Effects of rapid thermal annealing on the local environment, electronic structure and magnetic properties of Mn doped TiO2 thin films

Abstract

In this report, nanostructure thin films of Ti0.97 Mn0.03 O2−δ were deposited on Si (001) substrate by pulsed laser deposition (PLD) technique, followed by rapid thermal annealing (RTA) in different ambient gases; O2, N2 and Ar. The RTA treatment dramatically affected the surface defects, local environment, electronic structure and magnetic properties of the films, where, annealing of the film in O2 gas recovered the dislocated atoms of oxygen vacancy (Vo) at the surfaces and induced a diamagnetic phase, whereas the annealing of film in N2 gas introduced Vo, reduced the valence state of Ti4+ (TiO2) into Ti3+ (Ti2O3) and induced ferromagnetic (FM) signal. Therefore, the change in the local atomic defects of Vo at the surface texture is likely to be responsible for the magnetic response. The presence of oxygen vacancy has been traced by means of Raman scattering and near edge X-ray absorption fine structure (NEXAFS) spectroscopy measurements. The NEXAFS spectra were carried out in synchrotron facility at Ti/Mn L3,2 and O-K edges. The spectral feature at Ti L3,2 edges revealed the rutile phase of TiO2. The asymmetry of t2g and eg bands at the O-K edge confirmed the formation of Vo and reflected the modification in the O co-ordination around Ti4+cations. The Mn L3,2 edges revealed the incorporation of Mn ions in the TiO2 network with Mn2+ and Mn3+ mixed valence states. This different valance states (Mn2+ and Mn3+) is the basis for the ferromagnetism induced by Stoner spin-splitting of the local density of defects state at Fermi level (EF). The change in the magnetic moment of the films after RTA process is related to the change in the local density of defects band. Crystal field and charge transfer parameters were extracted roughly by using atomic multiplet calculations (theory).