Temperature dependence of Van Vleck susceptibility in pyrite (FeS2)

We present here a discussion of the temperature (T) dependence of Van Vleck susceptibility (χ_νν) in pyrite. Using the reported experimental values of energy gap E_G and χ_νν at various temperatures, we evaluate the percentage change in χ_νν with respect to temperature due to changes in E_G and a term due to virtual magnetic dipole transitions between states on the same site. This study leads us to believe that, with increase in temperature, the contribution to percentage change in χ_νν due to such transitions is negative while the contribution from E_G is positive.