Magnetic Ordering of Itinerant Systems in Modified CPA Approximation

TL;DR

This paper develops a modified CPA approach incorporating band degeneration and two-site correlations to analyze itinerant ferromagnetism, revealing how correlations influence magnetic ordering and energy reduction.

Contribution

It introduces a novel modified CPA method with two self-energies and band degeneration to better understand itinerant ferromagnetism.

Findings

Correlation factors decrease kinetic energy and promote ferromagnetism.

Modified CPA captures band shape changes due to correlations.

Inter-site correlations significantly reduce the effective Hartree field.

Abstract

We will analyze the itinerant model for ferromagnetism with both single-site and two-site electron correlations. We will include band degeneration into the model. This will allow us to consider the on-site exchange interactions in the Hamiltonian. The modified Hartree-Fock approximation for the two-site interactions will be used. This approximation will give the relative one spin band broadening with respect to the other, in addition to the shift in position of majority and minority spin bands. Next, we will use the coherent potential approximation technique (CPA) with two self-energies. One will describe the on-site correlation and the second one the inter-site correlation. We will separate from these self-energies, the linear terms arising from Hartree-Fock approximation and the higher order terms. The on-site linear term will contribute only to the effective molecular field. The inter-site linear term will contribute to the effective molecular field and to the inter-site correlation factor. The Green function technique and CPA decoupling will allow for the change in the shape of spin bands, which has been described by the correlation factors and which will decrease the kinetic energy of the system. The gain in kinetic energy due to the on-site and inter-site correlation factors will drive the ferromagnetism and significantly reduce the effective Hartree field necessary to create this ordering.