/Os/FePt/Ta thin films exhibited very strong (1 1 1) orientation in all the cases. As the thickness of Os spacer layer between 1 and 3 A˚, it appeared an antiferromagnetic exchange coupling (AFC) behavior. The exchange coupling transforms to ferromagnetic when the thickness of Os spacer layer was thicker than 4 A˚. Although the J value for FePt/Os/FePt thin films is 0.22 erg/cm2, which is relatively small than that of Co/Ru system, the GMR effects could be observed. In general, the AFC in multilayer system shows the first maximum of AFC peak occurring at spacer thickness between 8 and 11 A˚ for most spacer materials. In our investigation, this is the first time that a clear AFC behavior observed in FePt/Os/FePt system with Os spacer thickness at 2 A˚. r 2006 Published by Elsevier B.V. PACS: 71.25.Hc; 75.60.Ej; 75.70.Pa Keywords: FePt; Osmium; GMR; Interlayer exchange coupling; Magnetoresistance
In the last decade, considerable attention has been paid to the giant magnetoresistance effect in spin-valve systems which are composed of two ferromagnetic layers separated by a non-magnetic spacer layer, since the effect was first observed in Fe/Cr superlattices [1]. The antiferromagnetic coupling of the moments is due to the oscillatory exchange coupling phenomenon which reported by Parkin [2]. The ferromagnetic layers in the multilayer structure exhibit an exchange coupling that oscillates between ferromagnetic and antiferromagnetic which depends on the thickness of the spacer layer. Several non-magnetic transition metals spacer layer such as Cr, Cu and Ru have stimulated considerable interest [3–5]. Because Os has very high melting and boiling point, which is predicted to have good effect on preventing interdiffusion between layers, and the disordered FePt is a very soft ferromagnetic material. In
Corresponding author. Tel.: +886 2 2789 6717;fax: +886 2 2651 0530.
E-mail addresses: [email protected], [email protected] (Y.D. Yao). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.10.1105
this investigation, we attempt to study on the effect of the Os layer to the FePt layers. The FePt/Os/FePt thin films were deposited on natural oxidized Si(1 0 0) substrates with a 5 nm Ta buffer layer at room temperature by an ultra-high vacuum dc-magnetron sputtering system with a base pressure less than 1 108 Torr. The thickness of the ferromagnetic layers was 100 A˚ in all cases and a series of trilayers with Os spacer varied from 0 to 9 A˚. The FePt ferromagnetic layers were deposited using Fe50Pt50 alloying target and the composition of Pt and Fe checking by inductively coupled plasma spectra are 48.6% and 51.4 %, respectively. The crystal structures were characterized by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). Magnetic properties of the films were measured by a vibrating sample magnetometer (VSM). The electric resistance and magnetoresistance (MR) were measured with a standard DC four-point probe technique. The magnetic layer of FePt in the Si//Ta/FePt/Os/FePt/ Ta thin films exhibited very strong (1 1 1) orientation in all the cases as indicated by X-ray data near 40.81 as shown in
ARTICLE IN PRESS S.Y. Chen et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1914–1916
Fig. 1. X-ray patterns of Si//Ta/FePt/Os/FePt/Ta thin films.
Fig. 2. The M–H loops of Si//Ta/FePt/Os/FePt/Ta thin films with various Os spacer thickness (a), The squareness (b) and saturation field (c) vs. various Os thickness are plotted, respectively.
