Yu-ichiro Matsushita and Hung Ba Tran published the paper on SSRN.
L10 FePt nanoparticle is one of the most promising materials for magnetic recording and nano-magnet applications. In nanoparticles, the superparamagnetic, where the thermal fluctuation is comparable with energy barrier for magnetic flips, strongly affects the stability of magnetic recording. The temperature dependence of magnetic anisotropy constant in L10 FePt nanoparticles is a crucial factor to estimate the relaxation time of magnetic flips in a magnetic nanoparticle. However, the comprehensive simulation at the atomic level for energy barrier for magnetic flips in the L10 FePt nanoparticles is lacking. Here, we derived the Heisenberg model Hamiltonian from first-principles calculations by considering the Liechtenstein formula. Our simulation quantitatively reproduces the size dependence on Curie temperature of nanoparticles in experiment works. The superparamagnetic effect of nanoparticles is observed during thermal equilibrium in the atomistic Monte Carlo simulations for the first time. Moreover, the surface effect, where the magnetization decreases due to the loss of magnetic exchange interaction pair in the surface, is also clarified at atomic resolution. The temperature and size dependence of the energy barrier for magnetic flips of L10 FePt nanoparticles is obtained in this work, demonstrating that the critical diameter of nanoparticle for long-term storage is 3.7 nm.
