Electromigration in thin tunnel junctions with ferromagnetic/nonmagnetic: nanoconstrictions, local heating, and direct and wind forces

TL;DR

This study investigates how electromigration affects resistance switching in FM/NM/I/FM tunnel junctions, revealing the roles of heating, direct, and wind forces, and how a non-magnetic Ta layer influences the CIS effect.

Contribution

It provides new insights into electromigration mechanisms in tunnel junctions with non-magnetic layers, highlighting the dominance of direct forces in Ta and wind forces in FM layers.

Findings

Resistance switching increases with current up to a plateau at 65 mA.

High currents cause irreversible barrier degradation.

Electromigration direction is opposite in FM/NM/I/FM compared to FM/I/FM due to force competition.

Abstract

Current Induced Resistance Switching (CIS) was recently observed in thin tunnel junctions with ferromagnetic (FM) electrodes \emph{i.e} FM/I/FM. This effect was attributed to electromigration of metallic atoms in nanoconstrictions in the insulating barrier (I). Here we study how the CIS effect is influenced by a thin non-magnetic (NM) Ta layer, deposited just below the AlO$_x$ insulating barrier in tunnel junctions of the type FM/NM/I/FM (FM=CoFe). Enhanced resistance switching occurs with increasing maximum applied current ($\Imax$), until a plateau of constant CIS is reached for $\Imax\sim65$ mA (CIS$\sim$60%) and above. However, such high electrical currents also lead to a large ($\sim$9%) irreversible resistance decrease, indicating barrier degradation. Anomalous voltage-current characteristics with negative derivative were also observed near $\pm\Imax$ and this effect is here attributed to heating in the tunnel junction. One observes that the current direction for which resistance switches in FM/NM/I/FM (clockwise) is opposite to that of FM/I/FM tunnel junctions (anti-clockwise). This effect will be discussed in terms of a competition between the electromigration contributions due to the so called direct and wind forces. It will be shown that the direct force is likely to dominate electromigration in the Ta (NM) layers, while the wind contribution likely dominates in the CoFe (FM) layers.