Magnetic interactions in CoBr2·6H2O

High-precision measurements of the heat capacity of Co${\mathrm{Br}}_2$·6${\mathrm{H}}_2$O were performed between 1.2 and 11 K. A fit of the high-temperature magnetic specific heat to the high-temperature-series expansion for a two-dimensional $\mathrm{XY}$ antiferromagnet yielded $\frac{J}{k}=-2.5\mathrm{}$ K. A combination with antiferromagnetic-resonance results gave $J_{1xx}=-2.4\mathrm{}$ K, $J_{1yy}=-2.3\mathrm{}$ K, $J_{1zz}=-0.3\mathrm{}$ K for the intersublattice interaction and $J_{2xx}=-0.5\mathrm{}$ K, $J_{2yy}=-0.5\mathrm{}$ K, and $J_{2zz}=-0.07\mathrm{}$ K for the intrasublattice interaction. A calculation of the Curie constant, the perpendicular susceptibility, and the paramagnetic phase transition based upon these values gave satisfactory agreement with the experimental data. From the critical entropy (57%), as well as the critical behavior of the sublattice magnetization ($\beta =0.31\mathrm{}$), a somewhat bidimensional character could be concluded. The effect of deuteration will be briefly considered.