Size dependence of vortex-type spin torque oscillation in a Co₂Fe_0.4Mn_0.6Si Heusler alloy disk

This paper reports the systematic investigation of vortex-type spin torque oscillation in circular disks of highly spin-polarized Co₂Fe_0.4Mn_0.6Si (CFMS) Heusler alloys. We fabricated the current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) devices with various disk diameters (D) using the layer stack of CFMS/Ag₃Mg/CFMS. The gyrotropic motion of the vortex core was successfully excited for the CFMS circular disks with 0.2 μm D 0.3 μm. The CPP-GMR device with D = 0.2 μm exhibited the Q factor of more than 5000 and the large output power of 0.4 nW owing to the high coherency of vortex dynamics and the high spin-polarization of CFMS. However, the Q factor was remarkably decreased as D was reduced from 0.2 μm to 0.14 μm. The comparison with the calculated resonance frequencies suggested that this degradation of the Q factor was due to the transition of the oscillation mode from the vortex mode to other modes such as the low-coherent out-of-plane precession mode. The present experimental results also suggest that there exists an adequate disk size for the enhanced Q factor of the vortex-type spin torque oscillation.