title: Paramagnetism of cobalt-doped ZnO nanoparticles obtained by microwave solvothermal synthesis.
abstract: Zinc oxide nanopowders doped with 1-15 mol % cobalt were produced by the microwave solvothermal synthesis (MSS) technique. The obtained nanoparticles were annealed at 800 °C in nitrogen (99.999%) and in synthetic air. The material nanostructure was investigated by means of the following techniques: X-ray diffraction (XRD), helium pycnometry density, specific surface area (SSA), inductively coupled plasma optical emission spectrometry (ICP-OES), extended X-ray absorption fine structure (EXAFS) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and with magnetometry using superconducting quantum interference device (SQUID). Irrespective of the Co content, nanoparticles in their initial state present a similar morphology. They are composed of loosely agglomerated spherical particles with wurtzite-type crystal structure with crystallites of a mean size of 30 nm. Annealing to temperatures of up to 800 °C induced the growth of crystallites up to a maximum of 2 μm in diameter. For samples annealed in high purity nitrogen, the precipitation of metallic α-Co was detected for a Co content of 5 mol % or more. For samples annealed in synthetic air, no change of phase structure was detected, except for precipitation of Co3O4 for a Co content of 15 mol %. The results of the magentometry investigation indicated that all as-synthesized samples displayed paramagnetic properties with a contribution of anti-ferromagnetic coupling of Co-Co pairs. After annealing in synthetic air, the samples remained paramagnetic and samples annealed under nitrogen flow showed a magnetic response under the influences of a magnetic field, likely related to the precipitation of metallic Co in nanoparticles.
authors.affiliation: Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland.
authors.forename: Jacek
authors.initials: J
authors.lastname: Wojnarowicz
authors.affiliation: Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland.
authors.forename: Sylwia
authors.initials: S
authors.lastname: Kusnieruk
authors.affiliation: Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland.
authors.forename: Tadeusz
authors.initials: T
authors.lastname: Chudoba
authors.affiliation: Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland.
authors.forename: Stanislaw
authors.initials: S
authors.lastname: Gierlotka
authors.affiliation: Institute of High Pressure Physics, Polish Academy of Sciences, Sokolowska 29/37, 01-142 Warsaw, Poland.
authors.forename: Witold
authors.initials: W
authors.lastname: Lojkowski
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland.
authors.forename: Wojciech
authors.initials: W
authors.lastname: Knoff
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland.
authors.forename: Malgorzata I
authors.initials: MI
authors.lastname: Lukasiewicz
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland.
authors.forename: Bartlomiej S
authors.initials: BS
authors.lastname: Witkowski
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland.
authors.forename: Anna
authors.initials: A
authors.lastname: Wolska
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland.
authors.forename: Marcin T
authors.initials: MT
authors.lastname: Klepka
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland.
authors.forename: Tomasz
authors.initials: T
authors.lastname: Story
authors.affiliation: Institute of Physics, Polish Academy of Sciences, Al. Lotników 32/46, 02-668 Warsaw, Poland.
authors.forename: Marek
authors.initials: M
authors.lastname: Godlewski
keywords: cobalt-doped zinc oxide; ferromagnetism; magnetic properties; microwave solvothermal synthesis (MSS); paramagnetism