Atomistic simulation of collective excitations in Bcc iron with vacancy defects
Mudrick, Mark Stephen
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Utilizing an atomistic computational model which handles both translational and spin degrees of freedom, we have performed combined molecular and spin dynamics simulations to investigate the effect of vacancy defects on spin and lattice excitations in ferromagnetic iron. Interatomic interactions are described using an embedded atom potential and magnetic interactions are governed by a Heisenberg-like Hamiltonian with a coordinate dependent exchange interaction. Fourier transforms of space and time-displaced correlation functions yield the dynamic structure factor, providing characteristic frequencies and lifetimes of the spin wave modes. Comparison of the system containing a 5% randomly distributed vacancy concentration with pure lattice data shows a decrease in frequency as well as a decrease in lifetime for all accessible transverse spin wave excitations. By constructing the spin wave dispersion curve, we observe a decrease in the spin wave stiffness parameter with the introduction of vacancy defects, in agreement with experimental neutron scattering data. Additionally, a rugged spin wave line shape for low-$q$ excitations indicates the presence of multiple localized modes near the defect sites. These induced excitations result in reduced excitation lifetimes due to increased magnon-magnon scattering. We observe further evidence of increased magnon-magnon scattering as additional two-spin-wave annihilation peaks appear in the longitudinal spin wave spectrum under these conditions of impurity. Single vacancy defects, or voids, of varying sizes are introduced into the system, resulting in sharp splitting of long-wavelength excitation line shapes. This splitting behavior is shown to be strongly affected by defect size as well as the size of the surrounding system. Localized correlation function measurements are made in the vicinity of the defect site, showing the existence of a dominant excitation mode in this region. The longitudinal magnetic excitation spectrum contains additional modes which are not present in the pure system due to the increased number of available spin wave annihilation processes. The longitudinal spectrum also reveals splitting of the magnon-phonon coupling mode caused by the defect center.