The temperature variation of the low-field magnetic susceptibility χ for antiferromagnetic nanoparticles of ferritin and ferrihydrite in the superparamagnetic regime is shown to follow the equation $\chi ={\chi }_0+(C/T).$ The theoretical basis for this equation is the low-field limit of the modified Langevin variation of the magnetization M given by ${M=M}_0\mathcal{L}\left({\mu }_p{H/k}_{B}T\right)+{\chi }_{a}H$ (H is the applied field, ${\mu }_p$ is the magnetic moment/particle, and ${\chi }_a$ is the antiferromagnetic susceptibility of the core). Self-consistent values of C, ${\mu }_p,$ and ${\chi }_0$ are obtained for ferritin and ferrihydrite nanoparticles.

• Received 16 March 2001

DOI:https://doi.org/10.1103/PhysRevB.64.132410