Date of Award

Spring 2021

Document Type

Open Access Dissertation

Department

Physics and Astronomy

First Advisor

Thomas M. Crawford

Abstract

Bi-phasic composite multiferroics couple piezoelectric and magnetostrictive proper- ties via an interfacial strain, allowing for control of their magnetic properties with electric fields and vice versa. Nanofibers have a larger area-to-volume ratio than thin films and are not affected by substrate clamping, leading to a predicted magnetoelec- tric coupling an order of magnitude higher than those found in thin-film multiferroics. Nanofibers have potential applications in photonics, nanoelectronics, biosensing, and optoelectronics. This work focuses on measuring magnetoelectric effects in electro- spun nanofibers made of barium titanate (BaTiO3) and cobalt ferrite (CoFe2O4). To transform the disordered as-spun mat of fibers into a functional architecture for devices, they are ground to various lengths with an average diameter of ∼800 nm, and then magnetically self-assembled in a polymer solution. Temperature-dependent magnetometry shows that cobalt ferrite and barium titanate are coupled, confirmed with an observed magnetization shift at ∼393 K. I studied the self-assembly of these fibers in an external magnetic field and observed that the fibers chain end-to-end with different dynamics compared to magnetic nanoparticles. Due to geometric and elec- trochemical effects, in-fluid chaining proved unsuccessful as an in-situ probe of the magnetoelectric coupling of these promising multiferroic nanomaterials. I successfully used a novel scattered magneto-optical Kerr effect geometry to probe voltage-induced changes in magnetization. The observed magnetoelectric effects show 50-110 Oe/V changes in coercivity, typically non-hysteretic behaviors, and “collapsing” hysteresis loops. In conclusion, I successfully tested new techniques to measure the magneto- electric effects in self-assembled multiferroic nanofiber aggregates.

Rights

© 2021, Bryan Lucas Chávez

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