Very high field magnetization and AC susceptibility of native horse spleen ferritin

The magnetization of native horse spleen ferritin protein is measured in pulsed magnetic fields to 55 T at T=1.52 K. The magnetization rises smoothly with negative curvature due to uncompensated Fe3+ spins and with a large high field slope due to the underlying antiferromagnetic ferritin core. Even at highest fields the magnetic moment is only ∼4% of the saturation moment of the full complement of Fe3+ in the ferritin molecule. The AC magnetic susceptibility, χAC(T,f), responding to the uncompensated spins, reaches a maximum near the superparamagnetic blocking temperature with the temperature of the maximum, TM, varying with excitation frequency, TM−1 α log f for 10⩽f⩽104 Hz.