[14p-2J-5~18] 10.2 Spin torque, spin current, circuits, and measurement technologies

▲ [14p-2J-13] Reduction of bias current density using Heusler alloy spin injection layer in mag-flip spin-torque oscillator devices for microwave-assisted magnetic recording

〇(P)Subrojati Bosu¹, Hossein Sepehri-Amin¹, Yuya Sakuraba¹, Masamitsu Hayashi¹, Kazuhiro Hono¹ (1.National Institute for Materials Science)

Keywords:MAMR,Heusler alloy,STO

The main challenges of microwave assisted magnetic recording (MAMR) for next generation high areal density magnetic recording are development of a mag-flip spin torque oscillator (STO) [1] consisting of the in-plane magnetized field generating layer (FGL) and the perpendicular magnetized spin-injection layer (SIL) that is able to generate a large H_ac from FGL with a frequency over 20 GHz at small bias current density J_C < 1.0 X 10¹² A/m² [2]. We have investigated the oscillation behavior of a mag-flip STO device (Fig. 1(a)) with 100 nm diameter circular pillar using highly spin polarized ferromagnetic Heusler alloy, Co₂Fe(Ga_0.5Ge_0.5) (CFGG), perpendicularly magnetized with FePt as SIL to reduce J_C, and also compared with a typical ferromagnetic alloy CoFe SIL. ΔR-H_ex curves in Fig. 1 (b) and (c) for CFGG and CoFe SILs, respectively, for various negative dc bias currents I_dc with H_ex applied perpendicular to film plane (θ ~0°) reveals higher MR values for CFGG SIL than CoFe SIL. Moreover, for CFGG SIL when |I_dc| > 5.5 mA a sudden jump to the intermediate resistance state at high H_ex region in the ΔR-H_ex curves indicates excitation of magnetization dynamics by the reflected spin current from the SIL interface. On the other hand, for CoFe SIL intermediate resistance state appears for |I_dc| > 13 mA. Such difference in MR and I_dc seems to be due to the higher spin polarization in CFGG than CoFe. Figures 1(d), and (e) show, ΔR-H_ex for |I_dc| > 9 mA, and corresponding rf signals for CFGG SIL respectively, with H_ex slightly tilted θ ~7° from the film normal. A maximum f ~12 GHz has been observed for H_ex ~ 10 kOe, which systematically decreases following Kittle’s equation. Moreover, the blue shift of f with I_dc (not shown here) also confirms detection of oscillation perpendicular to plane for the bias current density J_C ~ 0.95 to 1.15 × 10¹² A/m², which is close to the desired J_C for application.