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NiFe films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e699–e701 www.elsevier.com/locate/jmmm
Field-angle dependence of magnetic resonance in Pt/NiFe films H.Y. Inoue, K. Harii, E. Saitoh Department of Applied Physics, Keio University, Hiyoshi 3-14-1, Yokohama city 223-8522, Japan Available online 20 November 2006
Abstract Ferromagnetic resonance in NiFe/ amorphous Pt bilayer thin films was investigated with changing the external field direction. The spectral width of the ferromagnetic resonance depends critically on the external-magnetic-field direction. We found that the sample dependence of the spectral width is enhanced with deviation of external field direction from the direction along the film plain, implying an important role of spin directions in field-induced spin-decoherence mechanism in Pt. r 2006 Elsevier B.V. All rights reserved. PACS: 76.50.+g Keywords: Ferromagnetic resonance; Spectral width; Spintronics; Spin pumping
There has been a rapidly growing interest in the field of spintronics, which is a new device-technology manipulating the electron’s spin degree of freedom rather than the electric charge. Especially, the utilization of a spin current, a flow of electron spins in a solid [1–5], is an essential spintronics technology that realizes an efficient magnetic memories and computing devices based on new architectures. In ferromagnetic/ paramagnetic bilayer metallic systems there is a method to realize a pure spin-current injection into the paramagnetic layer from the ferromagnetic layer, the spin pumping induced by ferromagnetic resonance (FMR). The magnetization-precession damping, which maintains steady magnetization precession in FMR, is partly due to the transfer of angular momentum of the precession local spins to the conduction electrons [1–6]. This transfer polarizes the spin of conduction electrons and generates a pure spin current, which transports purely spins perpendicularly to the bilayer interface in the system [3,4]. The polarization of the pumped spin current r is given as r ¼ (M dM/dt), where M is the magnetization vector [7]. Then, the spin current propagates into the paramagnetic metal when two metallic layers are suitably connected. Thus, the spin pumping enables us to inject a pure spin current into the paramagnetic metal [8]. Recent studies Corresponding author. Tel.: +81 45 566 1821; fax: +81 45 566 1821.
E-mail address: [email protected] (H.Y. Inoue). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.992
have shown that the decay time of the magnetization precession in such a bilayer system reflects the spin decoherence and damping in the metallic layer [10,11]. In this paper, we report the FMR spectral width as a function of the angle y between the applied DC magnetic field direction and the direction along the plain of the film using Ni81Fe19/Pt bilayer thin films. We prepared three Ni81Fe19/Pt bilayer films comprising a 10 nm thick ferromagnetic Ni81Fe19 layer and 3, 4, and 5 nm-thick paramagnetic metal Pt layer. The bilayer was deposited on an oxidized silicon substrate. The sample dimension was 2.5 mm 0.5 mm in width. Pt was sputtered in Ar gas atmosphere at the sputtering speed of 0.1 nm/s. The Pt layer was shown to be amorphous by the thin-film X-ray diffraction. During the deposition, the pressure was below 6.0 106 Torr. Then 10 nm-thick Ni81Fe19 layer was deposited with an electron beam evaporator at the depositing speed of 0.1 nm/s. During the deposition, the pressure was below 6.2 107 Torr for each sample. The sample system is placed near the center of a TE011 microwave cavity at which the magnetic-field component of the microwave mode is maximized while the electric field component of the microwave mode is minimized. We utilized the o0 ¼ 9.42 GHz microwave with an output power of 4 mW. The configuration of a sample system and the applied DC magnetic field is depicted in Fig. 1(a). Here y denotes the angle between the external field and the
ARTICLE IN PRESS H.Y. Inoue et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e699–e701
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a Hex Ni81Fe19 (10 nm)
θ
Pt (3,4,5nm) SiO2 substrate
b
c 8
H (mT)
600
6
400 200 0 30
60
θ (deg) 100
300
500 τ (10-9 sec)
FMR signal (arbitrary unit)
0
3nm
4
4nm 5nm
100
300
500 2
0 100
300 H (mT)
500
0
30 θ (deg)
60
Fig. 1. (a) Schematic illustration of the sample-field configuration. (b) External magnetic field dependence of the FMR signal for Ni81Fe19/Pt(5 nm) bilayer at angle y ¼ 01, 301, and 601. (c) y dependence of t for each sample at angle y ¼ 01, 301, and 601. Inset displays y dependence of the resonance field of the obtained FMR signal for each sample at angle y ¼ 01, 301, and 601.
