Defect Induced Room Temperature Ferromagnetism in High Quality Co-doped ZnO Bulk Samples

TL;DR

This study demonstrates that controlled defect introduction in high-quality Co-doped ZnO bulk samples induces room temperature ferromagnetism, explained by magnetic polarons mediating Co ion coupling, supported by experimental and theoretical analysis.

Contribution

It provides evidence that defect engineering can induce ferromagnetism in Co-doped ZnO, clarifying the role of defects and magnetic polarons in this phenomenon.

Findings

Ferromagnetism increases with defect density.

Magnetic behavior explained by bound magnetic polaron model.

Defects mediate ferromagnetic coupling between Co ions.

Abstract

The nature of the often reported room temperature ferromagnetism in transition metal doped oxides is still a matter of huge debate. Herein we report on room temperature ferromagnetism in high quality Co-doped ZnO (Zn1-xCoxO) bulk samples synthesized via standard solid-state reaction route. Reference paramagnetic Co-doped ZnO samples with low level of structural defects are subjected to heat treatments in a reductive atmosphere in order to introduce defects in the samples in a controlled way. A detailed structural analysis is carried out in order to characterize the induced defects and their concentration. The magnetometry revealed the coexistence of a paramagnetic and a ferromagnetic phase at room temperature in straight correlation with the structural properties. The saturation magnetization is found to increase with the intensification of the heat treatment, and, therefore, with the increase of the density of induced defects. The magnetic behavior is fully explained in terms of the bound magnetic polaron model. Based on the experimental findings, supported by theoretical calculations, we attribute the origin of the observed defect-induced-ferromagnetism to the ferromagnetic coupling between the Co ions mediated by magnetic polarons due to zinc interstitial defects.