Hidemi Toyosaki

Exploration of oxide-based diluted magnetic semiconductors toward transparent spintronics

A review is given for the recent progress of research in the field of oxide-based diluted magnetic semiconductor (DMS), which was triggered by combinatorial discovery of transparent ferromagnet. The possible advantages of oxide semiconductor as a host of DMS are described in comparison with conventional compound semiconductors. Limits and problems for identifying novel ferromagnetic DMS are described in view of recent reports in this field. Several characterization techniques are proposed in order to eliminate unidentified ferromagnetism of oxide-based DMS unidentified ferromagnetic oxide (UFO). Perspectives and possible devices are also given.

Magnetic oxide semiconductors

Magnetic oxide semiconductors, oxide semiconductors doped with transition metal elements, are one of the candidates for a high Curie temperature ferromagnetic semiconductor that is important to realize semiconductor spintronics at room temperature. We review in this paper recent progress of research on various magnetic oxide semiconductors. The magnetization, magneto-optical effect and magneto-transport such as the anomalous Hall effect are examined from the viewpoint of feasibility to evaluate the ferromagnetism. The ferromagnetism of Co-doped TiO2 and transition metal-doped ZnO is discussed.

Evolution of ferromagnetic circular dichroism coincident with magnetization and anomalous Hall effect in Co-doped rutile TiO2

Magnetic circular dichroism (MCD) of rutile Ti1−xCoxO2−δ is systematically examined with various x and δ to reveal a phase diagram for the appearance of ferromagnetism at higher carrier concentration and Co content. The phase diagram exactly matches with that determined from anomalous Hall effect (AHE). The magnetic field dependence of MCD also shows good coincidence with those of the magnetization and AHE. The coincidence of these independent measurements strongly suggests single and intrinsic ferromagnetic origin.

Anomalous Hall effect in anatase Ti1−xCoxO2−δ at low temperature regime

Anomalous Hall effect (AHE) of a ferromagnetic semiconductor anatase Ti1−xCoxO2−δ thin film is studied from 10to300K. Magnetic field dependence of anomalous Hall resistance is coincident with that of magnetization, while the anomalous Hall resistance decreases at low temperature in spite of nearly temperature-independent magnetization. Anomalous Hall conductivity σAHE is found to be proportional to the square of Hall mobility, suggesting that charge scattering strongly affects the AHE in this system. The anatase Ti1−xCoxO2−δ also follows a scaling relationship to conductivity σxx as σAHE∝σxx1.6, which was observed for another polymorph rutile Ti1−xCoxO2−δ, suggesting an identical mechanism of their AHE.

A Ferromagnetic Oxide Semiconductor as Spin Injection Electrode in Magnetic Tunnel Junction

A magnetic tunnel junctions composed of room temperature ferromagnetic semiconductor rutile Ti1-xCoxO2-δ and ferromagnetic metal Fe0.1Co0.9 separated by AlOx barrier showed positive tunneling magnetoresistance (TMR) with a ratio of ~11% at 15 K, indicating that Ti1-xCoxO2-δ can be used as a spin injection electrode. The TMR decreased with increasing temperature and vanished above 180 K. TMR action at high temperature is likely prohibited by the inelastic tunneling conduction due to the low quality of the amorphous barrier layer and/or the junction interface.

Role of charge carriers for ferromagnetism in cobalt-doped rutile TiO2

Electric and magnetic properties of a high-temperature ferromag- netic oxide semiconductor, cobalt-doped rutile TiO2, are summarized. Cobalt- doped rutile TiO2 epitaxial thin films with different electron densities and cobalt contents were grown on r-sapphire substrates with laser molecular beam epitaxy. Results of magnetization, magnetic circular dichroism and anomalous Hall-effect measurements were examined for samples with systematically varied electron densities and cobalt contents. The samples with high electron densi- ties and cobalt contents show high-temperature ferromagnetism, suggesting that charge carriers induce the ferromagnetism.

Indication of intrinsic room-temperature ferromagnetism in Ti1-xCoxO2-δ thin film : An x-ray magnetic circular dichroism study

Soft x-ray magnetic circular dichroism (XMCD) measurements at the Co L2,3 edges of Co-doped rutile TiO2 at room temperature revealed clear multiplet features characteristic of ferromagnetic Co2+ ions coordinated by O2− ions, being in sharp contrast to the featureless XMCD spectrum of Co metal or metallic clusters. The absorption and XMCD spectra agree well with a full atomic-multiplet calculation for the Co2+ high-spin state in the D2h-symmetry crystal field at the Ti site in rutile TiO2. The results indicate that the ferromagnetism arises from the Co2+ ions substituting the Ti4+ ions.

A Scaling Relation of Anomalous Hall Effect in Ferromagnetic Semiconductors and Metals

A scaling relation of the anomalous Hall effect recently found in a ferromagnetic semiconductor (Ti,Co)O2 is compared with those of various ferromagnetic semiconductors and metals. Many of these compounds with relatively low conductivity σxx≤104 Ω-1cm-1 are also found to exhibit similar relation: anomalous Hall conductivity σAH approximately scales as σAH∝σxx1.6, that is coincident with a recent theory. This relation is valid over five decades of σxx irrespective of metallic or hopping conduction.

Epitaxial growth and physical properties of a room temperature ferromagnetic semiconductor: Anatase phase Ti1−xCoxO2

There has been considerable debate regarding the origin of ferromagnetism in the ferromagnetic semiconductor anatase phase TiO2 doped with Co, ranging from carrier induced ferromagnetism in a framework of diluted magnetic semiconductor to merely the precipitation of ferromagnetic and metallic Co precipitates in the TiO2 matrix. This paper reports on a systematic investigation of the structures, together with the electronic and magneto-optical properties, of thin films of anatase phase TiO2, doped with Co. Films of anatase Ti1−xCoxO2 (x=0–0.10) were grown epitaxially on LaSrAlO4 (001) substrates, using pulsed laser deposition method. In order to control n-type carrier concentration, the oxygen pressure during the growth was systematically varied, yielding films with insulative, semiconductive (carrier concentration n∼1×1017 cm−3), and metallic (n∼2×1019 cm−3) properties. Reflection high energy electron diffraction patterns, atomic force microscopy, and transmission electron microscopy show no evidence of C...

Anomalous Hall effect in anatase Ti1−xCoxO2−δ above room temperature

Presence of spin polarized electrons was confirmed by observing anomalous Hall effect in a ferromagnetic semiconductor anatase Ti1−xCoxO2−δ up to 600K. The anomalous Hall resistivity exhibited apparent ferromagnetic hysteresis loop from 300to600K with insignificant change in the magnitude, indicating that the Curie temperature is higher than 600K. The measurements above 500K induced an annealing effect represented by the decrease in resistivity, whereas kept the anomalous Hall resistivity nearly constant.