Oscillation Stability of a Small Size Spin Torque Oscillator for MAMR

Oscillation behavior of a spin torque oscillator (STO) less than 40 nm in size was investigated by using micromagnetic simulations for microwave-assisted magnetic recording (MAMR) over 5 Tb/in². Decreasing the STO size from 40 to 20 nm deteriorates the stability of a reference layer (REF) and it shrinks the stable oscillation region of a field generation layer (FGL). Enhancing the magnetic anisotropy energy of the REF K_{u_REF} stabilizes the magnetization motion of the REF against the spin transfer torque (STT) effect and the external magnetic field H_z, and the stable oscillation region of the FGL expands within a relative lower applied voltage V_sto region, but random oscillation with fluctuating elevation angles appears at a higher H_z and V_sto. Similar phenomenon is observed when increasing the thickness of the REF t_REF. Enhancing both K_{u_REF} × t_REF and the thickness of FGL are found to stabilize oscillation of the FGL against the STT effect from a stable REF at a larger strength of H_z and a higher V_sto without the appearance of random oscillation, making it possible to apply about 20-nm-square STO to a magnetic head for over 5 Tb/in².