Dielectric breakdown in underoxidized magnetic tunnel junctions: Dependence on oxidation time and area

TL;DR

This study investigates how dielectric breakdown in underoxidized magnetic tunnel junctions depends on oxidation time and junction area, revealing different failure mechanisms linked to defects and unoxidized aluminum.

Contribution

It provides new insights into the failure mechanisms of underoxidized MTJs, highlighting the influence of oxidation time and junction size on breakdown behavior.

Findings

Large-area MTJs fail extrinsically at high oxidation times.

Small-area MTJs fail intrinsically at high oxidation times.

Extremely underoxidized MTJs fail extrinsically regardless of size.

Abstract

Magnetic tunnel junctions (MTJs) with partially oxidized 9 \AA AlO$_x$-barriers were recently shown to have the necessary characteristics to be used as magnetoresistive sensors in high-density storage devices. Here we study dielectric breakdown in such underoxidized magnetic tunnel junctions, focusing on its dependence on tunnel junction area and oxidation time. A clear relation between breakdown mechanism and junction area is observed for the MTJs with the highest studied oxidation time: samples with large areas fail usually due to extrinsic causes (characterized by a smooth resistance decrease at dielectric breakdown). Small area junctions fail mainly through an intrinsic mechanism (sharp resistance decrease at breakdown). However, this dependence changes for lower oxidation times, with extrinsic breakdown becoming dominant. In fact, in the extremely underoxidized magnetic tunnel junctions, failure is exclusively related with extrinsic causes, independently of MTJ-area. These results are related with the presence of defects in the barrier (weak spots that lead to intrinsic breakdown) and of metallic unoxidized Al nanoconstrictions (leading to extrinsic breakdown).