Classical and quantum magnetism in synthetic ferritin proteins

The magnetic properties of nanometer-scale particles are studied using the protein-complex ferritin as a vesicle for either an antiferromagnet or a ferrimagnet. For antiferromagnetic ferritin particles, the anisotropy energy is found to depend linearly on the particle volume, suggesting that bulk anisotropy dominates over surface anisotropy. Effects due to the bulk and surface spins are discerned at high magnetic fields (27 T). At very low magnetic fields (1 nT) and temperatures (20 mK), the tunneling frequency of the Neel vector is observed to scale exponentially with the particle volume, consistent with the linear dependence of the anisotropy barrier on volume and with theories of macroscopic quantum coherence. In the ferrimagnetic particles, the anisotropy barrier decreases for smaller particles while simultaneously displaying a slight increase in coercivity and a dramatic decrease in the remanence over three orders of magnitude. (C) 1996 American Institute of Physics.