Design of spin-injection-layer in all-in-plane spin-torque-oscillator for microwave assisted magnetic recording

摘要

A new type of spin-torque-oscillator (STO), all-in-plane STO, is introduced for microwave assisted magnetic recording (MAMR) that consist of a spin-injection-layer (SIL) and a field-generating-layer (FGL) with an effective in-plane easy axis due to the shape anisotropy separated with a metallic spacer. In this device, electrons are injected from SIL to FGL while the magnetization of the SIL and FGL is saturated to the out-of-plane by the external magnetic field of similar to 1.0 T. Micromagnetic simulations showed that the magnetization direction of SIL can be switched to the opposite direction to that of the applied external magnetic field by the use of spintransfer-torque. This results in a larger spin accumulation in FGL and its oscillation with a large cone angle at a low bias current density. We designed SIL to reduce the critical current density, J(c), required for the magnetization switching of SIL. Materials with a smaller saturation magnetization in SIL reduce J(c). Smaller spin polarization of SIL leads to a larger spin accumulation in SIL with an opposite direction to the magnetization, resulting in a reduction of J(c). This enables magnetization switching of SIL in small J(c) followed by oscillation of FGL with frequency above 20 GHz with a large out-of-plane oscillation cone angle of 45-50 degrees. The validity of this finding was studied experimentally by developing STO with two SIL materials, Co2Fe(Al0.5Si0.5) Heusler alloy and Fe67Co33, the former has the B2 crystal structure with a large spin polarization and the latter has the A2 crystal structure with a smaller spin polarization. The magnetization configuration of SIL and FGL in STO with a diameter of similar to 60 nm is investigated experimentally based on the field dependent resistance change and the oscillation behavior is discussed based on the micromagnetic simulations.