A theoretical study of magnetism and its extension to finite temperatures in random alloys

Abstract: This work presents new theoretical developments of atomistic spin simulations of magnetic materials at finite temperatures. Special focus is put on the description of longitudinal magnetic fluctuations and the application in random transition metal alloys. A new computational scheme is proposed for mapping total energies from electronic structure calculations to an extended atomistic spin model. The proposed model has some new appealing features from previous models. To be more specific, the proposed model successfully eliminates the reference state dependency of the mapping that previous models have suffered from. Moreover, the proposed model includes longitudinal magnetic fluctuations that gives an improved description of the magnetic properties over a larger temperature interval. The proposed model strives to find the right compromise between accuracy and computational feasibility and it is applied not only to the elemental systems Fe, Co and Ni, but also to a number of binary transition metal alloys such as Permalloy (Fe$_{20\%}$Ni$_{80\%}$) and Fe-Co systems.Electronic structure calculations of Gilbert damping and the closely related magnetodynamic properties, the saturation magnetization and exchange stiffness, have been conducted for a number of different magnetic systems including Permalloy with additional doping of $4d$ or $5d$ transition metal impurities and the full Heusler alloy Co$_2$FeAl. Regarding the Permalloy based systems, a systematic study of the magnetodynamic properties was performed and compared with existing experimental data. In general we found good agreement and manage to explain the main trends regarding the Gilbert damping across the series with a simple model that captures the most important material properties to the damping, namely the spin orbit coupling and density of states at the Fermi level. In Co$_2$FeAl, we calculated the Gilbert damping in different existing crystal structures and compare those with new experimental data and found good agreement between them.Magnon properties of random alloys, like Permalloy, are studied using two complementory methods, the adiabatic magnon spectra valid at zero temperature and from finite temperature atomistic spin dynamics through the dynamical structure factor. The influence of chemical disorder and temperature effects on the magnon properties are investigated that hopefully could motivate new experimental studies of these materials.