Hole transport across MgO-based magnetic tunnel junctions with high resistance-area product due to oxygen vacancies

TL;DR

This paper investigates hole transport in MgO-based magnetic tunnel junctions with high resistance-area products caused by oxygen vacancies, revealing how Fermi level shifts influence barrier heights and electronic symmetry conservation.

Contribution

It demonstrates that hole transport occurs across MgO barriers with oxygen vacancies and clarifies how Fermi level shifts affect barrier heights and symmetry conservation.

Findings

Hole transport involves oxygen vacancy states.

Fermi level shifts alter barrier heights.

High TMR observed despite oxygen vacancies.

Abstract

The quantum mechanical tunnelling process conserves the quantum properties of the particle considered. As applied to solid-state tunnelling (SST), this physical law was verified, within the field of spintronics, regarding the electron spin in early experiments across Ge tunnel barriers, and in the 90s across Al2O3 barriers. The conservation of the quantum parameter of orbital occupancy, as grouped into electronic symmetries, was observed in the '00s across MgO barriers, followed by SrTiO3 (STO). Barrier defects, such as oxygen vacancies, partly conserve this electronic symmetry. In the solid-state, an additional subtlety is the sign of the charge carrier: are holes or electrons involved in transport? We demonstrate that SST across MgO magnetic tunnel junctions (MTJs) with a large resistance-area (RA) product involves holes by examining how shifting the MTJ's Fermi level alters the ensuing barrier heights defined by the barrier's oxygen vacancies. In the process, we consolidate the description of tunnel barrier heights induced by specific oxygen-vacancy induced localized states. Our work opens prospects to understand the concurrent observation of high TMR and spin transfer torque across MgO-based nanopillars.