Magnetization Dynamics and Waves Generated by Spin Currents

Sergei Urazhdin, Emory University

May 29, 2020

Spin torques exerted on magnetic systems by spin polarized or pure spin currents can generate coherent spin waves with nanometer-scale wavelength, providing a route for efficient implementation of spin wave-based computing at nanoscale. In my talk, I will describe experimental studies of the mechanisms controlling the dynamical coherence of the magnetization states induced by spin currents, and approaches to utilizing them for coherent spin wave emission. Spectroscopic measurements show that, contrary to naïve expectations, for a typical microscopic magnetic system with in-plane magnetization, spin currents cannot generate coherent spin waves, due to two nonlinear effects: nonlinear frequency shift and nonlinear dynamical damping. The former results in localized oscillations [1], while the latter suppresses oscillations altogether [2]. I will describe the approaches that have been developed to minimize these nonlinear mechanisms, resulting in the generation of coherent, and in many cases directional, spin waves by spin current. Finally, I will discuss recent studies that demonstrated the possibility to suppress nonlinear damping [3], enabling coherent spontaneous magnetization dynamics in spatially extended magnetic systems.

[1] V. E. Demidov, S. Urazhdin, H. Ulrichs, V. Tiberkevich, A. Slavin, D. Baither, G. Schmitz, and S. O. Demokritov, Magnetic nano-oscillator driven by pure spin currents, Nature Mater. 11, 1028 (2012).
[2] V. Demidov, S. Urazhdin, E. Edwards, M. D. Stiles, R. McMichael and S. O. Demokritov, Control of magnetic fluctuations by spin current, Phys. Rev. Lett. 107, 107204 (2011).
[3] B. Divinskiy, S. Urazhdin, S. O. Demokritov, and V. E. Demidov, Controlled nonlinear magnetic damping in spin-Hall nano-devices, Nature Comm. 10, 5211 (2019).

Nonlinear damping prevents the onset of coherent spin current-driven auto-oscillation in a microscopic Py disk (left). Nonlinear damping is eliminated in a Co/Ni bilayer where demagnetizing anisotropy is compensated by PMA, resulting in coherent auto-oscillations (right).