Influence of Annealing Temperature on Spin Dynamics of Mn in Metal Oxides: Electron Spin Resonance Study

This work presents the influence of the annealing temperature (T-an) on electron spin resonance (ESR) spectra of 2.0 wt.% Mn-doped metal oxides MO (Mn : MO, with M = Mg, Ca, Sr, Ba, Zn and Ni) and TiO2 annealed at temperatures T-an = 400 degrees C - 1150 degrees C. Experimental data obtained reveal that their ESR spectra depend strongly on T-an. There is a temperature value T-tr located in between 600 degrees C and 800 degrees C, where characteristic spectra of samples have a change in shape. For samples MgO:Mn, CaO:Mn and ZnO:Mn, their spectra at temperatures T-an >= T-tr show the Mn2+ hyperfine lines, revealing the substitution of Mn2+ into Mg2+, Ca2+ and Zn2+ sites in the host lattices MgO, CaO and ZnO, respectively. However, such features are absent from samples SrO:Mn, BaO:Mn, NiO:Mn and TiO2:Mn, where their spectra can be a symmetrical single line (for SrO:Mn, BaO:Mn and TiO2:Mn) or an asymmetrical line (for NiO:Mn). At temperatures T-an < T-tr, the spectra of the samples usually consist of two lines, one is attributed to MnO2 with the paramagnetic behavior, and the other is assigned to a ferromagnetic (and/or antiferromagnetic) phase probably caused by a magnetic correlation present at the interface between Mn and MnO2 formed by the annealing.