Specific Heat of CoO nearTN: Anisotropy Effects

Abstract

The specific heat of CoO single crystals has been measured near the Néel temperature, using steady-state ac calorimetry. The critical exponents are found to be ${\alpha }^{\prime }=0.\mathrm{}05\pm 0.02$ and $\alpha =0.\mathrm{}12\pm 0.01$, which are very close to the predictions of the three-dimensional Ising model. Further, the actual values of the data (relative to pure copper) are fitted very closely above $T_N$ by the analytic form of the Ising specific heat. Recent theories have predicted Ising-like behavior near $T_N$ for slightly anisotropic exchange Hamiltonians. From a calculation of the effective spin Hamiltonian, it is found that a local pseudodipolar term is induced by magnetostriction. Use of a pair-cluster approximation shows that this local anisotropy is proportional to the short-range order parameter and persists above $T_N$. At $T_N$, the ratio $\frac{J_{\parallel }}{J_{\perp }}$ is found to be about 1. 02. This leads to the prediction of Ising-like behavior for $|\frac{T}{T_N}-1|\lesssim {10}^{-2\mathrm{}}$, which is borne out by the data.