Nanosized superparamagnetic precipitates in cobalt-doped ZnO

TL;DR

This study demonstrates that room temperature ferromagnetism in cobalt-doped ZnO is due to nanosized superparamagnetic cobalt precipitates, not carrier-mediated exchange, clarified through comprehensive magnetic and microstructural analysis.

Contribution

It provides definitive evidence linking ferromagnetic behavior in Co-doped ZnO to cobalt nano-inclusions rather than diluted magnetic moments.

Findings

Ferromagnetism arises from cobalt nano-inclusions.

No evidence of carrier-mediated ferromagnetism.

Microstructural analysis confirms cobalt precipitates.

Abstract

The existence of semiconductors exhibiting long-range ferromagnetic ordering at room temperature still is controversial. One particularly important issue is the presence of secondary magnetic phases such as clusters, segregations, etc... These are often tedious to detect, leading to contradictory interpretations. We show that in our cobalt doped ZnO films grown homoepitaxially on single crystalline ZnO substrates the magnetism unambiguously stems from metallic cobalt nano-inclusions. The magnetic behavior was investigated by SQUID magnetometry, x-ray magnetic circular dichroism, and AC susceptibility measurements. The results were correlated to a detailed microstructural analysis based on high resolution x-ray diffraction, transmission electron microscopy, and electron-spectroscopic imaging. No evidence for carrier mediated ferromagnetic exchange between diluted cobalt moments was found. In contrast, the combined data provide clear evidence that the observed room temperature ferromagnetic-like behavior originates from nanometer sized superparamagnetic metallic cobalt precipitates.