Interatomic exchange coupling of BCC iron

Abstract:

We performed first-principle calculations on the exchange interaction (EI) between atoms in BCC-Fe strained volumetrically. Our results show that the volume-dependence of the EI deviates considerably from the Bethe-Slater curve. This behavior is discussed in terms of the on-site and/or inter-site direct exchange interactions between electrons.

Introduction

In this paper, we perform ab initio calculations on Jij between different neighbors of BCC-Fe for various volume strains. The results are used to study the behavior of Jij and how it varies with the configuration of the neighboring atoms. We use spin polarized scalar-relativistic linear muffin-tin orbitals (LMTO) method combined with the Green’s function (GF) technique [22]. The accuracy of the electronic structure is cross checked with WIEN2k [23]. Many-body effects are found to be very significant in the magnitude of Jij. A pairwise functional representation of Jij independent of the neighboring environment is found to be inadequate. For the nearest neighbors, Jij generally decreases monotonically with  increasing separation Rij, except for an unexpected local minimum when Rij is slightly larger than the equilibrium distance. The magnitude of the MM also shows a sudden increase in the same regime. The values of Jij between second nearest neighbors are in general substantially smaller than the first nearest neighbors, except for distances larger than the equilibrium lattice distances. At smaller distances, on the other hand, the values of J2 decreases, rather than increases, with decreasing Rij even turn negative for distances sufficiently small. These results will be discussed and its implications analyzed.

Methodology

Only BCC iron is considered in this paper. For lattice constants ranging from 2.2Å to 3.2Å, we calculate the magnitude of atomic magnetic moment, densities of state, charge density maps and the exchange coupling constant Jij. We are aware that changing the atomic volume may induce phase transition [24] [25], nevertheless, this is beyond our present scope. Two computer codes are employed, WIEN2k [23], which is based on the full-potential linearized augmented plane-wave (FP-LAPW) method, and LMTO-GF [26], which is implemented within the atomic spheres approximation (ASA) [22]. While WEIN2k may be superior for electronic structure calculations, it is not equipped to calculation Jij, as LMTO-GF is. Both computer codes, however, can be used to calculate MM.

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https://arxiv.org/ftp/arxiv/papers/1005/1005.2931.pdf