Quantitative Measurements Of Spin Transfer Torque Induced Dynamics In Nanomagnets

Spin transfer torque is a torque exerted on a magnet by transferring spin angular momentum from a current to the magnet. It enables efficient manipulation of nanomagnets using current and can enable important applications. This dissertation focuses on doing measurements of magnetic dynamics using spintorque-driven ferromagnetic resonance (ST-FMR), a technique that gives quantitative information about device parameters. This technique not only leads to deeper understanding of spin torque devices but can also provide an improved way to characterize devices for applications. In a spin torque device such as a magnetic tunnel junction, a microwave current can drive small magnetic oscillations, which yields an oscillating resistance. If a DC current is applied at the same time, an oscillating voltage will be generated by Ohm's Law. The first project in this dissertation makes use of this RF frequency oscillating voltage to perform a quantitative measurement of spin torque and magnetic damping of the device. The second project discusses the possibility of making this voltage larger than the input voltage and thus producing a microwave amplifier. The same type of magnetic dynamics can be excited using nonlocal spin torque from a pure spin current. In this dissertation, I also discuss how to quantitatively measure the nonlocal spin torque in a 3-terminal device by adapting the DC-detected ST-FMR technique. Apart from being detected by electrical measurement, the same magnetic dynamics can be directly imaged using X-ray Magnetic Circular Dichroism. I will use a chapter in this dissertation to discuss our progress in doing so and studying magnetic normal modes, the fundamentals of magnetic dynamics. Last but not the least, in addition to measure conventional metal devices, I will talk about our effort in fabricating and measuring spin torque switching in ferromagnetic semiconductor devices.