X-ray and neutron scattering studies of the Rb₂MnF₄ and Cu₁₋õxMgx̳GeO₃ in an external magnetic field

Abstract

This thesis presents results of two scattering studies of low dimensional magnetic materials. The first is a neutron scattering study of Rb2MnF4, a nearly ideal two-dimensional square lattice Heisenberg antiferromagnet with S = 5/2. When this material is placed in a magnetic field on an extrapolation of the spin-flop phase boundary into the paramagnetic phase, Heisenberg critical behavior can be accessed to a much longer correlation length than had previously been achieved for a high-spin system. Comparison of our results with current theories indicate that the probed temperature region lies in a crossover region from classical to renormalized classical behavior. In addition, we show measurements of both the quasielastic and magnon dynamic behavior. The second part of this thesis presents extensive synchrotron x-ray scattering measurements of the high-field incommensurate phase in doped CuGeO3. Random-field behavior, including Lorentzian-squared lineshapes, the changing of a first order transition to second-order, and critical destruction of long-range order is observed in this continuous order parameter system.