Detection of bottom ferromagnetic electrode oxidation in magnetic tunnel junctions by magnetometry measurements

TL;DR

This study demonstrates how magnetometry measurements can detect surface oxidation in the bottom ferromagnetic electrodes of magnetic tunnel junctions, revealing its impact on exchange bias and coercivity, with annealing influencing oxygen migration.

Contribution

It introduces a magnetometry-based method to detect and analyze surface oxidation effects in ferromagnetic electrodes of MTJs, highlighting the sensitivity of coercivity to oxidation.

Findings

Surface oxidation affects exchange bias parameters.

Coercivity is highly sensitive to surface oxidation.

Annealing causes oxygen migration back to the tunnel barrier.

Abstract

Surface oxidation of the bottom ferromagnetic (FM) electrode, one of the major detrimental factors to the performance of a Magnetic Tunnel Junction (MTJ), is difficult to avoid during the fabrication process of the MTJ's tunnel barrier. Since Co rich alloys are commonly used for the FM electrodes in MTJs, over-oxidation of the tunnel barrier results in the formation of a CoO antiferromagnetic (AF) interface layer which couples with the bottom FM electrode to form a typical AF/FM exchange bias (EB) system. In this work, surface oxidation of the CoFe and CoFeB bottom electrodes was detected via magnetometry measurements of exchange-bias characterizations including the EB field, training effect, uncompensated spin density, and coercivity. Variations of these parameters were found to be related to the surface oxidation of the bottom electrode, among them the change of coercivity is most sensitive. Annealed samples show evidence for an oxygen migration back to the MgO tunnel barrier by annealing.