Magnetic nanoparticles as many-spin systems

We present a review of recent advances in the study of many-body effects in magnetic nanoparticles. Considering classical spins on a lattice coupled by the exchange interaction in the presence of the bulk and surface anisotropy, we investigate the effects of finite size, free boundaries, and surface anisotropy on the average and local magnetization for zero and finite temperatures and magnetic fields. Superparamagnetism of magnetic particles necessitates introducing two different, induced and intrinsic, magnetizations. We check the validity of the much used relation between them within different theoretical models. We show that the competition between the exchange and surface anisotropy leads to spin canting dependent on the orientation of the average magnetization with respect to the crystallographic axes and thus to a second-order effective anisotropy of the particle. We have also investigated the switching mechanism of the magnetization upon varying the surface anisotropy constant. Some cases of more realistic particles are also dealt with.