Electronic structure depiction of magnetic origin in BaTiO_(3-δ) thin film: A combined experimental and first-principles based investigation

With the motive of unraveling the origin of native vacancy induced magnetization in ferroelectric perovskite oxide systems, here we explore the consequences of electronic structure modification in magnetic ordering of oxygen deficient epitaxial BaTiO$_{3-\delta}$ thin films. Our adapted methodology employs state-of-the-art experimental approaches viz. photo-emission, photo-absorption spectroscopies, magnetometric measurements duly combined with first principles based theoretical methods within the frame work of density functional theory (DFT and DFT+\textit{U}) calculations. Oxygen vacancy (O$ _{V} $) is observed leading partial population of Ti 3\textit{d} (t$_{2g}$), which induces defect state in electronic structure near the Fermi level and reduces the band gap. The oxygen deficient BaTiO$_{2.75} $ film reveals Mott-Hubbard insulator characteristic, in contrast to the band gap insulating nature of the stoichiometric BaTiO$ _{3}$. The observed magnetic ordering is attributed to the asymmetric distribution of spin polarized charge density in the vicinity of O$ _{V} $ site which originates unequal magnetic moment values at first and second nearest neighboring Ti sites, respectively. Hereby, we present an exclusive method for maneuvering the band gap and on-site electron correlation energy with consequences on magnetic properties of BaTiO$_{3-\delta}$ system, which can open a gateway for designing novel single phase multiferroic system.