Cobalt-Doped Anatase Titanium Dioxide Thin Films Behave as Room-Temperature Magnetic Semiconductors
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Cobalt-Doped Anatase Titanium Dioxide Thin Films Behave as Room-Temperature Magnetic Semiconductors Scientists at Pacific Northwest National Laboratory (PNNL) have created a thinfilm semiconductor material made of titanium, oxygen, and cobalt. Their material demonstrated improvement of magnetic strength by nearly a factor of five over that currently demonstrated. In order to be practical, spintronics will need to use semiconductors that maintain their magnetic properties at room temperature. This is a challenge because most magnetic semiconductors lose their magnetic properties above critical temperatures that are well below room temperature, and would require expensive and impractical refrigeration in order to work in an actual computer. Scott Chambers, a chemist and PNNL senior chief scientist, and his team of scientists achieved these properties in a crystalline oxide film known as anatase titanium dioxide that is infused with a small amount of cobalt, a magnetic impurity. As
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each electron all pointed in the same direction or were aligned. The question, Awschalom said, was whether a cloud or bundle of electrons all spinning the same way would retain that same spinning when the cloud is moved to an adjacent semiconducting material. When transferring an electron spin across an interface between the semiconductors GaAs and ZnSe in a magnetic field, the researchers found that the spins stayed aligned, even as the temperature of the materials was raised, in some cases, to room temperature. Furthermore, the researchers observed that the GaAs semiconductor serves as a spin reservoir. Awschalom said that if spin was pulled from one material (e.g., GaAs) to another (e.g., ZnSe), the spins in the adjacent layer acquire the original spin frequency and lifetime of the reservoir. Therefore the total transferred spin current can have the properties of either the reservoir or the adjacent layer, and an external electric field gates the transition between the two very different regimes. Under electrical bias, the relative increase in spin-coherent injection was up to 500% in the n-GaAs/n-ZnSe junction. Significantly, this increase was nearly 4000% in the p-GaAs/n-ZnSe junction. The results in the n-n junction are due to the GaAs spin reservoirs whereas in the p-n junction, the data suggest that there is enhancement in spontaneous transfer mechanisms. These results, particularly for the p-n heterostructures, could point the way toward spin transistors.
described in a poster presentation at the 2001 Spintronics Workshop in Washington, D.C., in August, Chambers and his team created this magnetic semiconductor material using molecular-beam epitaxy. A team of scientists at IBM, led by research staff scientist Robin Farrow, then characterized the material’s magnetic properties.
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