Heisenberg Model and Magnetic Insulators

The magnetization and specific heat are calculated on the basis of a Heisenberg model which is thought to be appropriate for the description of the magnetic properties of four insulating copper salts. The calculation includes first- and second-neighbor interactions and is based on a quasimagnon approximation which goes beyond the Hartree-Fock theory. The low-temperature expansions are exact to order $T^4$, and the results obtained for the transition temperature are consistent with those obtained from the high-temperature series-expansion theory. Good agreement between theory and experiment is obtained over most of the temperature range $T<\mathrm{}{T}_c$.