Multidomain to single-domain transition for uniform Co80Ni20nanoparticles

Uniform fine Co 80 Ni 20 particles with diameters between 18 and 540 nm have been synthesized by polyol reduction of metallic salts. Their magnetic properties have been studied as a function of the particle size at 10 and 300 K. From the analysis of the experimental results at room temperature, a transition from polydomain structure to single-domain structure with the reduction of the particle size to around 40 nm is deduced. A considerable decrease of the saturation magnetization with respect to the bulk value has been found as the particle size decreases at both measuring temperatures. At low temperature, a strong increase of coercivity has been observed which is ascribed to an enhancement of the effective magnetic anisotropy induced by the surface layer.