Spin transfer torque (STT) induced magnetization switching and oscillation in nanometer scale magnetoresistance (MR) devices have been studied intensively due to its direct application in the non-volatile STT random access memory (STT-RAM) and its potential application in the high frequency spin torque oscillatior (STO). STO could be used in high-density microwave signal processor and chip-to-chip communication system due to its nanometer scale footprint and ultra high oscillation frequency. STO has also been suggested in the magnetic recoridng head for the microwave assisted magnetic recording (MAMR) and in the high-speed magnetic reader for future hard disk drive. However, several critical engineering challenges for those exciting STO applications are still remaining, including optimizing the operating condition, tuning the frequency, narrowing the linewidth and improving the output power.In this thesis work, the spin transfer torque induced oscillation is experimentally studied in MgO barrier based magnetic tunnel junctions (MTJs) with focus on the improvement of the key performances of the STO. The power angular dependence of spin torque oscillation is experimentally studied in dual MgO barrier MTJs based on the understanding of the relation between the MR and oscillation output power. It is proved that the STO electrical power increases with the polarizer canting angle. Meanwhile, the results also reveal a solution for extending the oscillation operating condition. Furthermore, the MTJ based STO device with a built-in hard axis polarizer is designed and studied. This design provides an external-field-free STO with high power, low critical current and extended operation range of the driving current for the first time. Additionally, two oscillation modes in the dual MgO barrier MTJs are observed and investigated. It is found from the field-dependent power spectra that the extra oscillation mode may come from the weakly-pinned top reference layer. A single-shot time-domain measurement to characterize the switching time of each switch under different voltages for MTJs in the nanosecond precessional regime was carried out too.
University of Minnesota Ph.D. dissertation. May 2014. Major: Electrical Engineering. Advisor: Jian-Ping Wang. 1 computer file (PDF); ix, 122 pages, appendix A.
Spin transfer torque induced oscillation and switching in magnetic tunnel junction.
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