Deviations from the Curie-law variation of magnetic susceptibility in antiferromagnetic nanoparticles

Abstract

The temperature variation of the low-field magnetic susceptibility \ensuremath{\chi} for antiferromagnetic nanoparticles of ferritin and ferrihydrite in the superparamagnetic regime is shown to follow the equation $\ensuremath{\chi}={\ensuremath{\chi}}_{0}+(C/T).$ The theoretical basis for this equation is the low-field limit of the modified Langevin variation of the magnetization M given by ${M=M}_{0}\mathcal{L}({\ensuremath{\mu}}_{p}{H/k}_{B}T)+{\ensuremath{\chi}}_{a}H$ (H is the applied field, ${\ensuremath{\mu}}_{p}$ is the magnetic moment/particle, and ${\ensuremath{\chi}}_{a}$ is the antiferromagnetic susceptibility of the core). Self-consistent values of C, ${\ensuremath{\mu}}_{p},$ and ${\ensuremath{\chi}}_{0}$ are obtained for ferritin and ferrihydrite nanoparticles.