Ground state properties of ferromagnetic metal/conjugated polymer interfaces

TL;DR

This paper models the ground state properties of ferromagnetic metal/conjugated polymer interfaces, focusing on charge and spin transfer, structural relaxation, and bipolaron formation using theoretical one-dimensional models.

Contribution

It introduces a combined theoretical framework using SSH and tight-binding models to analyze spin and charge transfer at ferromagnetic metal/polymer interfaces, including structural effects.

Findings

Electrons form bipolarons in the polymer bulk.

Spin density can localize near the interface.

Charge transfer depends on the Fermi energy adjustment.

Abstract

We theoretically investigate the ground state properties of ferromagnetic metal/conjugated polymer interfaces. The work is partially motivated by recent experiments in which injection of spin polarized electrons from ferromagnetic contacts into thin films of conjugated polymers was reported. We use a one-dimensional nondegenerate Su-Schrieffer-Heeger (SSH) Hamiltonian to describe the conjugated polymer and one-dimensional tight-binding models to describe the ferromagnetic metal. We consider both a model for a conventional ferromagnetic metal, in which there are no explicit structural degrees of freedom, and a model for a half-metallic ferromagnetic colossal magnetoresistance (CMR) oxide which has explicit structural degrees of freedom. The Fermi energy of the magnetic metallic contact is adjusted to control the degree of electron transfer into the polymer. We investigate electron charge and spin transfer from the ferromagnetic metal to the organic polymer, and structural relaxation near the interface. Bipolarons are the lowest energy charge state in the bulk polymer for the nondegenerate SSH model Hamiltonian. As a result electrons (or holes) transferred into the bulk of the polymer form spinless bipolarons. However, there can be spin density in the polymer localized near the interface.