Introduction

One current area of significant technological interest is the development of fast Magnetic Random Access Memory (MRAM) for computers. Currently, traditional forms of RAM utilize electric signals to read and write information, and require constant power to maintain a memory state, making the memory volatile. MRAM is an alternative, but currently requires a strong current to flip bits. Inducing spin-currents, a net flow of electrons that are spin biased, to apply a torque to the unit cell (known as spin-transfer torque, STT) in MRAM reduces the current to flip a bit allowing MRAM to be both more precise and use less power than traditional RAM. In addition, magnets are non-volatile in nature, since the induced magnetization will remain even after power is turned off, which means that MRAM could feasibly even be used as an ultra-high speed replacement for disk drives if produced cheaply enough.

The realization of MRAM, however, requires an appropriate magnetic material which has a strong enough STT response to flip the magnetization of a bit when a write operation is preformed. Ferromagnetic Res- onance (FMR) has long been used as a tool in order to probe the magnetic properties of materials that are potential candidates for MRAM [4]. It is a spectroscopic technique, which is preformed by injecting microwaves into a magnetic sample to drive a precession of the net magnetic moment about an applied external field. It is useful, as it allows for a direct measurement of the Gilbert Damping Constant, which would ideally be as low as possible to allow for STT to flip a bit efficiently with a low applied current density [8].

Theoretically, the Gilbert Damping of a pure ferromagnetic crystal should be very low. However, in practice, impurities in the crystal provide extrinsic mechanisms that can increase the damping of a magnetic sample to be much higher than what it would intrinsically be for a pure crystal. The effects of extrinsic damping become much more prominent at high input microwave power where nonlinear effects also become important in governing the precession of the magnetic moment. As such the goal of this experiment is two- fold: (1) FMR will be used to find the Gilbert Damping Constant of YIG to determine its applicability to MRAM applications, and (2) the input microwave frequency and power will be increased in order to better understand damping effects in the nonlinear FMR regime.