Stability study of spin torque oscillator for microwave assisted magnetic recording (MAMR)

MAMR is one of the technologies which could push recording density up to 3-4 Tb/in² [1-4]. The center part of MAMR is the spin torque oscillator (STO) for the generation of localized ac magnetic field in the microwave frequency regime of 20-30 GHz[5-6]. The STO is placed between the main pole and the trailing shield (Fig. 1a), where a strong magnetic field (gap field) exists. The gap field acts on the STO and greatly affects its performance [7]. Due to very low flying height of 2-3 nm, the STO also senses the field from recording media (Fig. 1a). The media field roughly lies in the magnetization-precession plane of the free layer (field generation layer or FGL) (Fig. 1b), which alters the FGL energy landscape and therefore disturbs its precession, causing STO instability. This is one of the key concerns for MAMR. Using micromagnetic simulation, we studied the STO stability against the gap field and the media stray field. It is found that the K_u of the reference layer, the gap field and the media stray field have a big effect on the STO stability. Possible approaches to enhancing the STO stability are proposed.