  Polycrystalline bulk Mn:ZnO ceramics with Mn nominal concentrations of 6, 11,
17 and 22 at.% were prepared trough solid-state reaction method and subjected
to a heat treatment in reducing atmosphere (Ar (95%) and H2 (5%)). The samples
were studied with particular emphasis on their compositions, structural, and
magnetic properties. A detailed microstructural and chemical analysis confirms
the Mn doping of the wurtzite ZnO structure mainly at the surface of the ZnO
grains. For the samples with higher Mn content, the secondary phases ZnMn2O4
and Mn1-xZnxO (Zn-doped MnO) were detected for the as prepared and the heat
treated samples, respectively. The structural change of the secondary phases
under heat treatment, from ZnMn2O4 to Mn1-xZnxO, confirms the effectiveness of
the heat treatment in to reduce the valence of the metallic ions and in the
formation of oxygen vacancies into the system. In spite of the induced defects,
the magnetic analysis present only a paramagnetic behavior with an
antiferromagnetic coupling between the Mn ions. In the context of the bound
magnetic polaron theory, it is concluded that oxygen vacancies are not the
necessary defect to promote the desired ferromagnetic order at room
temperature.
