Competing Order Parameters in Quenched Random Alloys:Fe1−xCoxCl2

A study is reported of the magnetic properties of the random alloy ${\mathrm{Fe}}_{1-x}{\mathrm{Co}}_x{\mathrm{Cl}}_2$, which represents an archetypal example of a system with competing orthogonal spin anisotropies. Behavior similar to previous experiments and theoretical predictions is found, but with important qualitative and quantitative differences; in particular the phase transition in one variable is drastically altered by the existence of long-range order in the other variable. It is hypothesized that this is due to microscopic random-field effects.