Anisotropy Distribution of FePt Nanoparticles With Twinned Structures

The existence of the five-fold twinned structures was observed in 3.4 nm FePt nanoparticles, and other n-fold structures were also been found in magnetic nano-grains in recording media. An analytical model, following the spirit of the Stoner-Wohlfarth model, is brought up to study the anisotropy field distribution of FePt or other magnetic nanoparticles with twinned or n-fold structures, based on simple statistical calculations of twin regions' volumes. It is found that the nanoparticle has an effective uniaxial anisotropy, and the average anisotropy field is substantially reduced compared to that of the single crystal. The calculated anisotropy distributions are also compared with the empirical distribution functions used in micromagnetic simulations.