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 TiO
₂
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 Mn
²⁺
hyperfine lines, revealing the substitution of Mn
²⁺
into Mg
²⁺
, Ca
²⁺
and Zn
²⁺
sites in the host lattices MgO, CaO and ZnO, respectively. However, such features are absent from samples SrO:Mn, BaO:Mn, NiO:Mn and TiO
₂
: Mn, where their spectra can be a symmetrical single line (for SrO:Mn, BaO:Mn and TiO
₂
: 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 MnO
₂
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 MnO
₂
formed by the annealing.