Spin-lattice dynamics study of vacancy formation and migration in ferromagnetic BCC iron

Spin-lattice dynamics (SLD) is performed to study the effect of ferromagnetism on vacancy formation and migration in BCC iron. Using a modified thermodynamic integration (TI) method, energies and entropies of mono-vacancy formation and migration are calculated from the SLD and MD phase-space trajectories. By comparing results among ferromagnetic, paramagnetic and non-magnetic cases, our analysis suggests that ferromagnetism significantly affects vacancy formation and migration in two ways. Firstly, scattering of magnons by the vacancy as a lattice imperfection results in an increase of the total free energy of the crystal, which depends on the vacancy configuration (equilibrium and saddle-point), and which thus contributes to vacancy formation and migration. Secondly, through the Heisenberg Hamiltonian, interatomic forces due to the exchange interaction of the electrons of neighboring atoms depend on the magnon state. The strong temperature dependence of the magnon distribution, particularly across the magnetic phase boundary, is carried through to all lattice properties, including the atomic volume, phonon frequency and lifetime, defect configurations and properties, giving rise to the observed non-Arrhenius behaviors of vacancy formation, migration, and self-diffusion. Our results are in good general agreement with experimental data, an exception being the migration energy derived from stage III recovery.