Date of Award

2018

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy in Materials Science and Engineering (PhD)

Administrative Home Department

Department of Materials Science and Engineering

Advisor 1

Yongmei Jin

Committee Member 1

Stephen Hackney

Committee Member 2

Keat Ong

Committee Member 3

Aleksey Smirnov

Abstract

Fe-Ga alloys, also referred to as Galfenol, have attracted great attention as new giant magnetostrictive materials for over fifteen years. They exhibit large magnetostriction, small hysteresis, good machinability, and high tensile strength, providing many advantages over other functional materials like magnetostrictive Terfenol-D, piezoelectric PZT, and magnetic shape memory Ni-Mn-Ga. In spite of the intense interest in Fe-Ga alloys, the domain phenomena that underlie their properties and functionalities are less known. Using the Bitter method, a systematic study of magnetic domain structures in Fe-Ga single crystals of different compositions and heat treatments has been performed. Our domain observation experiments reveal numerous previously unknown domain patterns and their responses to magnetic fields. Zigzag domain walls, previously observed in Galfenol, are reinterpreted as V-lines formed by the meeting of two domain walls. Two types of charged 90° domain walls are described for the first time in this material, including a new type of zigzag wall. As a cubic system of high magnetostriction and weak magnetocrystalline anisotropy, the domains on Fe-Ga surfaces are largely determined by the image forces (both magnetostatic and elastostatic) associated with the surfaces, making it challenging to infer inner bulk domains from surface observations, while the bulk domains dominate the material responses. Using domain theory, the observed domain structures and their evolutions near the surfaces were analyzed. The findings advance our understanding of the domain phenomena and mechanisms responsible for the magnetoelastic behaviors of Fe- Ga alloys.

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