Using density functional theory to describe slowly varying fluctuations at finite temperatures: local magnetic moments in Gd and the ‘not so local’ moments of Ni

Julie B Staunton,Arthur Ernst,Alberto Marmodoro

doi:10.1088/0953-8984/26/27/274210

Abstract

We briefly describe the density functional theory (DFT)-based ‘disordered local moment’ (DLM) picture for magnetism at finite temperatures. It shows how relatively slowly fluctuating local moments can emerge from the interacting electrons of many materials. Such entities have rigid magnitudes and fluctuate their orientations from atomic site to atomic site on a timescale long compared to other electronic times. We illustrate this theory with calculations of the magnetocaloric effect in Gd where we find excellent agreement with experiments. Fluctuating moments do not appear to establish naturally over such small regions for some other materials. We show how the DFT-DLM theory can be extended to these materials with the use of the Korringa–Kohn–Rostoker nonlocal coherent potential approximation (KKR-NLCPA) to allow for more extensive, slow magnetic fluctuations. We present the first application of this approach by revisiting the description of the magnetic fluctuations prevalent in the paramagnetic state of nickel. We find that local moments can emerge above Tc and that these form coherently over small clumps of atomic sites (4–8 sites).

Highlights

All real materials are disordered to some extent and abinitio modelling needs to address the influence this disorder has on electronic structure [1]
Apart from at very low temperatures, it is often possible to extract out from all the interacting electrons of a material those electronic degrees of freedom which are fluctuating much more slowly than most of the others. These can be modelled as classical entities and their statistical mechanics written down, as long as a way of determining how the remaining electronic degrees of freedom depend on them can be found
In many magnetic materials fluctuating local moments associated with each atomic site naturally emerge from the electronic glue and the density functional theory (DFT)-disordered local moment’ (DLM) method provides an ab-initio way of implementing this picture

Summary

Introduction

The theoretical formalism and applications of this relatively new method have been extended to materials with multi-atom per unit cell lattices so that it is in principle possible to describe complex systems in which several ordering phenomena are present [11]. This new method gives opportunities for improving the description of materials’ interacting electrons at finite temperature. To improve upon the success of DFT for ground state properties, one effective approach is to identify some attributes of the interacting electrons that vary relatively slowly and treat them as classical fluctuations. We outline the KKRNLCPA and show how it can improve the current scope of DFT-DLM which we back up by initial calculations for the paramagnetic state of nickel

Magnetism at finite temperatures and the disordered local moment picture

The KKR-NLCPA

Disordered local moments and the KKR-NLCPA

The ‘not so local’ moments of paramagnetic Ni

Findings

Conclusions