Fig. 1. Based on the lattice image, we learned that the FePt layer is polycrystalline and well orientated in (1 1 1) plane. We dealt with the FePt multilayer thin films without post annealing on purpose to achieve that the magnetism of FePt/Os/FePt thin films could tend to appear as antiferromagnetic exchange coupling (AFC) behavior. Fig. 2(a) shows the hysteresis loops of multilayer with Os spacer thickness varying from 0 to 9 A˚. As the thickness of Os spacer layer increased from 1 to 3 A˚, it appeared as an AFC behavior. The ferromagnetic behavior observed in samples with Os spacer layer of 0, 4, 5, 7 and 9 A˚. In the previous work of (Co,CoFe)/Os/(Co,CoFe) films [6,7], AFC appeared at spacer layer is 9 A˚ in thickness; however,
1915
in FePt/Os/FePt films, it appeared at 2 A˚. In general, the AFC in multilayer system shows the first maximum of AFC peak occurring at spacer thickness between 8 and 11 A˚ for most spacer materials [8]. In our investigation, a clear AFC behavior of the FePt/Os/FePt system could be observed at 2 A˚; this is the first time to be reported. The saturation magnetizations (Ms) of all FePt multilayer films were roughly 980 emu/cm3, which were relatively small in comparison with Ms ¼ 1900 emu/cm3 in CoFe/Os/CoFe films. The small Ms may be related to the AFC with a few mono layers of Os in FePt/Os/FePt system. From the M–H loops, the coercive force (Hc) of about several 10s of oersteds was found which indicated that the FePt has a disordered structure. The results showed the squareness value decreased from 0.77 to the minimum value of 0.04 at 2 A˚, and then increasing and dependence of saturation field on Os spacer layer thickness showed a maximum value of 2614 Oe at 2 A˚ as shown in Fig. 2(b) and 2(c), respectively. The interlayer coupling constant J can be derived from the in-plane saturation field (Hs) by using the simple relation: J ¼ DmMsHs/L, where Dm and Ms are the thickness and saturation magnetization of the magnetic layer, respectively. The factor L should be 2 when magnetic bilayers with two identical magnetic layers are used. The J value for FePt/Os/FePt thin films is 0.22 erg/cm2, which is relatively small than that of the Co/Ru system [8,9]. The anisotropic magnetoresistance is always existed in FePt multilayer system, even for samples with the Os thickness of 2 A˚, i.e. for sample with strongest antiferromagnetic coupling. Fig. 3(a) shows the variations of the DMR ratio with Os layer thickness. The measurements have been made with the magnetic field in the film plane and parallel to the current direction. The DMR ratio was defined as shown in the inset figure (DMR ratio ¼ 100(RpRo)/Ro), that Rp is the peak value of resistance due to interlayer exchange coupling. From the MR study, DMR ratio increased from 0.01% to the maximum value 0.14% at 2 A˚ and then decreased. The peak field of MR figures increased from 10 Oe to the maximum value 250 Oe at 2 A˚ and then decreasing as shown in Fig. 3(b). In summary, the FePt/Os/FePt thin films with Os layer varied between 1 and 9 A˚ were investigated. This is the first
Fig. 3. Relationship between DMR ratio and Os thickness (a) and the peak field strength vs. various Os thickness of Si//Ta/FePt/Os/FePt/Ta thin films (b).
ARTICLE IN PRESS 1916
S.Y. Chen et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1914–1916
time that a clear AFC behavior of the FePt/Os/FePt system could be observed for samples with Os at 2 A˚. The authors are grateful for financial support by the Acadmia Sinica and the National Science Council under Grant No. NSC 95-2112-M-001-059, Taiwan, ROC. References [1] M.N. Baibich, J.M. Broto, A. Fert, F. Nguyen Van Dau, F. Petroff, P. Etienne, G. Creuzet, A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472.
[2] S.S.P. Parkin, N. More, K.P. Roche, Phys. Rev. Lett. 64 (1990) 2304. [3] S.S.P. Parkin, Phys. Rev. Lett. 67 (1991) 3598. [4] S.S.P. Parkin, R. Bhadra, K.P. Roche, Phys. Rev. Lett. 66 (1991) 2152. [5] Y. Wang, P.M. Levy, J.L. Fry, Phys. Rev. Lett. 65 (1990) 2732. [6] S.Y. Chen, Y.D. Yao, J.M. Wu, J. Magn. Magn. Mater. 304 (2006) e37. [7] C.Y. Chou, Y.D. Yao, P.C. Kuo, S.F. Lee, J.J. Chou, J. Magn. Magn. Mater. 304 (2006) e349. [8] U. Hartmann, Magnetic Multilayers and Giant Magnetoresistance, Springer, Berlin, 2000. [9] S.V. Pietambaram, J. Janesky, R.W. Dave, J.J. Sun, G. Steiner, J.M. Slaughter, IEEE Trans. Magn. 40 (2004) 619.