Quasi-one-dimensional antiferromagnetism in MnCl2⋅H2O

Abstract

Recent magnetic susceptibility measurements on polycrystalline MnCl2⋅H2O indicate that it behaves like a quasi-one-dimensional antiferromagnet. The heat capacity of this compound has now been measured between ∼0.5 and 80 K in a liquid-3He-cooled vacuum calorimeter. Cp exhibits a large, sharp λ-type peak at TN = 2.18 ± 0.01 K marking the onset of long-range spin order. The critical entropy S(TN) is found to be ∼0.40R ln 6, confirming the short-range, low-dimensional nature of most of the spin ordering. An anisotropic elastic continuum model has been used to calculate the contribution of lattice vibrations to Cp. The data above ∼3 K are well represented by this model (with suitable choice of three parameters) plus the theoretical heat capacity of the 1D Heisenberg antiferromagnet with S =5/2 and exchange constant J/k=−0.45 K. Below ∼ 0.7TN, the magnetic part of Cp is well described by an anisotropic 3D spin-wave model with Jz/k = J/k = −0.45 K, Jx/k = Jy/k = −0.015 K, and an anisotropy field HA ≊ 3.3 kOe. The intra- and interchain exchange constants agree well with those inferred from χM. <lz>