Magnetic Properties of Nanometer-Scale Iron Structures and Proteins

Magnetic hysteresis is studied in arrays of nanometer -scale iron particles as small as 10 nm ({ ~}10^6 iron atoms per particle). Control of the aspect ratio and spacing of particles is realized by local organometallic deposition with a scanning tunneling microscope (STM). At low temperatures (5-100 K), the average magnetic properties of arrays of ~500 particles are studied with a two-dimensional electron gas Hall magnetometer in close proximity (100 nm) to the array. The angular dependence of the coercive field suggests that the particles are single domain, but with a distribution of magnetic anisotropies. At room temperature, individual particles are observed with a magnetic force microscope to switch discontinuously at different fields, complementing the results from low temperature measurements. For smaller length scales, a biochemical technique employs the globular protein ferritin as a host for either antiferromagnetic or ferrimagnetic iron oxide particles whose iron loading can be controlled from 100 to 4000 iron ions. The anisotropy energy for antiferromagnetic ferritin particles is found to depend linearly on the particle volume, implying 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 (30 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.