R&D: Design Spin-Injection-Layer in All-in-Plane Spin-Torque-Oscillator for Microwave Assisted Magnetic Recording

Journal of Magnetism and Magnetic Materials has published an article written by H.Sepehri-Amim,W.Zhou, S.Bosu, Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Tsukuba 305-0047, Japan, C.Abert, Christian Doppler Laboratory – Advanced Magnetic Sensing and Materials, University of Vienna, Austria, Y.Sakuraba, S.Kasai, D.Suess, and K.Hono, Research Center for Magnetic and Spintronic Materials, National Institute for Materials Science, Tsukuba 305-0047, Japan.

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Abstract: “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 ∼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 spin-transfer-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^o. The validity of this finding was studied experimentally by developing STO with two SIL materials, Co₂Fe(Al_0.5Si_0.5) 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 ∼60 nm is investigated experimentally based on the field dependent resistance change and the oscillation behavior is discussed based on the micromagnetic simulations“.