First-principle based simulations of longitudinal spin-fluctuations in metals and integration in classical space with variable spin amplitude

The use of the classical Heisenberg model which incorporates only transverse spin degrees of freedom has only limited success for description of the metallic magnetism at finite temperature, since temperature and magnetic disorder induced longitudinal variations of the atomic spin moments might become large in the itinerant electron systems away from the limit of localized moments. In order to incorporate the longitudinal spin fluctuations in finite temperature simulation schemes a simple extended version of the Heisenberg model which allows for an on-site spin magnitude variation controlled by the one-site energy terms is widely used during the recent decade for ab-initio mapping and statistical simulations. Here, we apply and discuss such ab-initio based scheme for the canonical itinerant ferromagnetic metals (Fe, Co, Ni) and recently discovered high temperature antiferromagnet - V3Al, in conjunction with standard spherical integration metrics in classical spin state and the recently proposed linear one. We also examine the dependence of the results on the choice of the exchange and correlation potential in ab-initio total energy calculations. We compare the respective uncertainties in the calculated values of the magnetic ordering temperature and temperature dependent spin moment magnitude to the difference in the results which relate to the choice of the metrics. (c) 2018 Author(s).