We numerically studied the spin transfer torque oscillation (STO) in a magnetic orthogonal configuration by introducing a strong biquadratic magnetic coupling 1. The advantage of the orthogonal configuration is the high efficiency of spin transfer torque leading the high STO frequency, but it is difficult to maintain the STO in a wide range of electric current. By introducing a biquadratic magnetic coupling into the orthogonal structure, it became possible to expand the electric current region realizing the stable STO, resulting that a high STO frequency. In this report, we systematically compared STO properties between the orthogonal magnetization disks with and without the biquadratic magnetic coupling, that is, B12=0.0 and -0.6. In our model, the orthogonal configuration is FePt 2 nm/spacer 2 nm/Co90Fe10, or Ni80Fe20, or Ni 2 nm. The time-domain magnetization precessions of the top layer Ni under current density J=0.5×107 A/cm2, 3.0×107 A/cm2, and 6.0×107 A/cm2 are shown in Fig.1 (a), (b) and (c). Only B12 was changed and A12 is set 0 for simplicity. In the case of the biquadratic magnetic coupling B12=0.0, the STO was observed only when J=0.5×107 A/cm2. Compared to this, in the case of B12=-0.6, the STO can be obtained in the wide range of J. This means that by introducing biquadratic magnetic coupling, we can expand the electric current region realizing the stable STO and increase the frequency. In Fig. 2, we show the STO frequency and the intensity as a function of the current density. First, even when the top layer is changed from Ni, to Ni80Fe20, or Co90Fe10, the current density region realizing the stable STO is widened by introducing the biquadratic magnetic coupling. In addition, we increased STO frequency and the intensity, meaning the stable STO.

References
1 C. Liu, et al. IEEE Trans. Mag., 58 (2022) 172504.