Magnetic hysteresis anomalies in ferritin

Ferritin, an antiferromagnetic nanoparticle with a net magnetic moment, exhibits some unusual magnetic behavior. Above its blocking temperature, ${\mathrm{T}}_{\mathrm{B}}$≈12 K, the magnetization can be fit with the Langevin function only by including an additional linear term. The saturation moment decreases linearly with increasing temperature, and suggests an ordering temperature of about 460 K. Below ${\mathrm{T}}_{\mathrm{B}}$, both zero-field-cooled and field-cooled hysteresis loops exhibit large coercivity (∼1800 Oe) and irreversibility in the loops is found up to H=35 kOe. An exchange field ${\mathrm{H}}_{\mathrm{e}}$ of few hundred Oe is observed at low temperatures. The dependencies of ${\mathrm{H}}_{\mathrm{e}}$ and ${\mathrm{H}}_{\mathrm{c}}$ on temperature were also studied.