Date of Award

Spring 2021

Document Type

Open Access Dissertation

Department

Electrical Engineering

First Advisor

Guoan Wang

Abstract

Time-domain measurements are made on a magnetite-based (Fe3O4) ferrofluid using an inductive technique. The constituent particles are 5.8% by volume, polydisperse, and have a nominal diameter of 10nm with a ~1nm-thick anionic hydrophobic coating. The ferrofluid is placed in a sealed channel on a coplanar waveguide (CPW) situated in an adjustable external magnetic bias field. A fast-rising step current in the CPW quickly reorients the local magnetic field above the signal trace causing the particles’ moments to align in the new field configuration. This changing magnetization induces a voltage in the CPW that is detected by a sampling oscilloscope. Precessional magnetization dynamics are observed as well as phenomenological damping predicted by the Landau-Lifshitz equation. Frequency analysis is done on the signals using a Fast Fourier transform (FFT) and reveal multiple resonances which vary as a function of the applied field. Computational models employing a macrospin approximation are shown to agree well with the time-domain data and reveal the response is due to interparticle interactions and chaining effects. The effective field of a particle in a typical chain structure is derived analytically and found to agree with the results. A small fraction of the particles exhibit an apparent increased anisotropy. This is postulated to result from surface effects of the smallest particles since the fitted effective surface anisotropy constant agrees well with previously published results. Evidence of intrinsic superparamagnetic behavior is also observed.

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