Multiscale modeling of ultrafast element-specific magnetization dynamics in FeNi ferromagnetic alloys

Herein, to understand the differences of the magnetization dynamics of Fe and Ni in Py, we develop a model based on a hierarchical multi-scale approach to investigate the sub-lattice dynamics of ferromagnetic alloys and to obtain a deeper insight into the underlying mechanisms. First, we construct and parametrize a spin model Hamiltonian for Py on the basis of first-principles calculations. This spin model Hamiltonian in combination with extensive atomistic spin computer simulations based on the stochastic Landau-Lifshitz-Gilbert equation are used to calculate the demagnetization process after the application of a step heat pulse. The second step of the presented multiscale model links the information gained from the atomistic spin model to the macroscopic two sub-lattices Landau-Lifshitz-Bloch (LLB) equation of motion recently derived by Atxitia et al. The analytical LLB models allow for cheap simulations, and most importantly, provide insight in the element-specific demagnetization rates of Py.