Finite size effects in L1o-FePt nanoparticles

Finite size effects on the temperature dependence of the uniaxial magnetic anisotropy, longitudinal and transverse susceptibilities and specific heat are examined for L1o-ordered FePt nanoparticles using an atomistic model based on an effective classical spin Hamiltonian. At low temperatures below criticality, we study the intrinsic uniaxial magnetic anisotropy energy (MAE) K1 and its scaling with magnetization K1(T)∼Ms(T)δ and using Langevin dynamics simulations we show that the dependence of the exponent δ on the size L and aspect ratio of the grain arises from decomposition of the MAE into bulk and surface dependent terms. Monte Carlo simulations in the critical regime near the Curie temperature Tc, show that the temperature variation of the specific heat and longitudinal susceptibility is given by finite size scaling relations c=Lα/νc̃(L1/νϵ) and χ=Lγ/νχ̃(L1/νϵ), respectively, where ϵ=(T−Tc)/Tc is the reduced temperature, and the susceptibility scaling function χ̃ can be approximated by a Lorentzian. Our estimates of the critical exponents α,γ, and ν appear to be in agreement with the universality class of the 3D Ising model.