Spin-Torque Oscillations in Magnetic Tunnel Junctions

Abstract

Spin-torque-induced magnetization oscillations in nanoscale magnetoresistive devices have been studied extensively due to their numerous potential applications, such as hard disk drive read head sensors, neuromorphic computing, and telecommunications. For real applications, the microwave signal should have high power, narrow linewidth, and tunable frequency. In this dissertation, I have investigated the spin-torque-induced magnetization oscillations by numerical simulations and experiments and studied the influence of size and shape on the spin-torque oscillators, which might be important for device optimization. I have also investigated the spin-torque oscillation modes of a composite synthetic antiferromagnetic free layer in MgO-based dual magnetic tunnel junctions (MTJs).The impact of device size on the characteristics of spin-torque oscillators has been investigated by studying the oscillator’s frequency and linewidth as a function of current density at room temperature. Upon reducing the device size from 40 down to 10 nm, the thermal noise flattens the frequency and linewidth trends with current density and lowers the threshold current density of the oscillation. The magnetization vector trajectories reveal that irregular oscillations exist in smaller devices. The findings suggest that a 20 × 20 nm2 spin-torque oscillator could be a viable candidate for a magnetic read sensor. Previous studies have indicated that the spin-torque oscillation linewidth depends on the temperature, current, and in-plane field angle. In this dissertation, the spin-torque oscillations in MgO-based MTJs have been investigated, and the impact of MTJ shape anisotropy on the threshold current has been demonstrated. The experimental results suggest that the linewidth is different in the MTJs with different shape anisotropy due to different threshold currents. The spin-torque oscillation modes of a composite synthetic antiferromagnetic free layer in MgO-based dual MTJs have been studied by analyzing the field- and current-dependent power spectra. Two oscillation modes are observed due to the unique layer structure. With increasing field, the first mode shows a frequency reduction at low magnetic fields followed by a frequency increase at high magnetic fields, while the second mode reveals a frequency increase at both low and high magnetic fields. The current-dependent power spectra of these two modes are also different. With increasing current, the frequency of the first mode drops at low magnetic fields and rises at high magnetic fields, while the other decreases at low and high magnetic fields. These two modes could be tentatively understood by considering the exchange coupling of the synthetic antiferromagnetic free layer.