Equilibrium susceptibilities of superparamagnets: longitudinal and transverse, quantumand classical

Abstract

The equilibrium susceptibility of uniaxial paramagnets is studied in a unified frameworkwhich permits us to connect traditional results of the theory of quantum paramagnets,S = 1/2,1,3/2,..., with molecularmagnetic clusters, S∼5,10,20 all the way up (S = 30,50,100,...,) to the theory of classical superparamagnets. This is done using standard tools ofquantum statistical mechanics and linear-response theory (the Kubo correlatorformalism). Several features of the temperature dependence of the susceptibility curves(crossovers, peaks, deviations from Curie law) are studied and their scalings withS identified and characterized. Both the longitudinal and transverse susceptibilities arediscussed, as well as the response of the ensemble with anisotropy axes oriented at random.For the latter case a simple approximate formula is derived too, and its range of validityassessed, which could be used in the modelization of experiments.