Tuesday, May 26th, 2015
Western Digital, 1710
Automation Parkway, San Jose, CA 95131
& Pizza at 6:45 P.M.
Presentation at 7:30 P.M.
Excitation and Detection of Magnetization Dynamics in Insulators by Spin Hall Effects
M. B. Jungfleisch1, W. Zhang1, J. Sklenar1,2, W. Jiang1, J. Ding1, H. Chang3, F. Y. Fradin1, J. E. Pearson1, J. B. Ketterson2, M. Wu3, and A. Hoffmann1
1 Materials Science Division, Argonne National Laboratory, Argonne, IL, U.S.A., email@example.com
2 Department of Physics and Astronomy, Northwestern University, Evanston, IL, U.S.A.
3 Department of Physics, Colorado State University, Fort Collins, CO, U.S.A.
Electric charge currents in nonmagnetic metals with high spin-orbit coupling give rise to transverse spin currents resulting in spin accumulations at the boundaries, a phenomenon know as spin Hall effect (SHE) . This spin accumulation can interact with the magnetization in an adjacent ferromagnet and thereby exert a spin-transfer torque, which can generate and manipulate magnetization dynamics. On the other hand, spin currents injected into nonmagnetic conductors can be transformed into electric charge currents by the inverse SHE . In combination with spin pumping this offers a unique method to electrically detect magnetization dynamics. Since no charge current is required to flow across the interface between the conductor and the ferromagnet, these mechanisms can even be used for insulating materials. Magnetic insulators such as yttrium iron garnet (YIG) have gained increased interest in the spintronics community due to the very low damping making them promising candidates for low power information transfer and processing.
We explored the propagation of spin-wave modes in micro-structured YIG stripes. Spin waves propagating perpendicularly to the long side of the stripe are detected by means of spatially-resolved Brillouin light scattering (BLS) microscopy. The propagation distance of spin waves is determined in the linear regime, where an exponential decay of 10 um corresponding to a Gilbert damping parameter of 8.8 x 10-4 is observed .
We also investigated the possibility of driving magnetization dynamics with SHEs in bilayers of YIG/Pt. For this purpose we adopted a spin-transfer torque ferromagnetic resonance approach. Here a rf charge current is passed through the Pt layer, which generates a spin-transfer torque at the interface from an oscillating spin current via the SHE. This gives rise to a resonant excitation of the magnetization dynamics. In all metallic systems the magnetization dynamics is detected via the homodyne anisotropic magnetoresistance of the ferromagnetic layer. However, since there is no charge flowing through ferromagnetic insulators there is no anisotropic magnetoresistance. Instead, we show that for the case of YIG/Pt the spin Hall magnetoresistance can be used. Our measured voltage spectra can be well fitted to an analytical model .
One of the key parameters in this model is the spin-mixing conductance, which describes the efficiency of the interfacial spin transfer. By performing measurements as a function of magnetic field direction we find that a significant imaginary component of the spin mixing conductance is required to model our data. Furthermore we investigated the lateral distribution of the spin dynamics using spatially-resolved BLS spectroscopy. We observe that the precession amplitude is not uniform across the sample and a strong localization only persists in the center of the sample.
The work at Argonne was supported by the U.S. Department of Energy, Office of Science, Materials Science and Engineering Division.
 A. Hoffmann, IEEE Trans. Magn. 49, 5172 (2013).
 M.B. Jungfleisch, W. Zhang, W. Jiang, H. Chang, J. Sklenar, S.M. Wu, J.E. Pearson, A. Bhattacharya, J.B. Ketterson, M. Wu, and A. Hoffmann, J. Appl. Phys. 117, 17D128 (2015).
 T. Chiba, G.E.W. Bauer, and S. Takahashi, Phys. Rev. Applied 2, 034003 (2014); T. Chiba, M. Schreier, G.E.W. Bauer, and S. Takahashi, J. Appl. Phys 117, 17C715 (2015).
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