J. F. Gregg

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.

All-linear time reversal by a dynamic artificial crystal

The time reversal of pulsed signals or propagating wave packets has long been recognized to have profound scientific and technological significance. Until now, all experimentally verified time-reversal mechanisms have been reliant upon nonlinear phenomena such as four-wave mixing. In this paper, we report the experimental realization of all-linear time reversal. The time-reversal mechanism we propose is based on the dynamic control of an artificial crystal structure, and is demonstrated in a spin-wave system using a dynamic magnonic crystal. The crystal is switched from an homogeneous state to one in which its properties vary with spatial period a, while a propagating wave packet is inside. As a result, a linear coupling between wave components with wave vectors k≈π/a and k′=k−2ππ/a≈−π/a is produced, which leads to spectral inversion, and thus to the formation of a time-reversed wave packet. The reversal mechanism is entirely general and so applicable to artificial crystal systems of any physical nature.

The art of sp↑n electron↓cs

Abstract A brief history is given of the nascent field of spin electronics in which the ability to differentially manipulate up- and down-spin current carriers is exploited. We discuss the impending marriage of spin-dependent effects with semiconductor technology and, in particular, the exploitation of spin-dependent transport in the semiconductors themselves. In this connection, preliminary experiments are described which explore spin transport in ion-implanted silicon. We conclude by evaluating various potential applications of the devices made possible by this exciting new development in electronic technology.

Oscillatory energy exchange between waves coupled by a dynamic artificial crystal.

We describe a general mechanism of controllable energy exchange between waves propagating in a dynamic artificial crystal. We show that if a spatial periodicity is temporarily imposed on the transmission properties of a wave-carrying medium while a wave is inside, this wave is coupled to a secondary counterpropagating wave and energy oscillates between the two. The oscillation frequency is determined by the width of the spectral band gap created by the periodicity and the frequency difference between the coupled waves. The effect is demonstrated with spin waves in a dynamic magnonic crystal.

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.

Magnonic crystal based forced dominant wavenumber selection in a spin-wave active ring

Spontaneous excitation of the dominant mode in a spin-wave active ring—a self-exciting positive-feedback system incorporating a spin-wave transmission structure—occurs at a certain threshold value of external gain. In general, the wavenumber of the dominant mode is extremely sensitive to the properties and environment of the spin-wave transmission medium, and is almost impossible to predict. In this letter, we report on a backward volume magnetostatic spin-wave active ring system incorporating a magnonic crystal. When mode enhancement conditions—readily predicted by a theoretical model—are satisfied, the ring geometry permits highly robust and consistent forced dominant wavenumber selection.

Effect of silicon crystal structure on spin transmission through spin electronic devices

Spin injection into and spin transport through silicon spacer layers in iron/silicon/cobalt structures has been investigated. Ultrahigh vacuum evaporated silicon spacers of varying crystal quality from amorphous to epitaxial of thicknesses from 10 to 200 A were shown to improve their electrical conduction with increasing crystallinity, but no spin dependent transport was observed through the structure. Silicon and iron interdiffusion was also observed at the interfacial region. Device quality silicon was studied using 460 and 540 μm doped silicon wafers of resistivity 0.1 and 1 Ω cm, respectively, polished on both sides, onto which were deposited iron and cobalt layers. Sharp metal-semiconductor interfaces were achieved in this way, but no spin dependent transport, putting an upper limit on the spin diffusion length in device quality silicon wafers.

Magnetoresistance in nanostructured Co-Ag prepared by mechanical-alloying

Fine cobalt and silver powders have been ball-milled to yield a granular solid of composition Co/sub 30/Ag/sub 70/ in the form of coarse powder. The material is an intimate mixture of grains of fcc Co in a silver matrix. The cobalt is in the single-domain size range, and samples exhibit coercivity (0.07 T at 296 K) and unusual thermomagnetic effects, including field-induced unidirectional anisotropy. There is a large, isotropic negative magnetoresistance, which exceeds 10% at 4.2 K. >

Further studies of the enhanced nuclear magnet HoVO4 - I. The crystal field and the Zeeman spectrum

A novel approach is adopted to fit the experimental results for the Van Vleck paramagnet HoVO4. Within the ground manifold 5I8, J = 8, the five parameters for a crystal field of tetragonal symmetry are adjusted to give values in agreement with the optical spectrum for the lowest energy levels: the ground singlet, the first excited doublet at 21 cm-1, and the (accidental) triplet at 47 cm-1. Within experimental error (of order 1 cm-1), this agreement is not impaired by a small modification in which all the crystal field parameters are multiplied by a factor 1.0225. This factor is introduced to give the correct value of the enhanced nuclear magnetic resonance frequency for the stable isotope 165Ho (I = 7/2), known to 0.3% (Bleaney et al. Proc. R. Soc. Lond. A 362, 179 (1978)). The optical Zeeman effect, calculated therefrom, is in good agreement with that observed experimentally for the lowest levels in magnetic fields up to 15 T, directed along the [100], [110] and [001] axes (Battison et al. Phys. Lett. A 55, 173 (1975); J. Phys. C 10, 323 (1977)).