Degree Type

Dissertation

Date of Award

2006

Degree Name

Doctor of Philosophy

Department

Materials Science and Engineering

First Advisor

Vitalij K. Pecharsky

Second Advisor

Karl A. Gschneidner, Jr.

Abstract

We undertook a detailed study of a single crystal of dysprosium metal, which is a classical example of a system where magnetic and crystallographic sublattices can be either coupled or decoupled from one another. Magnetocaloric effect, magnetization, ac magnetic susceptibility, and heat capacity of high purity single crystals of dysprosium have been investigated over broad temperature and magnetic field intervals with the magnetic field vector parallel to either the a- or c-axes of the crystal. Notable differences in the behavior of the physical properties when compared to Dy samples studied in the past have been observed between 110 K and 125 K, and between 178 K and ~210 K. A plausible mechanism based on the formation of antiferromagnetic clusters in the impure Dy has been suggested in order to explain the reduction of the magnetocaloric effect in the vicinity of the Neel point. Experimental and theoretical investigations of the influence of commensurability effects on the magnetic phase diagram and the value of the magnetocaloric effect have been conducted. The presence of newly found anomalies in the physical properties has been considered as evidence of previously unreported states of Dy. The refined magnetic phase diagram of dysprosium with the magnetic field vector parallel to the a-axis of a crystal has been constructed and discussed;The magnetic and crystallographic properties of Gd5Sb xGe4-x pseudo-binary system were studied by x-ray diffraction (at room temperature), heat capacity, ac-magnetic susceptibility, and magnetization in the temperature interval 5-320 K in magnetic fields up to 100 kOe. The magnetic properties of three composition (x = 0.5, 1, 2) were examined in detail. The Gd5Sb2Ge2 compound that adopts Tm5Sb2Si2-type of structure (space group is Cmca), shows a second order FM-PM transition at 200 K, whereas Gd 5SbxGe4-x compounds for x=0.5 and x=1 (Sm 5Ge4-type of structure, space group is Pnma) exhibit first order phase transformations at 45 K and 37 K, respectively;The detailed investigation of crystallographic, magnetic, thermal and electronic properties of Gd5Sb0.5Ge3.5 single crystal shows that the compound undergoes a coupled magnetic-structural transition at 40--70 K and an electronic transition around 60 K.

DOI

https://doi.org/10.31274/rtd-180813-10888

Publisher

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

Copyright Owner

Alexander S. Chernyshov

Language

en

Proquest ID

AAI3229060

File Format

application/pdf

File Size

124 pages

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