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°C - 1150°C. Experimental data obtained reveal that their ESR spectra depend strongly on Tan. There is a temperature value T_tr located in between 600°C and 800°C, where characteristic spectra of samples have a change in shape. For samples MgO:Mn, CaO:Mn and ZnO:Mn, their spectra at temperatures Tan ¿ 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.