Curie temperature suppression of ferromagnetic nanosolids

Based on the recent bond-order-bond-length-bond-strength correlation mechanism (Sun C Q, Chen T P, Tay B K, Li S, Huang H, Zhang Y B, Pan L K, Lau S P and Sun X W 2001 J. Phys. D: Appl. Phys. 34 3470) and the criterion of thermal-vibration-exchange-interaction energy equilibrium, an atomistic model has been developed for the Curie temperature (T_C) suppression of ferromagnetic nanosolids. At T_C, the atomic thermal vibration energy (E_V) overcomes the atomic cohesive energy (E_coh), which triggers the order-disorder transition of the spin-spin exchange interaction. Besides, the coordination-number (CN) imperfection at a surface enhances the strength of the bonds of the surface atoms. The CN reduction and bond-strength enhancement modifies the surface atomic E_coh from that of an atom inside the bulk. As such, the critical E_V for an atom at a free surface will be different from the bulk value and, hence, the T_C of a nanosolid will change with the portion of surface atoms. Matching between predictions and experimental observations on the T_C suppression of Fe, Ni and Co nanofilms evidences the validity of the current premise, in which no assumptions or freely adjustable parameters are involved.