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Improved Spin Transistor from Oxford University

August 15, 2002

Researchers at Oxford University’s Physics Department have developed an improved version of the “spin transistor,” a device which has the principle operating characteristics of a conventional transistor but with the added benefit of a current output dependent on the strength of the external magnetic field. This exciting new technology has potential applications in areas as diverse as magnetic field or position sensors and non-volatile memory chips.

Following the original spin transistor first described in 1993, this improved device is a type of transistor which relies on exploiting the presence of two populations of charge carriers, solely distinguished by the direction of their magnetic spin. In most semiconductor devices, such as transistors, the relative proportions of the up and down carrier types are equal. However, if a ferromagnetic material is used as the carrier source then the ratio can be deliberately skewed in one direction. An intervening layer of low coercivity paramagnetic material between the base and collector, which can be influenced by an externally applied magnetic field, can be used like a magnetically variable valve to throttle the flow of electrons through the device. After the electrons flow from the emitter electrode, the spin is “analysed” by another layer of ferromagnetic material located at the surface of the collector. Only electrons with the correct spin polarisation can travel successfully through the device, causing the device to behave like a transistor whose electrical properties are magnetically tuneable. In this way the amplification and/or switching properties of the device can be controlled by the external magnetic field applied to the device.

In early versions of the spin transistor, the ferromagnetic and paramagnetic metal parts were laid down directly onto the doped silicon base layer, which resulted in the formation of metal silicides at the interface. These metal silicides create a large depolarising effect on the flow of spin-polarised charge carriers, thereby severely reducing the magnetic sensitivity of the device and degrading its performance.

Oxford researchers have eliminated this particular drawback with the improved spin transistor, giving a significant reduction in the spin depolarisation, and hence an increase in device consistency and sensitivity. Additionally, the specific magnetic sensitivity of the device can be varied over a wide range by adjustment of the fabrication parameters used during manufacture, which allows the device characteristics to be tailored to particular target magnetic field strength ranges. The device therefore provides the benefits of a traditional transistor (i.e. a large amplification factor) but with the current gain dependent upon the local magnetic field. The improved spin transistor device is the subject of a patent application from Isis Innovation, Oxford University’s technology transfer company.

Isis Innovation Ltd

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