Curie transition of superparamagnetic nickel nanoparticles in silica glass: A phase transition in a finite size system

Macynetization of Ni nanoparticles (mean diameter of similar to 3 nm) in silica glass (SiO2) has been studied up to similar to 375 degrees C, in comparison with that of Ni film (thickness of similar to 120 nm). At room temperature, the nanoparticles (NPs) show a Langevin loop without hysteresis which is characteristic in superparamagnetic NPs. The saturation maanetization of the NPs decreases more steeply than the bulk with increasing temperature. However, a certain amount of magnetization remains in the NPs, even above the bulk Curie temperature. Quantum MonteCarlo simulation was carried out using an Ising spin model on the fcc lattice of L X L X L (L = 3 to 38). The experimental results are qualitatively reproduced with the model; i.e., the steep decrease of the saturation magnetization with increasing temperature and the residual magnetization above the bulk Curie temperature are both ascribed to the finite size effects.