Thermal dependence of the magnetization of antiferromagnetic copper(II) oxide nanoparticles

We report on the temperature dependence of the magnetization of nanoparticulate CuO prepared by both precipitation and solid-state reaction methods and having average particle sizes, D, ranging from 14 to 34 nm. The crystallographic and magnetic properties were studied by means of X-ray powder diffraction, X-ray photoelectron spectroscopy and magnetometry. We observed that the CuO sample having the smaller particle size (D=14 nm) showed a magnetic temperature transition of 170 K. This temperature is significantly lower than that measured in bulk CuO and in the CuO samples with D=27 and 34 nm (230 K in all three cases). This reduced transition temperature measured in the D=14 nm sample is compatible with the presence of a significant amount of oxygen defects in the core of the particles. We have also observed the occurrence of an increase in magnetization at low temperature, with a larger magnitude in the smaller particles and which we associate with the blocking of net local moments arising from the imbalance between the two antiferromagnetic sublattices linked to the presence of oxygen defects.