Nanoscale magnetic particles: Synthesis, structure and dynamics

In recent years, new synthetic routes, which include wet techniques and synthesis in confined geometries, have been developed for the preparation of nanoscale magnetic particles. The shape of the particles obtained is usually smooth and rounded all over, due to the influence of the surface energy. The final shape, however, depends on the preparation method and is much influenced by the substrate used, as this may stress and deform the particles in order that they adapt to it. Changes in lattice parameters, in comparison to the bulk, and surface modifications give way in many cases to changes in the magnetic and transport properties. For very small ferromagnetic metal particles (clusters), both enhanced magnetism, due to localization of the valence d-electrons, and giant magnetic moments for some nonmagnetic clusters are observed. Magnetic phenomena in real single-domain particles reveal that the statistics of the magnetization reversal cannot be described by a thermal activation over a single-energy barrier. Macroscopic quantum tunnelling of magnetization is observed at very low temperatures in several systems. Other interesting electrical and magnetic phenomena, such as finite size effects, coercivity enhancement due to surface effects, and giant magnetoresistance, are observed in fine particle systems.