Optical and Computational Studies of Structural/magnetic Phase Transitions

The Faraday rotation (FR) of the H = 0 excess magnetization in the field-cooled Ising antiferromagnet Fe_{0.47}Zn_{0.53 }F_2 has been measured for cooling fields 0 < H < 7T applied along the easy axis at temperatures 5K < T < 22K, well below the H = 0 phase transition occurring at T = T_{N}(= 37K). The relaxation of the excess magnetization, proportional to the Faraday rotation, does not follow the Nattermann-Vilfan theory in detail for this range of T and H. The experimentally determined time dependence of the excess magnetization is well described by M_{r}(t)=M _0exp{-A[ {rm ln}(t/t_0)]^{y} }. A simulation of the experiment using a Monte Carlo technique yields excellent fits with a slightly greater than unity value for y. The phase diagram for the dilute Ising antiferromagnet Fe_{0.31}Zn_{0.69 }F_2, which is an excellent realization of the d = 3 random field Ising model (RFIM) in the strong dilution limit, has been studied using the FR technique. The external field (H) dependence of the phase transition T_{c}(H) and the equilibrium boundary T_{eq}(H) shows the universal crossover behavior from the d = 3 random exchange Ising model to the RFIM for H < 1.5T. However, as the field is increased above H = 1.5T, a striking reversal of curvature of T_{eq }(H) vs T is observed. In the high field region T_{eq}(H) scales with H as does the de Almeida-Thouless replica symmetry breaking line in a conventional spin-glass which requires the randomness and the exchange-frustration. Since this system does not have exchange-frustration, the spin-glass-like behavior must be induced by the high external field. The structural phase transition of SrTiO _3, which belongs to d = 3 Heisenberg system, has been studied using the birefringence technique. The birefringence Delta n is supposed to behave as | t|^{ ~{beta}} with ~beta = 0.836 in the critical region below T_{c}. The data are consistent with this behavior, but the rounding of the transition due to a T_{c} gradient reduces the value attained for ~{beta}. Applying pressure along the < 110> direction in an attempt to see the crossover from Heisenberg to Ising behavior yielded anomalous hysteresis and time dependence.