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Most efficient spin injection ever

University of Arkansas physicists hit the highest efficiency ever in transferring polarized electrons into a semiconductor surface. In doing so, they also discovered some of the underlying mechanisms that prevent researchers from successfully injecting spin-polarized electrons into a semiconducting surface.

The researchers reported their findings in the May 25 issue of Science. Physicists hope to harness the power of electron spin to make multifunctional computational devices, where a single multifunctional device would replace hundreds of conventional devices, leading to faster, smaller electronics that consume less power.

For about 10 years, researchers have been exploring the idea of exploiting electron spin to enhance the performance of integrated circuits. Spins can rotate in a coherent manner and thus alter the resistance of a device in controlled ways. These properties may lead to greater storage capacity and information processing from spintronic devices.

Until now, however, injecting spin-polarized electrons into a semiconductor surface has not worked — a high percentage of the electrons change their spin orientation during the injection process. The highest spin efficiency recorded was 40% at 10° Kelvin, a temperature too low for effective use in electronic devices.

The U of A researchers saw an injection efficiency of 92% into a gallium arsenide (GaAs 110) surface at a temperature of 100°K, the temperature of liquid nitrogen. They used a technique that incorporates a magnetic nickel Scanning Tunneling Microscope (STM) tip to inject electrons that are all oriented in one direction. Measurements of polarization can determine whether or not the electrons retain their spin, a technique called spin-polarized tunneling induced luminescence microscopy (SP-TILM). The STM also lets the researchers correlate surface features in the topography of the semiconductor with the degree of spin disruption.

Areas with an atomic "step," a spot where the atoms do not form an even surface, cause spin disruption. The particular form of GaAs used in the experiments, GaAs (110), has few steps in it, accounting for the high degree of success in injecting spin-polarized electrons. The places where these steps occurred turned out to be the source of electron disruption, causing the spins to flip.

U of A researchers explain that it takes a free electron to scatter another electron's spin. Usually within a crystal all electrons are paired up, unless there is a broken bond. In the case of GaAs 110, all the electrons are in filled orbitals, so the spins are stable. There are plenty of surfaces, they say, where the electronic configuration is not as smooth and which would, therefore, be less efficient for use in spintronics.

The researchers plan to study other semiconductor and ferromagnetic surfaces using the same techniques.

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