in-plane direction. We used a goniometer to prepare every sample-field configuration at the desired angle y. Then, FMR signals were measured with the angle y at y ¼ 01, 30o and 60o at room temperature. Fig. 1(b) exemplifies the observed FMR signals at various external field angles y. We estimated the spectral width in each resonance structure of the FMR signal as the peak-to-dent DH field the obtained resonance spectral shape (see arrows in Fig. 1(b)). y dependence of the resonance field for each sample at angle y ¼ 01, 301 and 601 is shown inset to Fig. 1(c), which indicates that variation of the resonance fields of the samples are small. In contrast, the variation of the magnetization precession decay appears to be critically dependent on y. In Fig. 1(c), the field-direction dependence of t is plotted, where t1( ¼ Do) ¼ g DH 2o0 DH/(Hp+Hd), g is gyromagnetic
ratio, Hp is the field at the peak and Hd is the field at the dent. This y dependence is attributed partially to the shape magnetic anisotropy in the Ni81Fe19 layer [6]. At y ¼ 01 and 301 t’s of all the samples almost coincides. This indicates that these three samples exhibit similar magnetization-precession decay time when the angle between the external field and the in-plane direction is small. In spite of this agreement, t for each sample exhibits a considerable scatter at y ¼ 601. This indicates that the magnetization-precession damping depends critically on the angle between the field and the in-plane directions. Importantly, no correlation between t’s and the thickness of the Pt layer is observed, implying that the scatter at y ¼ 601 is due to the detail of the sample systems. This implies an important role of the polarization direction of a pumped spin current in the spin decoherence
ARTICLE IN PRESS H.Y. Inoue et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e699–e701
mechanism in this amorphous Pt layer. At y ¼ 601, the magnetization direction in the Ni81Fe19 layer deviates significantly from the external field direction due to the shape magnetic anisotropy of the thin film system. This deviation causes a disagreement between the external-field and the spin-current-polarization directions in the Pt layer, a situation which induces the precession of the spin-current polarization affected by the applied DC field. Since a decoherence in a diffusive system is generally affected by sample-dependent elastic scattering mechanism [9], which should reflect the detailed distribution of impurities in the metal, the precession of the diffusive spin current exhibit the sample-dependent decoherence [10]. This is in contrast to the small y case. Therefore, this field-induced spin decoherence is a candidate for the origin of the observed scatter; yet this unexpected angle dependence is left to be numerically elucidated. In summary, we measured FMR signals with changing the angle y between the applied magnetic field direction and the direction along the film-plain in Ni81Fe19/ amorphous Pt bilayer thin films. We found that the magnetization precession decay depends critically on the field direction and scatters at a large y, a situation which implies the field-induced decoherence of a pumped spin current in the diffusive Pt metallic layer.
e701
This work is supported by a Grant-in-Aid for Scientific Research on Encouragement of Young Scientists (A) from Ministry of Education, Culture, Sports, Science, and Technology and by a Strategic Information and Communications R&D Promotion Programme from Ministry of Internal Affairs and Communications. H.I.Y is grateful to Kohei. M. Itoh for the financial support in his grant of Japan Science and Technology Agency. References [1] R.H. Silsbee, A. Janossy, P. Monod, Phys. Rev. B. 19 (1979) 4382. [2] S. Murakami, N. Nagaosa, S.C. Zhang, Science 301 (2003) 1348. [3] Y. Tserkovnyak, A. Brataas, G.E.W. Bauer, Phys. Rev. Lett. 88 (2002) 117601. [4] A. Brataas, Y. Tserkovnyak, G.E.W. Bauer, B.I. Halperin, Phys. Rev. B. 66 (2002) 060404(R). [5] R. Urban, G. Woltersdorf, B. Heinrich, Phys. Rev. Lett. 87 (2001) 217204. [6] S. Mizukami, Y. Ando, T. Miyazaki, Phys. Rev. B. 66 (2002) 104413. [7] S. Maekawa, Concepts in Spin Electronics (Series on Semiconductor Science and Technology), Oxford Univ Press, Oxford, 2006, p. 296. [8] E. Saitoh, M. Ueda, H. Miyajima, Appl. Phys. Lett. 88 (2006) 182509. [9] R.A. Webb, S. Washburn, A.D. Benoit, C.P. Umbach, R.B. Laibowitz, Jpn. J. Appl. Phys. 26 (1986) 1926. [10] R.V. Shchelushkin, A. Brataas, Phys. Rev. B. 72 (2005) 073110.
