Abstract
We report the calculation of free energy with constrained
magnetization for $\lo$ FePt nanoparticles. We employ effective
spin Hamiltonian model constructed on the basis of constrained
density functional theory calculations for $\lo$ FePt. In this model
the Fe spins (treated as classical spins in this work) are coupled
"directly" and via induced Pt moments with both isotropic and
anisotropic interactions. Interactions mediated by the Stoner
enhanced Pt moment stabilize ferromagnetic order and lead to a
pronounced coordination dependence and long-range interactions.
The free energy of these nanoparticles, as a function of
the temperature and the constrained magnetization $F(T,M_z)$, is
calculated from the joint density of states $g(E,M)$, using the
extended Wang-Landau algorithm. The free energy barrier for
magnetization reorientation is found to depend fairly linearly on
the temperature in the ferromagnetic phase and vanishes in the
paramagnetic phase.