Successive Ising phase transitions in a random antiferromagnet with competing anisotropies.

Abstract

We report the results of an elastic neutron scattering study of successive Ising phase transitions in the magnetic alloy ${\mathrm{Fe}}_{0.4}$${\mathrm{Co}}_{0.6}$${\mathrm{Cl}}_{2}$\ensuremath{\cdot}2H 2O. Here the ${\mathrm{Fe}}^{2+}$ and ${\mathrm{Co}}^{2+}$ ions have orthogonal preferred spin directions so that this provides an example of a random magnet with competing anisotropies. The higher (${T}_{H}$=11.8 K) and the lower (${T}_{L}$=6.15 K) transition temperatures are equally sharp; furthermore, they both exhibit conventional critical scattering. These features are in contrast with those in all other competing anisotropy systems studied to date where the lower transitions are always broad and ill defined. Possible origins for the differing behavior in ${\mathrm{Fe}}_{0.4}$${\mathrm{Co}}_{0.6}$${\mathrm{Cl}}_{2}$\ensuremath{\cdot}2H 2O are discussed.