Controlling exchange bias in Co-CoOx nanoparticles by oxygen content

TL;DR

This study demonstrates how controlling oxygen content during fabrication influences the exchange bias in Co-CoOx nanoparticles, revealing a tunable magnetic interaction crucial for spintronic applications.

Contribution

It introduces a method to tune exchange bias in Co-CoOx nanoparticles by adjusting oxygen pressure during laser ablation, enabling control over magnetic properties.

Findings

Optimal oxygen pressure (~10^{-3} mbar) yields maximum exchange bias (~900 Oe).

Exchange bias arises from the interplay of ferromagnetic and antiferromagnetic phases.

High irreversible hysteresis loops are due to exchange coupling and AFM cluster blocking.

Abstract

We report on the occurrence of exchange bias on laser-ablated granular thin films composed of Co nanoparticles embedded in amorphous zirconia matrix. The deposition method allows controlling the degree of oxidation of the Co particles by tuning the oxygen pressure at the vacuum chamber (from 2x10^{-5} to 10^{-1} mbar). The nature of the nanoparticles embedded in the nonmagnetic matrix is monitored from metallic, ferromagnetic (FM) Co to antiferromagnetic (AFM) CoOx, with a FM/AFM intermediate regime for which the percentage of the AFM phase can be increased at the expense of the FM phase, leading to the occurrence of exchange bias in particles of about 2 nm in size. For oxygen pressure of about 10-3 mbar the ratio between the FM and AFM phases is optimum with an exchange bias field about 900 Oe at 1.8 K. The mutual exchange coupling between the AFM and FM is also at the origin of the induced exchange anisotropy on the FM leading to high irreversible hysteresis loops, and the blocking of the AFM clusters due to proximity to the FM phase.