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

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 χ=χ₀+(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=M₀L(μₚH/kBT)+χₐH (H is the applied field, μₚ is the magnetic moment/particle, and χₐ is the antiferromagnetic susceptibility of the core). Self-consistent values of C, μₚ, and χ₀ are obtained for ferritin and ferrihydrite nanoparticles.