Thermal Expansivity of RbMnF3 Near TN

Abstract

The thermal expansivity of single crystal RbMnF₃ has been measured in the vicinity of its Néel temperature, T_N≃83 K. A three‐terminal capacitive dilatometer was employed to measure length changes with a strain sensitivity ∼10⁻¹⁰. Above T_N, the magnetic contribution to the expansivity, α_m, exhibits a temperature dependence consistent with a logarithmic divergence at T_N for over two decades of reduced temperature and to within 4 mK of T_N. By utilizing previous magnetic specific heat (C_m) data, it is found that α_m and C_m are not directly proportional in the critical region. An asymptotic magnetic Gruneisen ``constant'' is evaluated for T→T_N⁺ with the result γ_m=−∂ log J/∂ log V=3.14±0.3. The pressure dependence of the Néel temperature dT_N/dP is 0.37 mK/bar.