Proceedings of Magneto-Optical Recording International Symposium '97, J. Magn. Soc. Jpn., Vol. 22 Supplement No. S2 (1998), pp.
13-14
©1997 by The Magnetics Society of Japan
THE ART OF SPIN ELECTRONICS
John F. Gregg
Clarendon Laboratory, Parks Road, Oxford, OXl 3PU, United Kingdom
Conventional electronics has ignored the spin of the electron. Indeed, with the exception of
solenoids, relays, Hall sensors and the occasional specialised microwave device, magnetism
has traditionally been the "poor relation" in the world of electronic circuitry. In the everyday
silicon transistor, the different families of electrical carrier are distinguished by their
different effective charges: however no practical use is made of the fact that some are spin-
up and others are spin-down.
The recognition of this distinction is the key which promises to unlock a whole new
generation of Spin Electronic devices whose operation relies upon differential manipulation
of independent families of current carriers with opposite spin polarisation.
The technical basis for Spin Electronics rests on the fact that electrical currents in
ferromagnets are carried by two distinct and independent "spin channels", one consisting of
electrons with spin parallel to the magnetisation and the other with spins antiparallel. The
relative independence of these two families of carriers arises since, although spin flip
processes are possible which move electrons from one channel to the other, they occur on a
timescale which is long compared with those of other processes. Moreover, the carrier
mobilities of the two channels are very different, and this implies that small ferromagnetic
elements incorporated into spin electronic devices may be used to encode magnetic
information onto the conduction electrons. The nature of the coded information may be
externally varied by application of magnetic fields which change the magnetisation states of
the encoder ferromagnets.
Spin Electronic devices work by transferring this spin information from one part of the
device to another where it is subsequently "read". The information is mediated by the
electrical carriers and it decays on a characteristic lengthscale (the spin diffusion length)
which is the average distance diffused by a carrier spin before flipping. This can vary from
nanometers to microns depending on material. An essential criterion for the creation of Spin
Electronics is thus the ability to engineer structures whose physical dimensions are of this
order or smaller. With the advent of modem thin film and nanofabrication technology this
dream is now a reality.
Potential applications of Spin Electronics lie in the data-storage industry as might be
expected, but also in programmable gate arrays, robotics, and precision engineering where
fast accurate position and motion sensing of mechanical components is a requirement. In
particular, there is a growing awareness of the need for such technology in the development
of improved fuel handling systems and electronic engine control in the automotive industry.
The requirement for robust, temperature stable, high signal to noise sensors extend to many
other aspects of automobiles including anti skid systems, speed controls and crash avoidance.
Gregg, J.F.: THE ART OF SPIN ELECTRONICS
The Oxford Spin Electronics Group is currently supported by the European Commission and
EPSRC. In collaboration with eight European physics laboratories and two major electronics
companies, work is in progress on novel 3- and 4-terminal spin electronic devices. Hybrid
designs using semiconductor/ferromagnetic metals are being developed with a view to
achieving enhanced gain, signal-to noise and field sensitivity. The research is wide-ranging
and includes fabrication of materials and structures, study of the fundamental transport
physics of semiconductor/ferromagnetic interfaces, charac!erisation of novel spin electronic
materials, theoretical modelling of interfacial processes and comparison with non-linear
optical studies of real interfaces, study of spin tunnelling processes, design of actual device
structures and even implementation of such devices in real automotive machinery.
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