Experimental Investigation of the Temperature-Dependent Magnon Density and Its Influence on Studies of Spin-Transfer-Torque-Driven Systems

Abstract

We present the temperature dependence of the thermal magnon density in a thin ferromagnetic layer. By employing Brillouin light scattering and varying the temperature, an increase of the magnon density accompanied by a lowering of the spin-wave frequency is observed with increasing temperature. The magnon density follows the temperature according to the Bose-Einstein distribution function which leads to an approximately linear dependency. In addition, the influence of this effect in spin-transfer-torque-driven systems is presented. In particular, the increase in the magnon density with temperature sets the limit for a suppression of magnons in charge current-driven systems. Hence, the maximum possible suppression of thermal magnons occurs at a finite current.

Origins of the Unidirectional Spin Hall Magnetoresistance in Metallic Bilayers

“…Strong nonlinearities in the magnon population have been observed by Brillouin light scattering as the current intensity approaches the damping compensation threshold in FM/NM bilayers [35]. Together with Joule heating, such nonlinear effects determine the nonequilibrium density of magnons in the FM [35,47], which ultimately affects R 2ω due to spin-flip processes. In Co/Pt, our fits of the current dependence suggest that the spin current (∝ I) modulates a thermalized magnon population ∝ (T +∆T ) ∝ (b(B)+c(B)I 2 ), where T is the ambient temperature and ∆T ∝ I 2 is the temperature increase due to Joule heating [46].…”

Realization of a Spin-Wave Switch Based on the Spin-Transfer-Torque Effect

“…Figure 1b exemplarily shows the time evolution of the µBLS intensity I µBLS (black curve) and the center frequency f Center of the thermal spin wave spectrum (blue squares) in the center of the waveguide for a 50 ns long current pulse of j = −48 mA applied at t = 0 and for an applied external magnetic field of µ 0 H ext = 70 mT. Here, the center frequency of the spectrum is determined as the weighted average [29]. The error bar in frequency corresponds to the channel width of 150 MHz used during the BLS measurement.…”

“…Due to the opposite signs of the respective spin-Hall angles of Pt and Cr [27], [28] in combination with the fact that the emerging spin currents enter the CMFS layer from opposite surfaces, the injected spin current is maximized while parasitic influences like, e.g., the Oersted fields of the charge currents partially compensate each other inside the CMFS layer. Thus, this trilayer design represents an efficient layer setup for the presented investigations on the SHE-STT effect [17], [29].…”