C.L. Dennis

Spin electronics—a review

An overview is given of the state of the art in spin electronics. The technical basis is reviewed and simple ideas of giant magnetoresistance discussed. The connection between spin electronics and mesomagnetism is explored. Three-terminal spin-electronic devices are introduced of various types including hot carrier and hybrid spin/semiconductor devices. Spin-tunnel devices are examined and single spin electronics is also treated. The paper concludes with an outlook on future prospects in this field.

The defining length scales of mesomagnetism: a review

This review is intended as an introduction to mesomagnetism, with an emphasis on what the defining length scales and their origins are. It includes a brief introduction to the mathematics of domains and domain walls before examining the domain patterns and their stability in 1D and 2D confined magnetic structures. This is followed by an investigation of the effects of size and temperature on confined magnetic structures. Then, the relationship between mesomagnetism and the developing field of spin electronics is discussed. In particular, the various types of magnetoresistance, with an emphasis on the theory and applications of giant magnetoresistance and tunnelling magnetoresistance, are studied. Single electronics are briefly examined before concluding with an outlook on future developments in mesomagnetism.

High current gain silicon-based spin transistor

A silicon-based spin transistor of novel operating principle has been demonstrated in which the current gain at room temperature is 1.4 (n-type) and 0.97 (p-type). This high current gain was obtained from a hybrid metal/semiconductor analogue to the bipolar junction transistor which functions by tunnel-injecting carriers from a ferromagnetic emitter into a diffusion driven silicon base and then tunnel-collecting them via a ferromagnetic collector. The switching of the magnetic state of the collector ferromagnet controls the collector efficiency and the current gain. Furthermore, the magnetocurrent, which is determined to be 98% (140%) for p-type (n-type) in −110 Oe, is attributable to the spin-polarized base diffusion current.

Spin injection efficiency in spin electronic devices

Abstract We examine the comparative importance of spin injection efficiency in different types of spin electronic devices. We analyse the devices using a “pseudo-density of states” treatment of spin currents coupled to the principle of spin balance. This approach affords a generality that readily encompasses both metal and semiconductor direct spin injection systems. This implies that the various spin electronic devices have differing criteria for assessing the quality of their performance and that in turn these criteria determine the importance of implementing high-efficiency spin injection. We examine the factors which determine the obtainable spin injection performance in each case. We conclude that for CPP trilayer devices with semiconductor interlayers, obtaining efficient spin injection should not be a problem so long as the semiconductor layer thickness is small compared with its own spin diffusion length. Direct-injected and tunnel-injected spin LEDs are predicted to have similar spin efficiencies. However, direct-injected SPICE-type spin transistors have superior spin performance to their tunnel-injected counterparts.

Evidence for electrical spin tunnel injection into silicon

Electrical spin injection into silicon was studied in a ferromagnet/insulator/silicon/insulator/ferromagnet structure, where the insulator is Si3N4. Si3N4 barriers conduct by hopping conduction at low voltages, but switch to Fowler-Nordheim tunneling at high voltages. In the Fowler-Nordheim tunneling regime a magnetic field dependence of the output current consistent with spin dependent transport through the silicon is observed; in the hopping conduction regime reduced magnetic field dependence of the output current is observed. This voltage dependence of the magnetic sensitivity strongly supports the existence of spin injection into silicon. After correction for Lorentz magnetoresistance, the magnitude of this signal is 4.1%+/- 0.5% (12%+/- 5%) for p-type (n-type) Si.

Comparative study of spin injection into metals and semiconductors

We present an analysis of spin injection efficiency that is of general application and relevant to a wide range of spin-electronic devices. By applying simple band structure ideas to a single interface between a metallic ferromagnet and a three-dimensional semiconductor, two conflicting figures of merit are identified - spin accumulation and polarization of injected current - and their validity to the analysis of different device types is discussed. The injected spin accumulation is smaller than the all-metal injection case by a factor (m*/me)3/2(kT/EF)1/2e^eD/kT. Moreover, the injected spin current at the interface is reduced by the factor (m*/me)3/2(kT/EF)(γS/γF) e^eD/kT, where γ for a particular material is the square root of its momentum scattering to spin-flip scattering ratio, m* and me are the effective and free masses, respectively, and eD is the donor binding energy. These results are consequent on the boundary condition that the spin channel electrochemical potentials are continuous at the interface. By inserting an insulating tunnel barrier between the ferromagnet and the semiconductor, not only is this boundary condition removed and the spin polarization of the injected current restored to the all-metal magnitude, but also the spin accumulations in the metal and the semiconductor even have opposite signs. This implies that thin or discontinuous tunnel barriers have the worst spin injection efficiency of any configuration. We finally note that for injected spin current into metals with polarization approaching 100%, the Fermi surface is polarized to a depth which exceeds the equilibrium carrier depth by a factor lSD/λ, hence >>1.

A novel high gain silicon based spin transistor

In this paper, we fabricate a novel high gain silicon based spin transistor and measured the minority carrier transport in the silicon. When measured with a magnetic field applied in the plane of the transistor and perpendicular to the direction of the current flow, the I-V characteristics show a variation in the collector current causing the transistor to behave as a magnetically tunable device with a field dependent gain.