MATERIALS SCIENCE COLLOQUIUM

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ABSTRACT: As semiconducting electronic devices are miniaturized to ever-smaller dimensions, power dissipation becomes an ever-increasing problem due to leakage charge currents. Spintronics may help addressing some of these issues by utilizing besides the charge degree of freedom also the electron spin. Towards this end, pure spin currents [1] may eliminate some of the limitations due to charge currents and their concomitant power dissipation. Besides diffusive spin currents in non-magnetic materials, especially spin-wave based spin currents have attracted interest due to the extremely low magnetization damping in ferrimagnetic insulators, such as Y₃Fe₅O₁₂ (YIG), which results in very low dissipative losses. Recently it has been shown that spin Hall effects, which convert charge currents in transverse spin currents and vice versa, can connect electric signals to the magnetization dynamics in these ferromagnetic insulators [2]. Our recent work has focused on characterizing spin Hall effects, which are the key enabling phenomena for this paradigm shift in spin-based devices. In order to gain insight into the underlying physical mechanism and to identify technologically relevant materials, it is important to quantify the spin Hall angle γ, which is a direct measure of the charge-to-spin conversion efficiency. We developed a measurement approach based on spin pumping, which enables us to quantify even small spin Hall angles with high accuracy. Spin pumping utilizes microwave excitation of a ferromagnetic layer adjacent to a normal metal to generate a homogeneous dc spin current over a macroscopic area, and we determined spin Hall angles for Pt, Pd, Au, and Mo at room temperature [3,4]. Of these materials Pt shows the largest spin Hall angle with γ = 0.013±0.002. Furthermore, we show how these spin Hall effects can be used to modify magnetization dynamics in an adjacent ferrimagnetic insulator, such as YIG. By using Pt/YIG bilayers, we show how charge currents in the Pt can either reduce or increase the linewidth of the ferromagnetic resonance in YIG [5]. Interestingly the current dependence of the line width shows a distinct threshold behavior.
Work at Argonne supported by DOE BES under Contract No. DE-AC02-06CH11357.

[1] A. Hoffmann, Phys. Stat. Sol. (c) 4, 4236 (2007).
[2] Y. Kajiware, et al., Nature 464, 262 (2010).
[3] O. Mosendz, J. E. Preason, F. Y. Fradin, G. E. W. Bauer, S. D. Bader, and A. Hoffmann, Phys. Rev. Lett. 104, 046601 (2010).
[4] O. Mosendz, V. Vlaminck, J. E. Preason, F. Y. Fradin, G. E. W. Bauer, S. D. Bader, and A. Hoffmann, Phys. Rev. B 82, 214403 (2010).
[5] Z. Wang, Y. Sun, Y.-Y. Song, M. Wu, H. Schultheiß, J. E. Pearson, and A. Hoffmann, Appl. Phys. Lett. 99, 162511 (2011).