Reduction of critical current density for out-of-plane mode oscillation in a mag-flip spin torque oscillator using highly spin-polarized Co_2Fe(Ga_(0.5)Ge_(0.5)) spin injection layer

摘要

We study spin torque oscillators comprised of a perpendicular spin injection layer (SIL) and a planar field generating layer to reveal the influence of the spin polarization of SIL material on the critical current density, J_C, to induce microwave oscillation. Two systems with different SIL are compared: one with a highly spin-polarized Heusler alloy, Co_2Fe(Ga_(0.5)Ge_(0.5)) (CFGG), and the other a prototypical Fe_2Co alloy. Cross sectional scanning transmission electron microscopy observations show the B2-ordered structure in a 3-nm-thick CFGG SIL, a prerequisite for obtaining half-metallic transport properties. Current induced microwave oscillations are found at frequencies of ～15 GHz for both systems. However, the current needed to cause the oscillations is ～50% smaller for films with the CFGG SIL compared to those of the Fe_2Co SIL. These results are in accordance with micromagnetic simulations that include spin accumulation at the SIL.