Size-Dependent Nonmagnetic Surface Layer of Ferromagnetic Nanoparticles

Based on the size-dependent melting temperature, we have deduced the analytic models for size-dependent Curie temperature T(c) and saturation magnetization at room temperature M(s) for ferromagnetic nanocrystals. The T(c)(D) and M(s)(D) functions decrease with decreasing size D. Agreements between model predictions and the corresponding experimental results enable us to determine the size-dependent thickness of nonmagnetic surface layer phi(D) and lambda(D) in describing T(c)(D) and M(s)(D), respectively. It is found that the surface layer should be an intrinsic property of ferromagnetic crystals since the nonmagnetic surface layer exists in the whole size range rather than vanishes as D approaches infinite.