Kazuhisa Inaba

A transparent metal: Nb-doped anatase TiO2

This Letter focuses on the discovery of a transparent conducting oxide (TCO), anatase Ti1−xNbxO2 films with x=0.002–0.2. The resistivity of films with x⩾0.03 is 2–3×10−4Ωcm at room temperature. The carrier density of Ti1−xNbxO2 can be controlled in a range of 1×1019to2×1021cm−3. The internal transmittance for films with x⩽0.03 (40nm thickness) is about 97% in the visible light region. The transport and optical parameters are comparable to those of typical TCOs, such as In2−xSnxO3 and ZnO.

Rutile-type oxide-diluted magnetic semiconductor: Mn-doped SnO2

Epitaxial films of an oxide-diluted magnetic semiconductor with rutile structure, Mn-doped SnO2, have been fabricated by pulsed-laser deposition. As the Mn content increases, systematic changes in lattice constants and in-gap absorption are observed. Magnetization measurements show almost paramagnetic behavior. The injection of n-type carrier over 1020 cm−3 is achieved by Sb doping. A Sn0.95Mn0.05O2:Sb film shows giant positive magnetoresistance as large as 60% at 5 K.

Ta-doped Anatase TiO2 Epitaxial Film as Transparent Conducting Oxide

We present electrical transport and optical properties of Ta-doped TiO2 epitaxial thin films with varying Ta concentration grown by the pulsed laser deposition method. The Ti0.95Ta0.05O2 film exhibited a resistivity of 2.5×10-4 Ω cm at room temperature, and an internal transmittance of 95% in the visible light region. These values are comparable to those of a widely used transparent conducting oxide (TCO), indium tin oxide. Furthermore, this new material falls into a new category of TCOs that utilizes d electrons.

Carrier induced ferromagnetism in Nb doped Co:TiO2 and Fe:TiO2 epitaxial thin film

We have investigated the carrier induced magnetic properties of anatase Ti1−x−yNbxMyO2 (M=Co,Fe) epitaxial films grown by the pulsed laser deposition technique. For Ti0.95−xNbxCo0.05O2, the n-type carrier density could be controlled in a wide range (4.9×1017cm−3to2.7×1021cm−3) by Nb doping (x=0–0.2). The temperature dependence of the resistivity showed metallic behavior, suggesting that Ti0.95−xNbxCo0.05O2 undergoes a semiconductor to metal transition along with a slight carrier doping less than x<0.03. In both Co-doped and Fe-doped films, we have confirmed hysteresis in M-H curves, and the anomalous Hall effect at room temperature. This strongly suggests that the charge carriers are spin polarized and mediate ferromagnetic interaction between local spins on transition metal ions. In the case of Ti0.94−xNbxFe0.06O2, ferromagnetism is sensitive to carrier concentration. That is, the x=0.002 film is nonmagnetic even at 3K, while room-temperature ferromagnetism appears at x=0.01.

Intrinsic Faraday spectra of ferromagnetic rutile Ti1−xCoxO2−δ

We have investigated the Faraday spectra of ferromagnetic rutile Ti1−xCoxO2−δ films grown on Al2O3 (0001). The Faraday spectra strongly depended on the film thickness, revealing the significant effect of the multiple reflections of light. The intrinsic Faraday spectra of rutile Ti1−xCoxO2−δ were corrected for this multiple reflection effect on the basis of an optical model and compared with the absorption spectra in detail. The quantitatively good correspondence between intrinsic Faraday ellipticity and optical band edge strongly suggests that rutile Ti1−xCoxO2−δ is an intrinsic ferromagnetic semiconductor. The exchange parameters, N0α and N0β, were estimated as the order of ∼0.1eV.

Enhancement of Magneto-Optical Properties of Anatase Co:TiO2 Co-Doped with Nb

We have fabricated Ti0.95-xNbxCo0.05O2 thin films by the pulsed laser deposition technique. Carrier density was found to be almost identical to the Nb concentration up to x=0.06, indicating that doped Nb atoms generate carriers with >80% efficiency. In this doping range, magneto-optical properties were significantly enhanced by Nb-doping. The remanent Faraday rotation at a wavelength of 405 nm was as high as 5.6 ×103 deg/cm, which is approximately one order of magnitude higher than that of a film free of Nb-doping. The present results support the hypothesis that conduction electrons play an essential role in the ferromagnetism of Co-doped TiO2.

Magnetic Properties of Rutile Ti1-xFexO2 Epitaxial Thin Films

Growth conditions for pulsed laser deposition of ferromagnetic Ti1-xFexO2 films with the rutile structure have been optimized on the basis of magnetooptical Kerr effect (MOKE) measurements. Thus, it was found that Ti0.94Fe0.06O2 films prepared under very limited growth conditions, at a substrate temperature of 650–675°C and an oxygen pressure of ~1 ×10-6 Torr, show ferromagnetism at room temperature. The optimized films revealed a saturation magnetization of 1.3 µB and a coercive field of 0.14 kOe. From XPS measurements, the valence state of Fe was determined to be 3+, ruling out the possibility that ferromagnetism may arise from Fe3O4 nanoparticles.

FERROMAGNETISM IN ANATASE TiO2 CODOPED WITH Co AND Nb

The magnetic moment and the coercive force of ferromagnetic anatase TiO2 films, codoped with Co and Nb, are enhanced and controlled by carrier doping. The carrier density can be varied independent of the Co concentration by Nb doping amount in this transparent room-temperature ferromagnetic degenerate semiconductor. The film with Co5% and Nb6% showed a Faraday rotation angle of about 1.5×10 deg/cm at a wavelength of 400 nm, showing a good linear dependence on Nb doping. An anomalous Hall effect is observed in magnetotransport measurements, showing that the charge carriers are spin polarized, revealing the magnetic interaction between itinerant electrons and localized Co spins.