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Salamon, Myron B. Ketelsen, Leonard Jan-Peter 2011-06-03T18:20:21Z 2011-06-03T18:20:21Z 10000-01-01 1985 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. Submitted by Carolyn Mead (cmead2@illinois.edu) on 2011-06-03T18:20:21Z No. of bitstreams: 1 1985_ketelsen.pdf: 5814106 bytes, checksum: 6d11e9ea28298e5cb6ad9c867329b9fb (MD5) Made available in DSpace on 2011-06-03T18:20:21Z (GMT). No. of bitstreams: 1 1985_ketelsen.pdf: 5814106 bytes, checksum: 6d11e9ea28298e5cb6ad9c867329b9fb (MD5) Previous issue date: 1985 Item marked as restricted to the 'UIUC Users [automated]' Group (id=2) by Carolyn Mead (cmead2@illinois.edu) on 2011-06-03T18:20:21Z Item is restricted indefinitely. Restriction data tranferred 2014-07-01T11:15:19-05:00 Original Data Group with Access UIUC Users [automated] Release Date: none Reason: Thesis Thesis U of I Only 864932 http://hdl.handle.net/2142/25274 en 1985 Leonard Jan-Peter Ketelsen spin-glass phase diagram mean-field theories magnetic relaxation processes Almeida-Thouless instability line relaxation times Experimental study of the spin-glass phase diagram Dissertation / Thesis text Physics Physics University of Illinois at Urbana-Champaign Dissertation Ph.D.