Giant exchange bias and ferromagnetism in the CoO shell of Co/CoO-MgO core-shell nanoparticles

TL;DR

This study demonstrates giant exchange bias and ferromagnetism in Co/CoO-MgO core-shell nanoparticles, revealing how the CoO shell's magnetic properties and the exchange bias field depend on composition and impurities.

Contribution

It reports the synthesis and characterization of Co/CoO-MgO nanoparticles showing unprecedented exchange bias and ferromagnetism linked to the CoO shell, with insights into the effects of impurities.

Findings

Giant exchange bias field HE=2460 Oe at 2K observed.

Ferromagnetic signal detected from CoO shell via x-ray dichroism.

Reducing Mg-impurities decreases FM signal and HE.

Abstract

Using magnetron sputtering, we produced a series of Co/CoO-MgO nanoparticles on Si(100) substrates. High-resolution transmission electron microscopy (HRTEM) image shows that small isolated Co-clusters (core) covered with CoO (shells) with a size of a few nm embedded in a MgO matrix. Resistivity as a function of Co atomic ratio exhibits a distinct percolation threshold with a sharp decrease around 69% Co content. Across the threshold, the resistivity drops about 7 orders of magnitude. For a sample at this percolation critical threshold, we have observed a giant exchange bias field HE=2460 Oe at T= 2K, and using soft x-ray magnetic circular dichroism at the Co-L2,3 edge, we have detected a ferromagnetic (FM) signal originating from the antiferromagnetic CoO shell. Moreover, decreasing the Mg-impurities will reduce the FM signal from CoO shell (namely the uncompensated spin density) and the size of HE, thus directly support the uncompensated spin model.