Oxygen-vacancy-induced ferromagnetism in undoped SnO₂ thin films

Анотація

We investigated the possible formation and segregation of oxygen vacancies near the surface of SnO₂ thin films from oxygen K-edge x-ray emission and absorption spectra and found that the distribution of O 2p unoccupied states for ferromagnetic SnO₂ thin films is different from that of postannealed SnO₂ films under oxygen atmosphere showing diamagnetic behavior. This spectroscopic result suggests that oxygen vacancies can be the source of the surface-induced magnetism in SnO₂ thin films. This possibility was then explored by calculating the lowest energy levels of the structural defects (impurities or neutral vacancies) with two localized carriers near the surface of SnO₂ film using a quantum-mechanical approach combined with the image charge method. A magnetic triplet state is found to be the ground state of those defects in the vicinity of the SnO₂ surface, whereas the nonmagnetic singlet is the ground state of bulk SnO₂. Surface-induced ferromagnetic order can appear at room temperature via 2D magnetic percolation once the vacancy concentration is greater than 3*10¹⁶ m^-2.