Journal of Magnetism and Magnetic Materials 310 (2007) e699–e701 www.elsevier.com/locate/jmmm
Field-angle dependence of magnetic resonance in Pt/NiFe films H.Y. Inoue, K. Harii, E. Saitoh Department of Applied Physics, Keio University, Hiyoshi 3-14-1, Yokohama city 223-8522, Japan Available online 20 November 2006
Abstract Ferromagnetic resonance in NiFe/ amorphous Pt bilayer thin films was investigated with changing the external field direction. The spectral width of the ferromagnetic resonance depends critically on the external-magnetic-field direction. We found that the sample dependence of the spectral width is enhanced with deviation of external field direction from the direction along the film plain, implying an important role of spin directions in field-induced spin-decoherence mechanism in Pt. r 2006 Elsevier B.V. All rights reserved. PACS: 76.50.+g Keywords: Ferromagnetic resonance; Spectral width; Spintronics; Spin pumping
There has been a rapidly growing interest in the field of spintronics, which is a new device-technology manipulating the electron’s spin degree of freedom rather than the electric charge. Especially, the utilization of a spin current, a flow of electron spins in a solid [1–5], is an essential spintronics technology that realizes an efficient magnetic memories and computing devices based on new architectures. In ferromagnetic/ paramagnetic bilayer metallic systems there is a method to realize a pure spin-current injection into the paramagnetic layer from the ferromagnetic layer, the spin pumping induced by ferromagnetic resonance (FMR). The magnetization-precession damping, which maintains steady magnetization precession in FMR, is partly due to the transfer of angular momentum of the precession local spins to the conduction electrons [1–6]. This transfer polarizes the spin of conduction electrons and generates a pure spin current, which transports purely spins perpendicularly to the bilayer interface in the system [3,4]. The polarization of the pumped spin current r is given as r ¼ (M dM/dt), where M is the magnetization vector [7]. Then, the spin current propagates into the paramagnetic metal when two metallic layers are suitably connected. Thus, the spin pumping enables us to inject a pure spin current into the paramagnetic metal [8]. Recent studies Corresponding author. Tel.: +81 45 566 1821; fax: +81 45 566 1821.
E-mail address: [email protected] (H.Y. Inoue). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.992
have shown that the decay time of the magnetization precession in such a bilayer system reflects the spin decoherence and damping in the metallic layer [10,11]. In this paper, we report the FMR spectral width as a function of the angle y between the applied DC magnetic field direction and the direction along the plain of the film using Ni81Fe19/Pt bilayer thin films. We prepared three Ni81Fe19/Pt bilayer films comprising a 10 nm thick ferromagnetic Ni81Fe19 layer and 3, 4, and 5 nm-thick paramagnetic metal Pt layer. The bilayer was deposited on an oxidized silicon substrate. The sample dimension was 2.5 mm 0.5 mm in width. Pt was sputtered in Ar gas atmosphere at the sputtering speed of 0.1 nm/s. The Pt layer was shown to be amorphous by the thin-film X-ray diffraction. During the deposition, the pressure was below 6.0 106 Torr. Then 10 nm-thick Ni81Fe19 layer was deposited with an electron beam evaporator at the depositing speed of 0.1 nm/s. During the deposition, the pressure was below 6.2 107 Torr for each sample. The sample system is placed near the center of a TE011 microwave cavity at which the magnetic-field component of the microwave mode is maximized while the electric field component of the microwave mode is minimized. We utilized the o0 ¼ 9.42 GHz microwave with an output power of 4 mW. The configuration of a sample system and the applied DC magnetic field is depicted in Fig. 1(a). Here y denotes the angle between the external field and the
ARTICLE IN PRESS H.Y. Inoue et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e699–e701
e700
a Hex Ni81Fe19 (10 nm)
θ
Pt (3,4,5nm) SiO2 substrate
b
c 8
H (mT)
600
6
400 200 0 30
60
θ (deg) 100
300
500 τ (10-9 sec)
FMR signal (arbitrary unit)
0
3nm
4
4nm 5nm
100
300
500 2
0 100
300 H (mT)
500
0
30 θ (deg)
60
Fig. 1. (a) Schematic illustration of the sample-field configuration. (b) External magnetic field dependence of the FMR signal for Ni81Fe19/Pt(5 nm) bilayer at angle y ¼ 01, 301, and 601. (c) y dependence of t for each sample at angle y ¼ 01, 301, and 601. Inset displays y dependence of the resonance field of the obtained FMR signal for each sample at angle y ¼ 01, 301, and 601.
