Mesoscopic effects in magnetism: Submicron to nanometer size single particle measurements

Low temperatures magnetization measurements of individual ferromagnetic particles and wires are presented (O.1<T(K)<6). The detector was a Nb micro-bridge-dc-SQUID, fabricated using electron-beam lithography. The angular dependence of the magnetization reversal could be explained approximately by simple classical micromagnetic concepts. However, our measurement evidenced nucleation and propagation of domain walls except for the smallest particles of about 20 nm. The switching field distributions as a function of temperature and held sweeping rate and the probabilities of switching showed that the magnetization reversal was thermally activated. These measurements allowed us to estimate the "activation volume" which triggered the magnetization reversal. Our measurements showed for the first time that the magnetization reversal of a ferromagnetic nanoparticle of good quality can be described by thermal activation over a single-energy berner as originally proposed by Néel and Brown.