Large surface magnetic contribution in amorphous ferromagnetic nanoparticles

We present magnetization measurements on similar to3-nm (Fe0.26Ni0.74)(50)B-50 ferromagnetic amorphous nanoparticles. Our results show evidence of the surface contribution to the magnetic properties in the M(H), M(T) and relaxation measurements. We observe a large increase in the magnetization with decreasing temperature, a nonsaturated component in the M(H) curves, the presence of two maxima in the zero-field-cooling M(T) data, and two viscosity regimes in magnetization relaxation data. The results have been interpreted by a simple model where we consider each single-domain particle as a core plus shell system with a uniaxial anisotropy acting on the core and a surface anisotropy acting on the shell. Monte Carlo simulations based in this core-shell model qualitatively reproduce all the observed phenomena.