Spin-polarized tunneling currents through a ferromagnetic insulator between two metallic or superconducting leads

TL;DR

This paper models spin-polarized tunneling currents through a magnetic insulator between metallic or superconducting leads using the Keldysh formalism, revealing dependence on voltage and material properties with qualitative experimental agreement.

Contribution

It introduces a theoretical model combining Keldysh formalism, Kondo Lattice Model, and BCS theory to analyze spin-polarized tunneling in hybrid structures with localized moments.

Findings

Spin polarization depends strongly on voltage and material properties.

Qualitative agreement with experimental observations.

Model captures effects of magnetic insulator on tunneling currents.

Abstract

Using the Keldysh formalism the tunneling current through a hybrid structure where a confined magnetic insulator (I) is sandwiched between two non-magnetic leads is calculated. The leads can be either normal metals (M) or superconductors (S). Each region is modelled as a single band in tight-binding approximation in order to understand the formation of the tunneling current as clearly as possible. The tunneling process itself is simulated by a hybridization between the lead and insulator conduction bands. The insulator is assumed to have localized moments which can interact with the tunneling electrons. This is described by the Kondo Lattice Model (KLM) and treated within an interpolating self-energy approach. For the superconductor the mean-field BCS theory is used. The spin polarization of the current shows a strong dependence both on the applied voltage and the properties of the materials. Even for this idealized three band model there is a qualitative agreement with experiment.