Curie temperature suppression of ferromagnetic nanosolids

Abstract

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 (TC) suppression of ferromagnetic nanosolids. At TC, the atomic thermal vibration energy (EV) overcomes the atomic cohesive energy (Ecoh), 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 Ecoh from that of an atom inside the bulk. As such, the critical EV for an atom at a free surface will be different from the bulk value and, hence, the TC of a nanosolid will change with the portion of surface atoms. Matching between predictions and experimental observations on the TC suppression of Fe, Ni and Co nanofilms evidences the validity of the current premise, in which no assumptions or freely adjustable parameters are involved.