Equilibrium susceptibilities of superparamagnets: longitudinal & transverse, quantum & classical

TL;DR

This paper unifies the understanding of equilibrium susceptibilities in superparamagnets across quantum and classical regimes using statistical mechanics, analyzing temperature-dependent features and response anisotropies.

Contribution

It develops a comprehensive framework connecting quantum and classical superparamagnet susceptibilities using standard statistical mechanics tools.

Findings

Identifies scaling laws for susceptibility features with spin magnitude $ ext{S}_m$.

Derives an approximate formula for ensemble response with random anisotropy axes.

Analyzes temperature dependence and crossover phenomena in susceptibility curves.

Abstract

The equilibrium susceptibility of uniaxial paramagnets is studied in a unified framework which permits to connect traditional results of the theory of quantum paramagnets, $\Sm=1/2, 1, 3/2$, ..., with molecular magnetic clusters, $\Sm\sim5, 10, 20$, all the way up, $\Sm=30, 50, 100$,... to the theory of classical superparamagnets. This is done using standard tools of quantum statistical mechanics and linear response theory (the Kubo correlator formalism). Several features of the temperature dependence of the susceptibility curves (crossovers, peaks, deviations from Curie law) are studied and their scalings with $\Sm$ identified and characterized. Both the longitudinal and transverse susceptibilities are discussed, 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 validity assessed, so it could be used in modelization of experiments.