Spin momentum transfer effects for spintronic device applications
Abstract: The recent discovery that a spin-polarized current can exert a large torque on a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of electrical current controlled manipulation of magnetic moment in nanoscale magnetic device structures. This so-called spin torque effect holds great promise for two applications, namely, spin torque oscillators (STOs) for wireless communication and radar communication, and spin transfer torque RAM (STT-RAM) for data/information storage. The STO is a nanosized spintronic device capable of microwave generation at frequencies in the 1-65 GHz range with high quality factors. Although the STO is very promising for future telecommunication, two major shortcomings have to be addressed before it can truly find practical use as a radio-frequency device. Firstly, its very limited output power has to be significantly improved. One possibility is the synchronization of two or more STOs to both increase the microwave power and further increase the signal quality. Synchronization of serially connected STOs has been suggested in this thesis. In this configuration, synchronization relies on phase locking between the STOs and their self-generated alternating current. While this locking mechanism is intrinsically quite weak, we find that the locking range of two serially connected spin-valve STOs can be enhanced by over two orders of magnitude by adjusting the circuit I-V phase to that of an intrinsic preferred phase shift between the STO and an alternating current. More recently, we have also studied the phase-locking of STOs based on magnetic tunnel junctions (MTJ-STO) to meet the power specifications of actual application where the rf output levels should be above 0 dBm (1 mW). In addition to the spin torque terms present in GMR spin valves, MTJs also exhibit a significant perpendicular spin torque component with a quite complex dependence on both material choices and applied junction bias. We find that the perpendicular torque component modifies the intrinsic preferred I-V phase shift in single MTJ-STOs in such a way that serially connected STOs synchronize much more readily without the need for additional circuitry to change the I-V phase. Secondly, equal attention has been focused on removing the applied magnetic field for STO operation, which requires bulky components and will limit the miniaturization of STO-based devices. Various attempts have been made to realize STOs operating in zero magnetic field. By using a tilted (oblique angle) polarizer (fixed layer) instead of an in-plane polarizer (standard STO), we show zero field operation over a very wide polarizer angle range without sacrificing output signal. In addition, the polarizer angle introduces an entirely new degree of freedom to any spin torque device and opens up for a wide range of additional phenomena. The STT-RAM has advantages over other types of memories including conventional MRAM in terms of power consumption, speed, and scalability. We use a set of simulation tools to carry out a systematic study on the subject of micromagnetic switching processes of a device for STT-RAM application. We find that the non-zero k spin wave modes play an important role in the experimentally measured switching phase boundary. These may result in telegraph transitions among different spin-wave states, and be related to the back-hopping phenomena where the switching probability will decrease with increasing bias in tunnel junctions.
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