Magnetic Conductive Outer Layer in Oxygen-Deficient TiO₂ Single Crystals

A magnetic conductive outer layer at insulating rutile TiO₂ crystals is obtained by high-temperature annealing at low oxygen pressure. The conductivity here is closely dependent on the annealing temperature and time and originates from oxygen vacancies, which has been verified by the observed mixed valence of Ti^3+/4+. The most striking observation is the occurrence of ferromagnetism demonstrated by ferromagnetic hysteresis and anomalous Hall effect. The ferromagnetic hysteresis is weakly temperature dependent, which implies a defect-induced ferromagnetism. The anomalous Hall effect provides transport evidence for the existence of a long-range ferromagnetic order. The present work presents a prototypical case of designing surface physics from the interplay of multiple factors.