in-plane direction. We used a goniometer to prepare every sample-field configuration at the desired angle y. Then, FMR signals were measured with the angle y at y ¼ 01, 30o and 60o at room temperature. Fig. 1(b) exemplifies the observed FMR signals at various external field angles y. We estimated the spectral width in each resonance structure of the FMR signal as the peak-to-dent DH field the obtained resonance spectral shape (see arrows in Fig. 1(b)). y dependence of the resonance field for each sample at angle y ¼ 01, 301 and 601 is shown inset to Fig. 1(c), which indicates that variation of the resonance fields of the samples are small. In contrast, the variation of the magnetization precession decay appears to be critically dependent on y. In Fig. 1(c), the field-direction dependence of t is plotted, where t1( ¼ Do) ¼ g DH 2o0 DH/(Hp+Hd), g is gyromagnetic
ratio, Hp is the field at the peak and Hd is the field at the dent. This y dependence is attributed partially to the shape magnetic anisotropy in the Ni81Fe19 layer [6]. At y ¼ 01 and 301 t’s of all the samples almost coincides. This indicates that these three samples exhibit similar magnetization-precession decay time when the angle between the external field and the in-plane direction is small. In spite of this agreement, t for each sample exhibits a considerable scatter at y ¼ 601. This indicates that the magnetization-precession damping depends critically on the angle between the field and the in-plane directions. Importantly, no correlation between t’s and the thickness of the Pt layer is observed, implying that the scatter at y ¼ 601 is due to the detail of the sample systems. This implies an important role of the polarization direction of a pumped spin current in the spin decoherence
ARTICLE IN PRESS H.Y. Inoue et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e699–e701
mechanism in this amorphous Pt layer. At y ¼ 601, the magnetization direction in the Ni81Fe19 layer deviates significantly from the external field direction due to the shape magnetic anisotropy of the thin film system. This deviation causes a disagreement between the external-field and the spin-current-polarization directions in the Pt layer, a situation which induces the precession of the spin-current polarization affected by the applied DC field. Since a decoherence in a diffusive system is generally affected by sample-dependent elastic scattering mechanism [9], which should reflect the detailed distribution of impurities in the metal, the precession of the diffusive spin current exhibit the sample-dependent decoherence [10]. This is in contrast to the small y case. Therefore, this field-induced spin decoherence is a candidate for the origin of the observed scatter; yet this unexpected angle dependence is left to be numerically elucidated. In summary, we measured FMR signals with changing the angle y between the applied magnetic field direction and the direction along the film-plain in Ni81Fe19/ amorphous Pt bilayer thin films. We found that the magnetization precession decay depends critically on the field direction and scatters at a large y, a situation which implies the field-induced decoherence of a pumped spin current in the diffusive Pt metallic layer.
e701
This work is supported by a Grant-in-Aid for Scientific Research on Encouragement of Young Scientists (A) from Ministry of Education, Culture, Sports, Science, and Technology and by a Strategic Information and Communications R&D Promotion Programme from Ministry of Internal Affairs and Communications. H.I.Y is grateful to Kohei. M. Itoh for the financial support in his grant of Japan Science and Technology Agency. References [1] R.H. Silsbee, A. Janossy, P. Monod, Phys. Rev. B. 19 (1979) 4382. [2] S. Murakami, N. Nagaosa, S.C. Zhang, Science 301 (2003) 1348. [3] Y. Tserkovnyak, A. Brataas, G.E.W. Bauer, Phys. Rev. Lett. 88 (2002) 117601. [4] A. Brataas, Y. Tserkovnyak, G.E.W. Bauer, B.I. Halperin, Phys. Rev. B. 66 (2002) 060404(R). [5] R. Urban, G. Woltersdorf, B. Heinrich, Phys. Rev. Lett. 87 (2001) 217204. [6] S. Mizukami, Y. Ando, T. Miyazaki, Phys. Rev. B. 66 (2002) 104413. [7] S. Maekawa, Concepts in Spin Electronics (Series on Semiconductor Science and Technology), Oxford Univ Press, Oxford, 2006, p. 296. [8] E. Saitoh, M. Ueda, H. Miyajima, Appl. Phys. Lett. 88 (2006) 182509. [9] R.A. Webb, S. Washburn, A.D. Benoit, C.P. Umbach, R.B. Laibowitz, Jpn. J. Appl. Phys. 26 (1986) 1926. [10] R.V. Shchelushkin, A. Brataas, Phys. Rev. B. 72 (2005) 073110.