Can spin-lattice dynamics model hysteresis loops?

DOS SANTOS, GONZALO; ROMÁ, FEDERICO; TRANCHIDA, JULIEN; BRINGA, EDUARDO M.

Congreso; International Seminar on Condensed Matter Physics and Statistical Physics; 2022

The different responses of magnetic materials to external magnetic fields largely define the range of applicability of these materials in different technological or bio-medicinal fields among others. In particular, hysteresis loops are crucial, for example, in magnetic hyperthermia as they represent a measure of the ability to dissipate heat of a given material or nanostructure. In this sense, it is important to have reliable simulation methods that allow explaining and even guiding experiments based on the modeling of magnetic effects at the atomic scale with the possibility of simulating real materials including for example defects, free surfaces, etc. Along these lines is the simulation method known as Spin-Lattice Dynamics (SLD). This approach is a semiclassical method of molecular dynamics coupled to spin dynamics that allows simulating the simultaneous and coupled evolution of the degrees of freedom of the spins and the atoms of the lattice. The main objective of this work is to determine if it is possible (and under what conditions) to reliably simulate hysteresis loops using SLD, trying finally to lay the foundations for this type of simulations. We simulate hysteresis loops in bulk Fe at low temperatures for various angles between the external magnetic field and the “easy” anisotropy axis. We explore the effect of various physical and computational parameters such as the field frequency or sweep rate, the Gilbert damping, the magnitude and type of anisotropy (cubic, uniaxial), the intensity of the spin-spin coupling, the size of the system and the effect of the coupling the dynamics of the lattice to the dynamics of the spins. We find that the type of anisotropy and spin-spin coupling have little effect on the hysteresis curves. However, the frequency of the applied external field and the value of the Gilbert damping have a substantial effect. Our results generally indicate that a careful selection of parameters allows obtaining hysteresis loops with SLD that show excellent agreement with the semi-analytical Stoner-Wohlfarth model at low temperatures. We find that at low temperatures, the coupling of the lattice degrees of freedom to the spin dynamics has little influence on the coercive field obtained. However, for higher temperatures it is expected that the coercive field is noticeably affected by the inclusion of the coupled dynamics of SLD.