Macrospin and micromagnetic simulations of spintronic devices for magnetic sensors and oscillator applications

Abstract

In this thesis, by categorizing the application of spintronic devices with the standard of signal type being processed, the spintronic devices based magnetic field sensors which process the D.C. signal are studied with micromagnetic simulation and the spintronic devices based spin-torque oscillators which process the A.C. signal are studied with macrospin simulation.

By conducting micromagnetic simulation, the thermally excited mag-noise in spintronic device based magnetic field sensors is systematically studied. In magnetic tunnel junctions (MTJ) based magnetic field sensor, the spatial distribution of the thermally excited mag-noise indicates that the edges are the main contributor of thermal mag-noise in the free layer (FL). Both hard bias (HB) field and applied field could suppress the thermal mag-noise in edges. A relatively high applied field will decrease the influence of HB field on mag-noise in edges. The edge effect is not applicable for MTJ sensors with circular cross section. In ferromagnetic ring structure based magnetic field sensor, the saturated state, triangle state, half triangle state, onion state, and vortex state are explored and studied, respectively. The mag-noise calculation shows that triangle state is the main reason for the mag-noise exhibiting 1/f tendency in both the low-frequency range and high-frequency range in relaxed state, while the onion state explains why a noise peak appears in high-frequency range in the relaxed state. It is proved that the area of the ferromagnetic rings is not the determining factor for the thermal mag-noise distribution in the saturated state. In dual-vortex structure based magnetic field sensor, the combination of the dual-vortex motion and the magnetic noise properties make it possible to measure the external field (along hard bias direction) through measuring the FMR peak positions or the integrated thermal mag-noise, which indicated two novel field sensing mechanisms using elliptical permalloy single layer. Besides the study of the thermally excited mag-noise in spintronic device based magnetic field sensors, the spintronic device based spintorque oscillators (STOs) is fully investigated by macrospin simulation. Conclusions demonstrate numerically and analytically how a STO locks to a microwave field (Hac). A magnetic energy based analysis is used to explain this phenomenon. This result provides a possible way to synchronize serially connected STOs by tuning each single STO’s phase shift with external microwave field, which could finally enhance the locking efficiency, locking range and output power of serially connected STOs. Meanwhile, the capacitance effect on the oscillation characteristics and the switching characteristics of the STOs has also been studied.

The micromagnetic simulation of the noise sources in traditional GMR/TMR based magnetic field sensor and novel spintronic device based magnetic field sensors not onlyprovide reliable explanations of noise-related phenomenon in magnetic field sensors but also offer guidance on how to fabricate magnetic field sensors with relative low thermal magnetic noise and high performance. Meanwhile, the macrospin study of the spin-torque oscillators which process A.C electrical current signal has provided theoretical fundamentals for next generation microwave generator.