High-Resolution Specific Heat Measurements on MnCl2·4H2O near Its Néel Temperature

Abstract

Specific heat measurements have been made on large single crystals of Mn${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}4${\mathrm{H}}_{2}$O in the immediate neighborhood of its N\'eel temperature, with a temperature resolution of 2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$\ifmmode^\circ\else\textdegree\fi{}K. For $|1\ensuremath{-}\frac{T}{{T}_{N}}|=t<{10}^{\ensuremath{-}2}$ the results can be fitted by the relations: $C={A}_{+}{t}^{\ensuremath{-}\ensuremath{\alpha}}+{B}_{+}$, for $T>{T}_{N}$, and $C={A}_{\ensuremath{-}}{log}_{10}t+{B}_{\ensuremath{-}}$, for $T<{T}_{N}$, where $\ensuremath{\alpha}=0.35\ifmmode\pm\else\textpm\fi{}0.02$. A rounding of the data occurred over approximately 100\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$\ifmmode^\circ\else\textdegree\fi{}K near the peak. From these results, along with the recently reported thermal expansion coefficient measurements on Mn${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}4${\mathrm{H}}_{2}$O, it is possible to calculate the dependence of the exchange constant on pressure at low pressures. The resulting value of $\frac{(\frac{1}{J})dJ}{\mathrm{dP}}=9\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}6}$ ${\mathrm{atm}}^{\ensuremath{-}1}$ is about a factor of 2 smaller than the values for Fe${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O and Co${\mathrm{Cl}}_{2}$\ifmmode\cdot\else\textperiodcentered\fi{}2${\mathrm{H}}_{2}$O found by Narath and Schirber by direct measurement. The temperature dependence of the data is discussed in relation to the most recent three-dimensional Ising model calculations.