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Title:Experimental study of the spin-glass phase diagram
Author(s):Ketelsen, Leonard Jan-Peter
Doctoral Committee Chair(s):Salamon, Myron B.
Department / Program:Physics
Subject(s):spin-glass phase diagram
mean-field theories
magnetic relaxation processes
Almeida-Thouless instability line
relaxation times
Abstract:The spin-glass phase diagram is examined as a function of temperature, magnetic field, random anisotropy, and uniaxial anisotropy and the results are compared to recent mean-field theories. It is clearly demonstrated through measurements of the magnetic relaxation processes in the amorphous spin-glass (FeO.64MnO.36)75P16B6A13 that the onset of strong magnetic irreversibilities occurs at a field dependent temperature. The observed behavior agrees with the de Almeida-Thouless instability line predicted by mean-field theory. This line does not represent a phase transition, but instead marks a crossover from short to long relaxation times in the spin-glass phase. The presence of weak random anisotropy in CuMn4 at. % is found to make the spin-glass order parameter accessible through a measurement of the transverse ac susceptibility. The order parameter obeys thermodynamic scaling relations which show that the spin-glass transition occurs along the Gabay-Toulouse line, as expected from mean-field theory. The scaling of the transverse susceptibility data is strong evidence that the spin-glass phase is a true phase transition. In the moderately anisotropic spin-glass CuMn4 at. %Au1 at. % the anisotropic interactions distort the transition line. In small fields the anisotropy dominates, making the system behave in an Ising-like manner. At large fields, however, the anisotropy is overwhelmed and the system behaves as a Heisenberg spin-glass. This behavior agrees with recent mean-field results for anisotropic spin-glasses. The transition in the XY spin-glass YTb3 at. % is thoroughly examined using neutron diffraction, equilibrium susceptibility, non-linear susceptibility, and magnetic relaxation. This system exhibits typical spin-glass behavior, including: absence of long-range magnetic order, unidirectional an1sotropy, pseudo-logarithmic decay of the remanent magnetization, and scaling of the non-linear susceptibility. The spin-glass transition is clearly shown to occur only in the basal plane of this hexagonal system. The results of this work show that mean-field theory is a useful tool in examining real spin-glasses.
Issue Date:1985
Genre:Dissertation / Thesis
Rights Information:1985 Leonard Jan-Peter Ketelsen
Date Available in IDEALS:2011-06-03
Identifier in Online Catalog:864932

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