Structural and magnetic properties of wurtzite CoO thin films

Hexagonal CoO thin films on (0 0 0 1) Al2O3 and ZnO are prepared by off-axis and on-axis pulsed laser deposition. The structural properties were studied by in situ reflection high-energy electron diffraction and ex situ x-ray diffraction. The magnetic properties of the films depend on the mode of growth. The films grown in a normal geometry are a mixture of Co3O4 and CoO and show ferromagnetism at room temperature, whereas the films grown off-axis are single phase and present a Curie-Weiss behaviour. Wurtzite CoO does not order magnetically because the antiferromagnetic Co2+-O2--Co2+ superexchange is geometrically frustrated. It is proposed that defective Co3O4 in thin films is ferrimagnetic, due to the appearance of high spin Co3+ on B sites.