Abstract
The existence of high-temperature ferromagnetism in thin films and nanoparticles of oxides containing small quantities of magnetic dopants remains controversial. Some regard these materials as dilute magnetic semiconductors, while others think they are ferromagnetic only because the magnetic dopants form secondary ferromagnetic impurity phases such as cobalt metal or magnetite. There are also reports in d0 systems and other defective oxides that contain no magnetic ions. Here, we investigate TiO2 (rutile) containing 1–5% of iron cations and find that the room temperature ferromagnetism of films prepared by pulsed-laser deposition is not due to magnetic ordering of the iron. The films are neither dilute magnetic semiconductors nor hosts to an iron-based ferromagnetic impurity phase. A new model is developed for defect-related ferromagnetism, which involves a spin-split defect band populated by charge transfer from a proximate charge reservoir—in the present case a mixture of Fe2+ and Fe3+ ions in the oxide lattice. The phase diagram for the model shows how inhomogeneous Stoner ferromagnetism depends on the total number of electrons Ntot, the Stoner exchange integral I and the defect bandwidth W; the band occupancy is governed by the d–d Coulomb interaction U. There are regions of ferromagnetic metal, half-metal and insulator as well as non-magnetic metal and insulator. A characteristic feature of the high-temperature Stoner magnetism is an anhysteretic magnetization curve, which is practically temperature independent below room temperature. This is related to a wandering ferromagnetic axis, which is determined by local dipole fields. The magnetization is limited by the defect concentration, not by the 3d doping. Only 1–2% of the volume of the films is magnetically ordered.
Highlights
The existence of high-temperature ferromagnetism in thin films and nanoparticles of oxides containing small quantities of magnetic dopants remains controversial
We develop a new general model based on our idea of charge transfer ferromagnetism [37]
Based on our study of the magnetism of Fe-doped TiO2, and recent results by others on related materials [29, 44, 48], we have proposed a new theory for the origin of ferromagnetic-like behaviour of thin films of dilute magnetic oxides and other d0 materials where there is no trivial explanation in terms of clusters of a ferromagnetic impurity phase
Summary
A series of films was prepared by pulsed-laser deposition from sintered targets made from 99.999% pure TiO2 and 95% isotopically enriched 57Fe2O3. Substrates were 5 × 5 × 0.5 mm squares of R-cut (1 ̄ı02) sapphire, polished on both sides. They were maintained at 750 ◦C during deposition, in 102 or 10−3 Pa ambient oxygen pressure, using a KrF excimer laser operating at 248 nm and 10 Hz. Laser fluence on the target was 2 J cm−2. The resulting films were 70–140 nm thick, with an rms roughness of 2.5 nm. The 57Fe Mössbauer spectra were collected at room temperature in transmission geometry with a 57Co (Rh) source. Optical spectra were recorded at room temperature using a Perkin-Elmer dual-beam spectrophotometer
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