Effects of Ar/H2 annealing on the microstructure and magnetic properties of CoO nanoparticles

Abstract

Wurtzite CoO nanoparticles were annealed at 200–400 °C for 1 h in Ar/H₂. The effects of the annealing temperature on the structure, morphology and magnetic properties were investigated in detail. As the annealing temperatures rise, the obtained samples change from the hcp-CoO/fcc-CoO complex phase to the fcc-CoO/fcc-Co complex phase to the fcc-Co single phase. Notably, the 300 °C- and 325 °C-annealed samples show the most distinct coexistence of ferromagnetic (FM) Co and antiferromagnetic (AFM) CoO components, and therefore display certain exchange bias (H_E = 284 and 250 Oe) and enhanced coercivity (H_C = 1583 and 1148 Oe) at 5 K. The magnetization M, H_C, H_E and Néel temperature T_N exhibit interesting change rules, which are determined by three factors: size effect, phase composition, and FM-AFM interface coupling effect. Obviously, as adjusting the annealing temperature can control the microstructure of CoO (or Co) particle samples, many superior magnetic properties can be expected.