High-Temperature Heat Capacity of Ferromagnets Having 1st- and 2nd-Neighbor Exchange: Application to EuS

Abstract

This paper describes the derivation of exact power series expansions of the high-temperature magnetic-heat capacity of Heisenberg ferromagnets having both first- and second-neighbor exchange interactions. The calculations employ the general diagramatic techniques developed by Rushbrooke and Wood as extended by Wojtowicz and Joseph to include the second-neighbor interaction. All mixed coefficients for terms through the fifth power of the inverse temperature have been computed for arbitrary spin and general lattice structure. The series expansions have been used to obtain certain information on the nature of the exchange interactions in ferromagnetic EuS. By the use of a spin-wave analysis Charap has been able to deduce a set of values for the exchange interactions which simultaneously fit the low-temperature magnetization and heat capacity data. It has been found that these same values substituted into the present theory are capable of reproducing the high-temperature heat capacity data of Moruzzi and Teaney. This result lends considerable support to the hypothesis that EuS is very nearly an ideal Heisenberg ferromagnet.