Heat Capacity of RbMnF3near the Antiferromagnetic Transition Temperature

Precision measurements of the heat capacity $C_p$ of the isotropic Heisenberg system RbMn${\mathrm{F}}_3$ near the antiferromagnetic transition temperature $T_c=83.08\mathrm{}$ K are reported. The measurements are for $76\le T\le 88$ K. The results are in every respect consistent with predictions based upon recent theories of critical phenomena. They imply that $C_p$ is finite and continuous at $T_c$. Under certain assumptions about the functional form of $C_p$, the critical exponents $\alpha ={\alpha }^{\prime }=-0.14\mathrm{}\pm 0.01$ and the amplitude ratio $\frac{A}{A^{\prime }}=1.40\mathrm{}\pm 0.04$ are obtained. These parameters are believed to be characteristic of systems with short-range interactions and three degrees of freedom in the order parameter. A comparison with thermal-expansion measurements yields $\frac{\mathrm{dP}}{d{T}_c}=1.69\mathrm{}\times {10}^3$ bar ${\mathrm{K}}^{-1\mathrm{}}$ and $\frac{dS}{d{T}_c}=0.51\mathrm{}$ J ${\mathrm{mole}}^{-1\mathrm{}}$ ${\mathrm{K}}^{-2\mathrm{}}$.