Multiple nearest-neighbor exchange model for the frustrated magnetic molecules { Mo72 Fe30 } and { Mo72 Cr30 }

Our measurements of the differential susceptibility H of the frustrated magnetic molecules { Mo72 Fe30 } and { Mo72 Cr30 } 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.