Room Temperature Spin Polarization in Oxide Semiconductors
In order to enable future levels of miniaturization and speed in computer circuits to be realized, researchers from around the world are considering alternative approaches for how devices process bits and store information. One such approach involves using the quantum mechanical spin of electrons to carry information, rather than charge as is currently used. This technology is called spin electronics, or spintronics. Practical spintronics requires a new generation of magnetic semiconductors that can propagate spin polarized currents well above room temperature. Conventional magnetic semiconductors, such as Mn:GaAs, are only magnetic at cryogenic temperatures, precluding their use in circuits without expensive refrigeration.
Researchers at PNNL are exploring a new class of magnetic oxide semiconductors that may hold promise for spin polarization above room temperature. By taking a careful materials science approach using state-of-the-art epitaxial oxide film growth at PNNL, along with various x-ray spectroscopies at DOE's Advanced Light Source (ALS) and Advanced Photon Source (APS), these researchers are able to synthesize and evaluate materials in a rational manner, thereby determining whether or not such materials possess the necessary properties to be useful in spintronics. In one recent study, the PNNL team has found that TiO2 anatase doped with a few atomic percent of Cr is ferromagnetic up to ~800 K, making it one of the most thermally robust of the recently discovered magnetically doped oxides. Moreover, recent x-ray spectroscopic measurements made at the ALS reveal that although the Cr dopant is strongly hybridized to the valence band, there is no spin polarization in the conduction band, where the free carriers are in n-type material. The physical causes of this result have been revealed by comparing spectroscopic data with detailed density functional calculations of the electronic structure of the material.