MICROWAVE STUDIES OF FERROMAGNETIC RESONANCE, MAGNONIC CRYSTALS, AND PASSIVE EARTH REMOTE SENSORS

Abstract

This thesis collects the work and results of three microwave experiments for materials science applications in the contexts of data storage, communications, and passive remote sensing. The first topic measures the Gilbert damping constant of magnetic materials at elevated temperatures. This is performed using a setup for cavity-based ferromagnetic resonance (FMR) measurements adapted to reach and measure at temperatures above 700 K. Significant effort is being made in the digital storage industry to increase the areal density of magnetic storage by using techniques such as heat-assisted magnetic recording (HAMR) and spin-transfer-torque (STT) recording. The Gilbert damping constant is directly related to the efficiency of magnetic storage because a greater damping constant corresponds to more energy required to flip the orientation of the magnetic dipole moment that represents a digital bit. Characterizing the performance at the high effective temperatures required by HAMR and expected in STT devices has not been widely available due to limitations of typical FMR measurement systems. This work demonstrates measurements of the Gilbert damping constant in STT magnetic random-access memory (MRAM) free layers at temperatures up to 560 K. A typical STT MRAM free layer design, which includes a relatively thick layer of tungsten, is compared to a novel design where the tungsten layer has been split into two thinner layers. The results show that the novel design has a reduced damping constant compared to the traditional design, and that this advantageof the novel design grows with inreasing temperature. Discussed second are measurements of spin wave propagation through thin films. The propagation is manipulated using magnonic crystals: spin-wave waveguides that have non-uniform magnetic properties affecting the transmission of the wave. For example, a thin film with spatially periodic features such as grooves will support destructively-interfering reflections satisfying the Bragg condition, resulting in wavenumbers that are forbidden to propagate. The effect of Bragg reflections is observed as strong dips in transmission amplitude (functioning as a notch filter) and as band gaps in the dispersion relation. The design space for magnonic crystals remains relatively unexplored, and a number of nonlinear effects are predicted to be observed in designs with increased complexity. In pursuit of this, a nanofabrication recipe for yttrium iron garnet thin films was developed using photolithography and wet etching. A doubly-periodic magnonic crystal with two alternating lattice periods was fabricated and the transmission of surface spin waves was measured using a vector network analyzer. It was observed that the transmission dips were stronger at wavenumbers satisfying the Bragg condition for both lattice periods, which is a distinct result from singly-periodic designs, and that the wavenumbers satisfying the Bragg condition corresponded to an effective period equal to the sum of the two periods. Presented third is the emissivity characterization of a novel blackbody calibration target for use in passive remote sensing microwave radiometers. State-of-the-art blackbody calibration targets are microwave-absorbing epoxies that are ferrite-loaded and arranged in arrays of pyramids to maximize loss. These targets are excellent approximations to a blackbody, meaning that the radiometric brightness temperature measured by the radiometer is approximately equal to the physical temperature of the target. However, state-of-the-art targets are disadvantaged by their mass and volume and by thermal gradients that exist across thepyramids, which is a source of uncertainty that can be difficult to characterize. Calibration targets based on metamaterial absorbers are presented that address both of the aforementioned disadvantages in pyramidal targets via their planar design. Monostatic measurements of the metamaterial-based blackbody calibration targets show their emissivity as a function of frequency in bands relevant to remote sensing of the Earth’s atmosphere, and nearly all of the targets reach an absorptivity target of 30 dB. A radiometer is used to support the result at the highest frequencies with measurements that directly determine emissivity from the physical and radiometric temperatures of the metamaterial.