Experimental evidence of surface effects in the magnetic dynamics behavior of ferrite nanoparticles

The magnetic dynamics behavior of copper and nickel ferrite nanoparticles used in the magnetic fluid elaboration and with mean sizes between 3.5 to 10.4 nm is investigated by measurements of magnetic hysteretic properties and zero field cooling (ZFC) susceptibility. The dependence of the irreversibility field, inversely proportional to the particle size, clearly indicates that the magnetic anisotropy of our particles finds its origin on the surface layer.