Andreev reflection at half-metal-superconductor interfaces with non-uniform magnetization

TL;DR

This paper investigates how non-uniform magnetization in a half-metallic ferromagnet affects Andreev reflection at the interface with a superconductor, revealing the dependence on magnetization orientation and energy.

Contribution

It provides a microscopic calculation of Andreev reflection amplitudes considering spatially varying magnetization, linking scattering and quasiclassical formalisms.

Findings

Andreev reflection amplitude depends on magnetization gradient orientation.

The energy and angle dependence of reflection amplitudes are characterized.

Connections are made between scattering approach and Green function symmetry properties.

Abstract

Andreev reflection at the interface between a half-metallic ferromagnet and a spin-singlet superconductor is possible only if it is accompanied by a spin flip. Here we calculate the Andreev reflection amplitudes for the case that the spin flip originates from a spatially non-uniform magnetization direction in the half metal. We calculate both the microscopic Andreev reflection amplitude for a single reflection event and an effective Andreev reflection amplitude describing the effect of multiple Andreev reflections in a ballistic thin film geometry. It is shown that the angle and energy dependence of the Andreev reflection amplitude strongly depends on the orientation of the gradient of the magnetization with respect to the interface. Establishing a connection between the scattering approach employed here and earlier work that employs the quasiclassical formalism, we connect the symmetry properties of the Andreev reflection amplitudes to the symmetry properties of the anomalous Green function in the half metal.