Effect of Reducing Atmosphere on the Magnetism of Zn1-xCoxO Nanoparticles

TL;DR

This study investigates how reducing atmospheres influence the magnetic properties of Zn1-xCoxO nanoparticles, revealing that oxygen vacancies induced by a forming gas atmosphere enable room-temperature ferromagnetism.

Contribution

It demonstrates that oxygen vacancies created in a reducing atmosphere are crucial for inducing room-temperature ferromagnetism in Zn1-xCoxO nanoparticles, a novel insight into their magnetic behavior.

Findings

Reducing atmosphere induces oxygen vacancies in Zn1-xCoxO nanoparticles.

Oxygen vacancies are essential for room-temperature ferromagnetism.

Air-annealed samples remain non-magnetic down to 77K.

Abstract

We report the crystal structure and magnetic properties of Zn1-xCoxO nanoparticles synthesized by heating metal acetates in organic solvent. The nanoparticles were crystallized in wurtzite ZnO structure after annealing in air and in a forming gas (Ar95%+H5%). The X-ray diffraction and X-ray photoemission spectroscopy (XPS) data for different Co content show clear evidence for the Co+2 ions in tetrahedral symmetry, indicating the substitution of Co+2 in ZnO lattice. However samples with x=0.08 and higher cobalt content also indicate the presence of Co metal clusters. Only those samples annealed in the reducing atmosphere of the forming gas, and that showed the presence of oxygen vacancies, exhibited ferromagnetism at room temperature. The air annealed samples remained non-magnetic down to 77K. The essential ingredient in achieving room temperature ferromagnetism in these Zn1-xCoxO nanoparticles was found to be the presence of additional carriers generated by the presence of the oxygen vacancies.