Assisted Tunneling in Ferromagnetic Junctions and Half-Metallic Oxides

TL;DR

This paper investigates various mechanisms affecting spin-dependent tunneling and tunnel magnetoresistance (TMR) in ferromagnetic junctions, highlighting the impact of surface states, defect resonances, magnons, and phonons, with implications for half-metallic systems.

Contribution

It provides a microscopic model analyzing different tunneling mechanisms and their effects on TMR, especially in half-metallic ferromagnets, including the suppression of one-magnon processes.

Findings

Direct tunneling causes about 30% resistance change in iron systems.

Resonant defect states significantly decrease TMR.

Magnons and phonons influence bias dependence of TMR.

Abstract

Different mechanisms of spin-dependent tunneling are analyzed with respect to their role in tunnel magnetoresistance (TMR). Microscopic calculation within a realistic model shows that direct tunneling in iron group systems leads to about a 30% change in resistance, which is close but lower than experimentally observed values. The larger observed values of the tunnel magnetoresistance (TMR) might be a result of tunneling involving surface polarized states. It is found that tunneling via resonant defect states in the barrier radically decreases the TMR by order of magnitude. One-magnon emission is shown to reduce the TMR, whereas phonons increase the effect. The inclusion of both magnons and phonons reasonably explains an unusual bias dependence of the TMR. The model presented here is applied qualitatively to half-metallics with 100% spin polarization, where one-magnon processes are suppressed and the change in resistance in the absence of spin-mixing on impurities may be arbitrarily large. Even in the case of imperfect magnetic configurations, the resistance change can be a few 1000 percent. Examples of half-metallic systems are CrO2/TiO2 and CrO2/RuO2.