Random fields and the weakly anisotropic Ising model:MnxZn1−xF2

Neutron-scattering measurements have been performed to study the effect of applying a magnetic field along the $c$ axis to the disordered weakly anisotropic antiferromagnet ${\mathrm{Mn}}_{0.65}$ ${\mathrm{Zn}}_{0.35}$ ${\mathrm{F}}_2$. The experiments were mostly performed by field cooling; that is, the field was applied in the paramagnetic phase and the sample was then cooled slowly. At high fields, there is a spin-flop transition and the system exhibits effectively long-range order throughout this phase. When, however, the system is cooled in lower fields, the scattering shows that there is not the transition to a long-range-ordered state. The scattering develops a Lorentzian-squared profile and the width of this Lorentzian-squared profile decreases with decreasing temperature. At the lowest temperatures and small fields the observed width is the experimental resolution but at fields close to those which produce the spin-flop phase, there is a significant increase in width of the distributions, suggesting that long-range order is not achieved. A detailed analysis yields values of the correlation lengths which are consistent with those measured in the strongly anisotropic system ${\mathrm{Fe}}_x{\mathrm{Zn}}_{1-x}{\mathrm{F}}_2$. We also find significant history-dependent effects, indicating that equilibration phenomena play an essential role up to temperatures very close to $T_N\mathrm{}\left(H\right)\mathrm{}$. We also describe preliminary measurements in the two-dimensional system ${\mathrm{Rb}}_2$ ${\mathrm{Mn}}_{0.5}$ ${\mathrm{Ni}}_{0.5}$ ${\mathrm{F}}_4$; the results are similar in that there is a slight increase in the width of a Bragg reflection when cooled in a field of 6.5 T. The effect is however much smaller than in the analogous Ising antiferromagnet ${\mathrm{Rb}}_2$ ${\mathrm{Co}}_{0.7}$ ${\mathrm{Mg}}_{0.3}$ ${\mathrm{F}}_4$.