High-Temperature Susceptibility of Ferrimagnetic Spinels

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Abstract
Power series expansions of the susceptibility and its inverse in ascending powers of the exchange divided by the temperature have been obtained for ferrimagnetic spinels by using the methods of Rushbrooke and Wood. The Heisenberg form of exchange is assumed, and in this paper, interactions between neighboring spins from different sublattices (A – B exchange) only are considered. The coefficients in the series are derived for arbitrary values of the spins on the two sublattices. The calculations have been carried out to terms including the fifth power of the exchange divided by the temperature; the molecular field theory by contrast is rigorously valid only to the first power term of its expansion. The explicit dependence of the Néel temperature on the spin values has also been deduced. The derived susceptibilities and Néel temperatures are compared to the results of earlier models. It is anticipated that these expansions will prove useful in the interpretation of accurate susceptibility data for the purpose of deriving meaningful values for the exchange interactions in the spinels.