Experimental study of the spin-glass phase diagram

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.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• Ketelsen, Leonard Jan-Peter

Contributors dc:contributor

• Salamon, Myron B.

• spin-glass phase diagram
• mean-field theories
• magnetic relaxation processes
• Almeida-Thouless instability line
• relaxation times

Statement dc:rights

• 1985 Leonard Jan-Peter Ketelsen

Language dc:language

en

Identifier

864932

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/25274