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 antiferromagn et or a ferrimagnet. For antiferromagnetic ferritin particles, the ani sotropy energy is found to depend linearly on the particle volume, sug gesting that bulk anisotropy dominates over surface anisotropy. Effect s due to the bulk and surface spins are discerned at high magnetic fie lds (27 T). At very low magnetic fields (1 nT) and temperatures (20 mK ), the tunneling frequency of the Neel vector is observed to scale exp onentially with the particle volume, consistent with the linear depend ence of the anisotropy barrier on volume and with theories of macrosco pic quantum coherence. In the ferrimagnetic particles, the anisotropy barrier decreases for smaller particles while simultaneously displayin g a slight increase in coercivity and a dramatic decrease in the reman ence over three orders of magnitude. (C) 1996 American Institute of Ph ysics.