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    Absence of ferromagnetic order in high quality bulk Co-doped ZnO samples.
    (2010) Carvalho, Hugo Bonette de; Godoy, Marcio Peron Franco de; Pais, Rafael Witter Dias; Mir, Mirta; Zevallos, Ângela Maria Ortiz de; Iikawa, Fernando; Brasil, Maria José Santos Pompeu; Chitta, Valmir Antônio; Ferraz, Wilmar Barbosa; Boselli, Marco Aurélio; Sabioni, Antônio Claret Soares
    Bulk Zn1−xCoxO samples were synthesized via standard solid-state reaction route with different Co molar concentrations up to 21%. A detailed microstructural analysis was carried out to investigate alternative sources of ferromagnetism, such as secondary phases and nanocrystals embedded in the bulk material. Conjugating different techniques we confirmed the Zn replacement by Co ions in the wurtzite ZnO structure, which retains, however, a high crystalline quality. No segregated secondary phases neither Co-rich nanocrystals were detected. Superconducting quantum interference device magnetometry demonstrates a paramagnetic Curie–Weiss behavior with antiferromagnetic interactions. We discuss the observed room temperature paramagnetism of our samples considering the current models for the magnetic properties of diluted magnetic semiconductors.
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    Evidence of defect-mediated magnetic coupling on hydrogenated Co-doped ZnO.
    (2013) Godoy, Marcio Peron Franco de; Mesquita, Alexandre; Avansi Junior, Waldir; Neves, P. P.; Chitta, Valmir Antônio; Ferraz, Wilmar Barbosa; Boselli, Marco Aurélio; Sabioni, Antônio Claret Soares; Carvalho, Hugo Bonette de
    Hydrogenated bulk Zn1_xCoxO samples were synthesized via standard solid-state reaction route with Co molar concentrations up to 15 at.%. Magnetic characterization demonstrates a room temperature ferromagnetic behavior associated to a paramagnetic Curie–Weiss component. Detailed microstructural analysis was carried out to exclude the presence of extrinsic sources of ferromagnetism. The magnetization increases linearly as a function of Co concentration. Hall measurements reveal an insulating character for the whole set of samples. In this context, the defect mediated magnetic coupling between the Co atoms under the scope of the bound magnetic polarons model is used to interpret the observed room temperature ferromagnetism.