MESOSCOPIC EFFECTS IN MAGNETISM - SUBMICRON TO NANOMETER-SIZE SINGLE-PARTICLE MEASUREMENTS

Low temperatures magnetization measurements of individual ferromagneti c particles and wires are presented (0.1<T(K)<6). The detector was a N b micro-bridge-dc-SQUID, fabricated using electron-beam lithography. T he angular dependence of the magnetization reversal could be explained approximately by simple classical micromagnetic concepts. However, ou r measurement evidenced nucleation and propagation of domain walls exc ept for the smallest particles of about 20 nm. The switching field dis tributions as a function of temperature and field sweeping rate and th e probabilities of switching showed that the magnetization reversal wa s thermally activated. These measurements allowed us to estimate the ' 'activation volume'' which triggered the magnetization reversal. Our m easurements showed for the first time that the magnetization reversal of a ferromagnetic nanoparticle of good quality can be described by th ermal activation over a single-energy barrier as originally proposed b y Neel and Brown. (C) 1997 American Institute of Physics.