High-frequency spin torque oscillation in orthogonal magnetization disks with strong biquadratic magnetic coupling

In this study, we numerically investigate the spin transfer torque oscillation (STO) in a magnetic orthogonal configuration by introducing a strong biquadratic magnetic coupling. The orthogonal configuration consists of top and bottom layers with in-plane and perpendicular magnetic anisotropy sandwiching a nonmagnetic spacer. The advantage of an orthogonal configuration is the high efficiency of spin transfer torque leading a high STO frequency; however, maintaining the STO in a wide range of electric current is challenging. By introducing biquadratic magnetic coupling into the orthogonal structure of FePt/spacer/Co₉₀Fe₁₀, Ni₈₀Fe₂₀ or Ni, we were able to expand the electric current region in which the stable STO is realized, resulting in a relatively high STO frequency. For example, approximately 50 GHz can be achieved in an Ni layer at a current density of 5.5 × 10⁷ A/cm². In addition, we investigated two types of initial magnetic state: out-of-plane and in-plane magnetic saturation; this leads to a vortex and an in-plane magnetic domain structure after relaxation, respectively. The transient time before the stable STO was reduced to between 0.5 and 1.8 ns by changing the initial state from out-of-plane to in-plane.