Electrical detection of spin Hall effect torques induced auto-oscillations in nanometer thick YIG/Pt stripe

Abstract

In recent years, Spin Orbit interaction as a source of spin current has been widely used through the physics of Spin Hall Effect (SHE) or that of Rashba effect. The peculiar symmetry of SHE allows creating a spin accumulation at the interface between a spin-orbit metal and a magnetic insulator that could lead to a net pure spin current flowing from the metal into the insulator. This spin current will induce a torque on the magnetization and eventually could drive it into steady motion. The magnetization dynamics can be detected electrically on the metal using the inverse spin Hall effect (ISHE). As a ferromagnetic insulator with a very low Gilbert damping, Yttrium Iron Garnet (YIG) is a very promising candidate to investigate pure spin current phenomena [1.2]. In a previous study, we have been able to observe auto-oscillation induced by SHE torques in 20 nm thick YIG disks [3]. In this work, we have gone towards 1D microstructure to observe the effect of SHE torques on this extended system that can be important for potential magnonics application [4]. The samples used are 1 ¿m wide YIG (20 nm)\Pt (7 nm) stripes that have been lithographically defined. DC electrical contacts separated by 80 ¿m are also defined on the top of the stripes to inject ac-current and detect the ISHE voltage. A lock-in detection technique is used to remove the Ohm¿s law contribution and we measure simultaneously two identical stripes in parallel with a bridge measurement configuration to observe any SHE torques signal in the second harmonic voltage (Fig. 1(a)). Due to Joule heating by passing current in the Pt layer, a spin Seebeck effect (SSE) signal can also be detected in the ISHE voltage [5,6]. The angular dependence of the second harmonic voltage shows mostly a 180° period signal in the film plane in agreement with the SHE (ISHE) symmetry of the injected (detected) spin current in the measurement configuration (Fig. 1(b)). A minor contribution also appears showing a dependence in agreement with the SSE signal where only the direction of the YIG magnetization is relevant. Thanks to symmetry consideration, we can disentangle any thermal effects from torques effects and notice a much larger signal coming from SHE torques. When we reach the threshold current for the onset of auto-oscillations that was observed by ¿-BLS on the same stripes [7], we obtain an abrupt decrease in the amplitude of the SHE voltage. We attribute this behavior to strong non-linear mode coupling in the excited magnetization dynamic. The interplay between thermal and coherent modes excited by SHE torques will be discussed. This study shows the excitation of YIG dynamics by SHE torques and its electrical detection via ISHE, separately from the SSE signal, in a YIG/Pt stripe.