Optical and Computational Studies of Structural/magnetic Phase Transitions
Abstract
The Faraday rotation (FR) of the H = 0 excess magnetization in the fieldcooled 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 NattermannVilfan 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 AlmeidaThouless replica symmetry breaking line in a conventional spinglass which requires the randomness and the exchangefrustration. Since this system does not have exchangefrustration, the spinglasslike 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.
 Publication:

Ph.D. Thesis
 Pub Date:
 1992
 Bibcode:
 1992PhDT.......159H
 Keywords:

 ANTIFERROMAGNET;
 MAGNETIZATION;
 Physics: Condensed Matter