Atomistic simulations of magnetic models with coupled translational and spin degrees of freedom
Perera, Dilina Niranga
MetadataShow full item record
Using an atomistic model that treats translational and spin degrees of freedom on an equal footing, we perform combined molecular and spin dynamics (MD-SD) and Monte Carlo (MC) simulations to study the dynamic and static (thermodynamic) properties of body-centered cubic (bcc) iron. The atomic interactions are modeled via an empirical many-body potential while spin-spin interactions are established through a Heisenberg-like Hamiltonian with a coordinate-dependent exchange interaction parameterized by first-principles calculations. In MD-SD simulations, the numerical solutions to the coupled equations of motion were obtained using an algorithm based on the second-order Suzuki-Trotter decomposition of the exponential time evolution operator. By calculating the Fourier transforms of the space- and time-displaced correlation functions, the characteristic frequencies and the linewidths of the vibrational and magnetic excitation modes were determined. Comparison of the results with that of the stand-alone molecular dynamics and spin dynamics simulations reveal that the dynamic interplay between the phonons and magnons leads to a shift in the respective frequency spectra and a decrease in the lifetimes. Moreover, in the presence of lattice vibrations, additional longitudinal magnetic excitations were observed with the same frequencies as the longitudinal phonons. A generic, phenomenological approach was developed for incorporating spin-orbit interactions into the MD-SD formalism. These interactions are modeled in terms of the local magnetic anisotropies that arise as the symmetry of the local crystal structure is broken due to phonons or crystallographic defects. Using canonical MD-SD simulations, we show that this novel extension overcomes the major shortcoming of the original method; namely, the inability to achieve the mutual thermalization of both the atomic and spin degrees of freedom via a heat bath coupled to the lattice subsystem. Using massively parallel replica-exchange Wang-Landau MC simulations, the magnetic phase transition in bcc iron was investigated with/without the impact of the phonons. The transition temperature as well as the amplitude of the peak in the specific heat curve is marginally affected by the lattice vibrations. However, the results were also found to be sensitive to the particular choice of the interatomic potential.