Magnetic Properties Of Al Doped TiO₂Films

Magnetic semiconductors are considered as an important kind of electronicmaterials in the21stcentury and caught great attention from researchers, onaccount of its combination of semiconductors and magnetic materials. Amongmagnetic semiconductors, oxide diluted magnetic semiconductor is the researchfocus, since its Curie temperature is higher than room temperature. But the originof its ferromagnetism is controversial, mainly because it’s difficult to exclude theinterferences from the magnetic metal clusters and oxide composed of magneticdoping ions.Non-magnetic Al-doped TiO₂thin films are perfect research objects todetermine the intrinsic origin of TiO₂based oxide diluted magneticsemiconductors’ ferromagnetism, because the un-intrinsic factors are excludednaturally due to both aluminum metal and oxide are not magnetic.The Al-doped TiO₂thin film samples with different concentration of Al areprepared by the pulsed laser deposition （PLD） under1Pa oxygen pressure. X-raydiffraction shows that all the films are rutile （200） epitaxial orientated, andconfirms the room temperature ferromagnetism of Al-doped TiO₂is confirmed bythe measurements of magnetic hysteresis curves at room temperature. X-rayphotoelectron spectroscopy shows that no aluminum metal clusters or oxide isproduced, but Ti³⁺ions. Tests of Hall effect show that the samples are n-typesemiconductor with¹017/cm-3carrier concentration. The first principlecalculation shows that oxygen vacancies play an important role in the origi n ofthe ferromagnetism, and magnetic changes can be explained by the boundmagnetic polarons theory. The doping of Al³⁺produces the oxygen vacancies, andthe adjacent Ti4+is changed into Ti³⁺with d electron by the oxygen vacancies,thereby Al³⁺-VO-Ti³⁺magnetic polarons are formed. As the doping concentrationof Al gets higher, the amount of Al³⁺-VO-Ti³⁺magnetic polarons increasesgradually, but after the doping concentration exceeds the threshold, Al³⁺-VO-Al³⁺structures emerge instead of Al³⁺-VO-Ti³⁺magnetic polarons and the magneticmoment decreases. Both experimental results and theoretical analysis revealed that the Al³⁺-VO-Ti³⁺magnetic polarons induced by Al doping in thin film is themain reason of the intrinsic ferromagnetism.To study the oxygen vacancy’s role in the ferromagnetism’s origin, themagnetic properties and transport properties of thin TiO₂films formed in vacuumare studied. All the films have rutile （200） epitaxial orientation, and the peakpositions move low angle, indicating large amounts of oxygen vacancies. Roomtemperature hysteresis curves measurements show that the Al-doped TiO₂ismagnetic under room temperature, and there are Ti³⁺ions according to X rayphotoelectron spectroscopy. Hall effect tests show that Al-doped samples aren-type semiconductor with¹021/cm-3carrier concentration. The magneticmoment changes according to not only different carrier concentration, but alsothe environment the samples were prepared in: the samples prepared in vacuumhave much larger magnetic moments than those prepared under oxygen pressure.This is because the carrier concentration is so high that the carriers jump betweenthe Ti³⁺and Ti4+ions, which constitutes the long range orderly arrangement ofd-electrons, leading to the ferromagnetic.In conclusion, Al-doped TiO₂thin film’s room temperature ferromagnetismis intrinsic, and oxygen defects play an important role: a small amount of carriersgenerated by a small amount of oxygen defects, the carriers with adjacent Ti ionsforms the magnetic polarons, leading to room temperature ferromagnetism; highconcentration of oxygen defects will produce a large number of carriers, whichjump between the Ti³⁺and Ti4+ions, hence the long range orderly arrangement ofd electrons and the ferromagnetism.