Degree Type

Dissertation

Date of Award

2004

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

First Advisor

David C. Jiles

Abstract

Thermal expansion (TE) and magnetostriction (MS) measurements have been conducted for Gd5(SixGe1-x)4 with a series of x values to study its critical behavior in the vicinity of transition temperatures.;It was found that the Curie temperature of Gd5(SixGe 1-x)4 for x: 0 ~ 0.5 is dependent on magnetic field, direction of change of temperature (Tc on cooling was lower than Tc on heating), purity of Gd starting material, compositions, material preparation methods, and also can be triggered by the external magnetic field with a different dT/dB rate for different x values. For Gd5(Si1.95Ge 2.05), Gd5(Si2Ge2), Gd5(Si 2.09Ge1.91), it was also found that the transition is a first order magneto-structural transition, which means the magnetic transition and crystalline structure transition occur simultaneously, and completely reversible. Temperature hysteresis and phase coexistence have been found to confirm that it is a first order transformation. While for Gd5(Si0.15Ge 3.85), it is partially reversible at some temperature range between the antiferromagnetic and the ferromagnetic state. For Gd5(Si 2.3Ge1.7) and Gd5(Si3Ge1), it was a second order transformation between the paramagnetic and ferromagnetic state, because no DeltaT have been found. Giant magnetostriction was only found on Gd5(Si1.95Ge2.05), Gd5(Si 2Ge2), Gd5(Si2.09Ge1.91) in their vicinity of first order transformation. MFM images have also been taken on polycrystal sample Gd5(Si2.09Ge1.91) to investigate the transformation process. The results also indicates that the Curie temperature was lower and the thermally-induced strain higher in the sample made from lower purity level Gd starting materials compared with the sample made from high purity Gd metal.;TE, MS, MFM and VSM measurements on single crystal Gd5(Si 2Ge2) and Gd5(Si2.3Ge1.7) samples have shown their strong anisotropic behaviors.

DOI

https://doi.org/10.31274/rtd-180813-10893

Publisher

Digital Repository @ Iowa State University, http://lib.dr.iastate.edu

Copyright Owner

Mangui Han

Language

en

Proquest ID

AAI3145643

File Format

application/pdf

File Size

178 pages

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