A simple model for the size and shape dependent Curie temperature of freestanding Ni and Fe nanoparticles based on the average coordination number and atomic cohesive energy

To study the effect of size and shape of metallic nanoparticle on their Curie temperature, an analytical model is proposed. The core average coordination number (CAC) and surface average coordination number (SAC) of freestanding nanoparticles are considered in the model. Clusters of icosahedral (IC) and body centred cubic (BCC) structure without any vacancies and defects are modelled. A critical Curie temperature is introduced for metallic clusters with a diameter of 2-3 nm. This critical diameter is related to clusters which the ratio of surface atoms to total atoms is about 50%. The `` shape effect'' is shown to be important at sizes less than 20 nm. The obtained results are supported by available experimental results for nickel and iron nanoparticles. (C) 2011 Elsevier B.V. All rights reserved.