Evaluation of interaction between local magnetization dynamics and spin waves measured by ST-FMR

Abstract

The control of magnetization dynamics is an important issue for the development of high-frequency devices based on spintronics. There are several ways to excite magnetization dynamics. In our previous study, we reported the amplification of magnetization dynamics derived from the interaction between local magnetization dynamics and spin waves by using a slot line waveguide [1].In this paper, we report the study on the evaluation of the interaction by measuring ST-FMR. The spin orbit torque is caused by the spin current induced by the spin hall effect in ferromagnetic and non-ferromagnetic bilayer system. The ST-FMR is originally used for the estimation of the spin orbit torque via the measurement of the spin hall magneto resistance. We found that the ST-FMR signals for the samples described below delivered information on the interaction between local magnetization dynamics and spin waves. Fig.1 shows the sample structure and measurement set up for the ST-FMR. We used yttrium iron garnet (YIG) as a ferromagnetic layer. Coplanar waveguides were prepared on the YIG film. The signal line consisted of Cu layer (200 nm) , and was partially replaced by 10nm thick Pt layer. In the separated Pt areas, additional local magnetization dynamics was excited by the spin orbit torque, emitting spin waves. The distance between Pt areas was systematically varied, leading the change of the strength of the interaction between the local magnetization dynamics and spin waves at the Pt areas. Fig.2 shows the distance between Pt areas dependence of ST-FMR signal measured at 7.0 GHz. The sample with one Pt shows symmetrical peaks. On the other hand, the samples with two Pt areas indicate that the asymmetry of the peaks changes with the distance of Pt areas. The symmetry and asymmetry of the peaks reflect the contribution of torques to the magnetization dynamics from the spin orbit torque and RF magnetic field respectively at Pt areas [2]. It should be noted that the strength of RF magnetic field was fixed for all the samples, meaning that the increase in the asymmetry was caused by an additional torque to the magnetization. Another factor is speculated from the interaction between local magnetization dynamics and spin waves because the direction of the torque for the interaction is considered the same as the direction for the RF Oersted field, leading the asymmetry of the peak for the ST-FMR measurement. The asymmetry of the curve showed strong dependence on the distance between Pt areas, reflecting the strength of the interaction. Those experimental results give a way to design the artificially controlled magnetization dynamics effectively. We thank GRANOPT Co,Ltd. for their support of YIG single crystal thin films. This work was supported in part by JSPS KAKENHI Grant Number JP18K14114. This work was supported in part by CSRN, Tohoku University. Part of this work was carried out under the Cooperative Research Project Program of the Research Institute of Electrical Communication, Tohoku University. A part of this work was carried out at Nano-Fabrication Support Laboratory, Yamaguchi University supported by "Nanotechnology Platform Project" of the Ministry of Education，Culture，Sports，Science and Technology (MEXT)，Japan. **