Spin dynamics of spin torque oscillator in zero-field regime

Abstract

Spin torque oscillator has been considered as next generation microwave oscillator as it has small size that enables on-chip integration and good oscillation signal qualities. Although it is most desirable to have working spin torque oscillator in zero external magnetic bias condition for on-chip integration, the requirement of such condition accompanies with a serious degradation in working frequency range of the device. In this paper we show that very small spin torque oscillators shows high frequency oscillation behavior in zero field regime due to stray magnetic field generated by magnetic loop made in anti-parallel state of magneto tunnel junction. The oscillator shows three mode of oscillation under zero-field condition as the applied current level increases. Each of the modes operates in different frequency range with different power which enables them to be used for different application purposes. Effect of shape on the oscillation properties and modes are also speculated. In addition new way to tune the frequency of spin torque oscillator is suggested using mechanical stress. The analysis reveals that real-time mode optimization is possible in similar way that was done by shape anisotropy. It is also shown that frequency modulation of spin torque oscillator is more dominant when ferromagnetic layer of device is composed of material with lower saturation magnetization.