Size dependence of the Curie temperature of L1o-FePt nanoparticles

Monte Carlo simulations using the single histogram method are performed to study the effects of finite size and Cu doping on the Curie temperature of a single FePt nanoparticle. An atomistic model is used with an effective classical spin Hamiltonian constructed on the basis of first-principles calculations for L1o FePt. The Curie temperature, evaluated from the peak in the temperature variation of the magnetic susceptibility, is consistent with the finite-size scaling relation Tc(L)=Tc(∞)(1−xoL−1/ν), where L is the in-plane grain size, xo is found to depend on the grain aspect ratio, and the critical exponent of the correlation length is ν=1.06±0.06. A theoretical relation is obtained for the Tc dispersion arising from a lognormal grain size distribution. Addition of Cu results in reduction of the Curie temperature proportional to the dopant content.