Excitation of localized and propagating spin wave modes in spin-Hall Nano-oscillators

The first experimental observation of persistent magnetization oscillation driven by spin-transfer torque from spin-Hall effect is due to Liu, et al.1 In that experimental framework, the torque from spin-Hall effect originating from a bias current flowing in a Tantalum (Ta) strip was large enough to compensate the natural magnetic losses due to the Gilbert damping in a CoFeB free layer of a magnetic tunnel junction (MTJ) etched over the strip. The persistent magnetization precession was also measured in a simpler device structure by Demidov et al2 by means of micro-focus Brillouin light scattering (BLS) spectroscopy and by R. H. Liu et al3 via the conversion of the magnetization precession to the anisotropic magnetoresistive signal. The main advantages of those last device configurations 2,3 over the one studied in 1 are the simple realization process and the possibility to read the signal from both optical and electrical measurements, on the other hand the output power of the magnetization precession is much smaller. In the present study, we concentrate our attention on the micromagnetic aspects related to those experimental frameworks. We performed a systematic study to understand the origin of the exited modes and how the dynamical behavior changes as function of geometrical properties, field and currents.