Magnetic properties of undoped Cu₂O fine powders with magnetic impurities and/or cation vacancies

Fine powders of micron-and submicron-sized particles of undoped Cu ₂O semiconductor, with three different sizes and morphologies, have been synthesized by different chemical processes. These samples include nanospheres 200nm in diameter, octahedra of size 1νm and polyhedra of size 800nm. They exhibit a wide spectrum of magnetic properties. At low temperature, T = 5K, the octahedron sample is diamagnetic with the magnetic susceptibility χ_OH = -9.5 × 10^-6emug^-1Oe ^-1. The nanosphere is paramagnetic with χ_NS = 2.2 × 10^-5emug^-1Oe^-1. The other two polyhedron samples synthesized in different runs by the same process are found to show different magnetic properties. One of them exhibits weak ferromagnetism with T_C∼455K and saturation magnetization M_S∼0. 19emug^-1 at T = 5K, while the other is paramagnetic with χ = 1.0 × 10^-5emug^-1Oe^-1. The total magnetic moment estimated from the detected impurity concentration of Fe, Co and Ni, is too small to account for the observed magnetism by one to two orders of magnitude. Calculations by density functional theory (DFT) reveal that cation vacancies in the Cu₂O lattice are one of the possible causes of induced magnetic moments. The results further predict that the defect-induced magnetic moments favour a ferromagnetically coupled ground state if the local concentration of cation vacancies, n_C, exceeds 12.5%. This offers a possible scenario to explain the observed magnetic properties. The limitations of the investigations in the present work, in particular in the theoretical calculations, are discussed and possible areas for further study are suggested.