2009

Electron-Spin Filters Would Offer Spin Polarization >1

Net spin flux could be generated with little net electric current.

A proposal has been made to develop devices that would generate spin-polarized electron currents characterized by polarization ratios having magnitudes in excess of 1. Heretofore, such devices (denoted, variously, as spin injectors, spin polarizers, and spin filters) have typically offered polarization ratios having magnitudes in the approximate range of 0.01 to 0.1. The proposed devices could be useful as efficient sources of spin-polarized electron currents for research on spintronics and development of practical spintronic devices.

The polarization ratio in question — denoted the current spin polarization — is a standard measure of efficiency of a spin-polarizing device. It is defined in terms of current densities along a given coordinate axis, by means of the following equation:

PJ> = (J↑ – J↓)/(J↑ + J↓),

where J↑ is current density of electrons in the “up” spin state and J↓ is the current density of electrons in the “down” spin state. If J↑ and J↓ can be made to have opposite signs — in other words, if electrons in opposite spin states can be made to move in opposite directions — then, as desired, it is possible to obtain |PJ|>1. By making |PJ|>1, one would make it possible to obtain a net spin flux with little net electric current.

altA spin-polarizing device according to the proposal would be based largely on the same principles as those of the devices described in “Electron-Spin Filters Based on the Rashba Effect” (NPO-30635), NASA Tech Briefs, Vol. 28, No. 10 (October 2004), page 58. To recapitulate: The Rashba effect is an energy splitting, of what would otherwise be degenerate quantum states, caused by a spin-orbit interaction in conjunction with interfacial electric fields in an asymmetrical semiconductor heterostructure. The magnitude of the energy split is proportional to the electron wave number. Theoretically, electron-energy states would be split by the Rashba effect, and spin-polarized currents would be extracted by resonant quantum-mechanical tunneling. Accordingly, a spin-polarizing device based on these principles would be denoted an asymmetric resonant interband (or intraband, as the case may be) tunneling diode [a-RITD].

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