Local and long-range magnetic order of the spin − 3 2 system CoSb 2 O 6

The ${\mathrm{Co}}^{2+}$ ions of ${\mathrm{CoSb}}_{2}{\mathrm{O}}_{6}$ exhibit local magnetic order below $\ensuremath{\sim}80$ K followed by long-range antiferromagnetic order below ${T}_{N}=13.45$ K. Analysis of the magnetic susceptibility above ${T}_{N}$ using an Ising model of Co-Co dimers yields a magnetic exchange coupling ${J}_{\ensuremath{\parallel}}/{k}_{\mathrm{B}}=\ensuremath{-}10.603(6)$ K (${k}_{\mathrm{B}}$ is the Boltzmann constant). The transition at ${T}_{N}$ is accompanied by a spin gap in the heat capacity $[{\mathrm{\ensuremath{\Delta}}}_{2}/{k}_{\mathrm{B}}=33.92(9)$ K]. Highly anisotropic behavior of the thermal expansion is observed, and the influence of local magnetic order is evident. A critical exponent of $\ensuremath{\alpha}=0.103(3)$ is obtained from analysis of the critical behavior of the heat capacity and thermal expansion near ${T}_{N}$, similar to the value expected for the three-dimensional Ising universality class.