Spin Tunneling in Conducting Oxides

TL;DR

This paper analyzes various tunneling mechanisms in ferromagnetic junctions, highlighting how surface states, defects, and inelastic processes like magnons and phonons influence tunneling magnetoresistance (TMR) and its bias dependence.

Contribution

It provides a comprehensive theoretical model comparing direct, impurity-assisted, surface state, and inelastic tunneling contributions to TMR, with specific focus on half-metallic systems.

Findings

Direct tunneling accounts for about 30% resistance change in iron systems.

Surface polarized states can significantly increase TMR values.

Resonant defect states decrease TMR to around 4%, while resonant tunnel diodes yield about 8%.

Abstract

Direct tunneling in ferromagnetic junctions is compared with impurity-assisted, surface state assisted, and inelastic contributions to a tunneling magnetoresistance (TMR). Theoretically calculated direct tunneling in iron group systems leads to about a 30% change in resistance, which is close to experimentally observed values. It is shown that the larger observed values of the TMR might be a result of tunneling involving surface polarized states. We find that tunneling via resonant defect states in the barrier radically decreases the TMR (down to 4% with Fe-based electrodes), and a resonant tunnel diode structure would give a TMR of about 8%. With regards to inelastic tunneling, magnons and phonons exhibit opposite effects: one-magnon emission generally results in spin mixing and, consequently, reduces the TMR, whereas phonons are shown to enhance the TMR. 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 CrO$_2$/TiO$_2$ and CrO$_2$/RuO$_2$, and an account of their peculiar band structures is presented. The implications and relation of these systems to CMR materials which are nearly half-metallic, are discussed.