Investigation of local atomic structure of Ni doped SnO₂ thin films via X-ray absorption spectroscopy and their magnetic properties

Nanostructured Ni (10 at.%) doped SnO₂ thin films were grown on Si (100) substrate via pulsed laser deposition technique in ultrahigh vacuum (UHV) chamber and oxygen partial pressure (Po₂) environment. The influence of UHV and Po₂ growth conditions on the ferromagnetic (FM) ordering, electronic states and short-range structure around Ni ions embedded in the SnO₂ network has been investigated. Synchrotron X-ray diffraction results revealed the single-phase nature of SnO₂ rutile structure without any foreign peak, and the mean crystallite sizes were found to be 12 and 25 nm for UHV and Po₂ deposited films respectively. The crystal growth in UHV chamber, introduced deliberately the oxygen vacancy (Vo) and reduced partially the valence state of Sn⁴⁺ (SnO₂) ions to Sn³⁺ (Sn₂O₃), whereas the Po₂ environment optimized the crystallinity and enhanced the oxygen stoichiometry (O:Sn = 2:1) by healing the oxygen vacancies. These details have been obtained by means of Raman spectra, near edge X-ray absorption fine structure at Ni L_3,2, O K edges and XANES spectra at Ni K edge. The films showed FM response and the saturation moments increase clearly from 4.4 emu/cm³ (0.33 µ_B/Ni) for Po₂ deposited film to 5.9 emu/cm³ (0.45 µ_B/Ni) for the film grown in UHV condition. Hence, the enhanced magnetization in UHV condition gives clear evidence on the importance of oxygen vacancies to activate the FM ordering. The role of Vo²⁺, in the first-shell of oxygen coordination around Sn/Ni ions, to achieve the FM response has been discussed on the basis of bound magnetic polaron and charge transfer percolation mechanisms.