Multiple nearest-neighbor exchange model for the frustrated magnetic molecules{Mo72Fe30}and{Mo72Cr30}

Abstract

Our measurements of the differential susceptibility $\ensuremath{\partial}M/\ensuremath{\partial}H$ of the frustrated magnetic molecules ${{\text{Mo}}_{72}{\text{Fe}}_{30}}$ and ${{\text{Mo}}_{72}{\text{Cr}}_{30}}$ reveal a pronounced dependence on magnetic field $(H)$ and temperature $(T)$ in the low $H$--low $T$ regime, contrary to the predictions of the existing models. Excellent agreement with experiment is achieved upon formulating a nearest-neighbor classical Heisenberg model where the 60 nearest-neighbor exchange interactions in each molecule, rather than being identical as has been assumed heretofore, are described by a two-parameter rectangular probability distribution of values of the exchange constant. We suggest that the probability distribution provides a convenient phenomenological platform for summarizing the combined effects of multiple microscopic mechanisms that disrupt the idealized picture of a Heisenberg model based on a single value of the nearest-neighbor exchange constant.