Low-Temperature Specific Heat of Ni(NO3)2 · 6H2O: Antiferromagnetic Interactions

Abstract

Measurements of the specific heat of nickel nitrate hexahydrate for $0.5<T<\mathrm{}12\mathrm{}$ K reveal a Schottky anomaly with $C_{p max}=1.373\mathrm{}$ cal/mole K at $T_{max}=2.35\mathrm{}$ K. The experimental data are explained by the complete splitting of the $S=1\mathrm{}$ ground-state triplet, with levels separated successively by 4.80 and 3.26 K. The lattice contribution is found to be $4.11\times {10}^4{T}^3$ cal/mole K. The discrepancies between the level spacings, as deduced from these measurements and the reported powder susceptibility data, are removed by taking into account a small antiferromagnetic exchange interaction ($\frac{A}{k}=+0.6\mathrm{}$ K) in a molecular-field approximation. The total magnetic entropy change is found to be within 0.6% of the theoretically predicted value $R\ln 3$ for a spin-1 system.