The magnetism of wurtzite CoO nanoclusters

TL;DR

This study investigates whether surface spins of wurtzite CoO nanoclusters can explain room temperature ferromagnetism in Co-doped ZnO, using computational methods to analyze size and defect effects on magnetic properties.

Contribution

It combines density functional theory and Monte Carlo simulations to show that nanocluster size and defects lower the critical temperature, challenging the explanation of room temperature ferromagnetism.

Findings

Critical temperature decreases with cluster size.

Defects further reduce the critical temperature.

Room temperature ferromagnetism is unlikely in ZnO:Co due to these effects.

Abstract

The possibility that the apparent room temperature ferromagnetism, often measured in Co-doped ZnO, is due to uncompensated spins at the surface of wurtzite CoO nanoclusters is investigated by means of a combination of density functional theory and Monte Carlo simulations. We find that the critical temperature extracted from the specific heat systematically drops as the cluster size is reduced, regardless of the particular cluster shape. Furthermore the presence of defects, in the form of missing magnetic sites, further reduces $T_\mathrm{C}$. This suggests that even a spinodal decomposed phase is unlikely to sustain room temperature ferromagnetism in ZnO:Co.