High-frequency phenomena in magnetic recording and inductive devices

At high frequencies (>1 GHz), ferromagnetic materials and associated electronic circuitry show interesting and sometimes undesirable behavior. In this dissertation, we examine high-frequency effects in magnetic recording and magnetic inductive devices. We analyze "impedance profiling" of the disk drive interconnect, as a way of shaping the write current waveform. This proves to be useful under somewhat limited conditions (for write head with low impedance, characteristic time of the shaped waveform less than the one-way interconnect propagation delay). We then analyze a buffer amplifier (consisting of a single transistor in an emitter-follower configuration) as a means of improving the electronic signal to noise ratio (SNR) associated with high-resistance read sensors. We develop and utilize a "matched filter bound" SNR for assessing the performance of the disk drive read-path. For a hypothetical recording system at an areal density of 1 terabit/in2, the buffer amplifier improves SNR anywhere from 0.5 dB for 670 Mb/s up to 1 dB for 4.17 Gb/s. We then present measurements and quantitative analysis for magnetic fluctuation noise in read sensors. The analysis is enabled by rigorous calibration of the noise measurement setup. We are able to explain the behavior of the mag-noise (primary) resonance frequency versus bias current and externally-applied field, by using a micromagnetic model (NIST-OOMMF) where we also account for sensor heating and associated reduction in free-layer and biasing magnet saturation moment. We then analyze the behavior of multi-domain magnetic materials and the associated inductive device behaviors. First we utilize micromagnetic modeling to calculate the spin-resonance modes associated with multi-domain films. We find agreement in trend between the modeling results and experimentally-observed sub-FMR permeability resonances, particularly that both model and experiment predict a power-law dependence of frequency on the ratio of thickness to mode number. Following this, we utilize finite-element modeling software (Ansoft HFSS) to model the circuit behavior of multi-domain inductive devices. We find that the inductance/quality factor of "closed" magnetic devices (like toroids) are substantially worsened by the presence of a multi-domain pattern, while the multi-domain pattern has practically no impact on circuit behavior of "open" magnetic devices (like solenoids). We specifically model CoFeHf-O being used in a solenoidal device, and we find that the degraded "soft" magnetic material properties when deposited on rough PCB substrate, contributing to especially large spin-dynamic damping, have more impact on the circuit response than the multi-domain pattern.