Specific-heat study of random-field and competing-anisotropy effects in ${\mathrm{Fe}}_{1\mathrm{-}\mathrm{x}}$${\mathrm{Co}}_{\mathrm{x}}$${\mathrm{Cl}}_2$

We report a comprehensive specific-heat study of the mixed antiferromagnetic system ${\mathrm{Fe}}_{1\mathrm{-}\mathrm{x}}$ ${\mathrm{Co}}_{\mathrm{x}}$ ${\mathrm{Cl}}_2$ with x=0.286, 0.366, and 0.604. Four different issues related to competing anisotropies and random fields are discussed. (i) In zero field, a single sharp peak at the Néel transition $T_N$ is seen in each sample. There is no evidence for additional transitions at lower temperatures. We suggest that there are differences between systems with competing Ising-XY anisotropies and those with Ising-Ising anisotropies. Some new neutron scattering data are presented.