External magnetic field effect on BCC iron at elevated temperatures by spin-lattice dynamics simulation

Abstract

The thesis investigates the thermomechanical properties of ferromagnetic body-centered cubic iron under various external magnetic field strengths, using spin-lattice dynamics (SLD) simulations implemented with graphics processing units, emphasizing the external field effect on the magnetic phase transition around the transition temperature regime. Simulation results are presented in terms of ensemble averages of thermodynamic quantities and their derivatives, together with the spectra derived from lattice and spin motions. Then, three major perspectives are sought to analyze the external field effect: classical spins, magnons, and spin-lattice coupling. From the analysis of classical spins, the simulation results show that an external field can reinforce the long range magnetic ordering reflected by the atomic magnetization, and can increase the magnetic phase transition temperature. In addition, the application of an external field allows the SLD magnetization to emulate the mean field magnetization because spin precession modeled by SLD is then suppressed. According to the magnon analysis, an external field inhibits magnon-magnon interaction and maintains the spin stiffness, whose effects are more discernible around the transition temperature regime. Finally, an external field has an insignificant effect on spin-lattice coupling, originated from the more harmonic interatomic potential adopted in our simulations, which cannot reproduce the structural phase-transition from BCC to FCC at 1183K, thus showing stiff lattice vibration. However, both the variation of transition temperature and the magneto-volume effect are observed more prominently around the critical region. As a result, it can be said classically that an external magnetic field varies the separation dependence of the interatomic potential, which leads to the reduction of the atomic volume. Alternatively in quantum physics, an external field alters the atomic volume because it alters the magnon-magnon interaction, bringing about the change in phonon scattering by phonon-magnon interaction. In short, SLD can model the magneto-volume effect of ferromagnetic materials under an external field, and can help to estimate the resulting transition temperature due to the increased magnetic ordering and inhibited magnon-magnon interaction.