Experimental evidence of the Neel-Brown model of magnetization reversal

Presented are the first magnetization measurements of individual ferromagnetic nanoparticles (15-30 nm) at very low temperatures (0.1-6 K). The angular dependence of the hysteresis loop evidenced the single domain character of the particles. Waiting time, switching field, and telegraph noise measurements showed for the first time that the magnetization reversal of a well prepared ferromagnetic nanoparticle can be described by thermal activation over a single energy barrier as originally proposed by Neel and Brown. The ''activation volume'' estimated by these measurements was close to the particle volume.