With the rapid advance of
miniaturization, data processing using electric currents faces tough
challenges, some of which are insurmountable. Magnetic spin waves are a
promising alternative for the transfer of information in even more compact
chips. Scientists at the Helmholtz-Zentrum Dresden-Rossendorf (HZDR), as part
of an international research venture, have now succeeded in generating spin
waves with extremely short wavelengths in the nanometer range - a key feature
for their future application.
Smaller, faster, more energy-efficient
- this is the mantra for the further development of computers and mobile
telephones which is currently progressing at a breathtaking pace. However, Dr.
Sebastian Wintz of the HZDR Institute of Ion Beam Physics and Materials
Research knows only too well, how difficult it already is to achieve any
further degree of miniaturization. "One major problem with current
technologies," he said, "is the heat which is generated when data are
transmitted with the aid of electric currents. We need a new concept." The
physicist is working with international colleagues on so-called spin waves
(magnons) which are set to replace moving charges in the future as information
carriers. The scientists have now succeeded for the first time in generating
spin waves of such short wavelengths that they have potential for future
applications in data processing.
Spin waves replace
electric current
The spin denotes a property which lends
the particles a magnetic moment. They then act like tiny magnets which run
parallel to each other in ferromagnetic materials. If one of the spins then
changes direction, this has a knock-on effect on its neighbors. A chain
reaction gives rise to a spin wave.
The processing of information is
presently based on electric currents. The charged particles speed
through a network of wires which are squeezed closer and closer together,
driven by the desire for ever more compact chips. On their way, the electrons
collide with atoms, causing them to rock to and fro in the crystal lattice
thereby generating heat. If the wires are too close together, this heat can no
longer be dissipated and the system breaks down. "The great advantage of
spin waves is that the electrons themselves don't move," explained Wintz,
"therefore precious little heat is produced by the flow of data."
Magnetic vortex as a
nano-antenna
The traditional approach adopted to
generate spin waves is to use small metal antennas which generate magnons when
driven by a high-frequency alternating current. The smallest wavelength which
can be generated in this way will be about the size of the antenna which is
used. This is precisely where the major problem lies in that small wavelengths
on the nanometer scale are required in order to satisfy the demand for ever
greater miniaturization. It is not currently possible, however, to make such
small high-frequency antennas.
The research team from Germany,
Switzerland and the USA has now succeeded in generating extremely
short-wavelength spin waves in an entirely new way. As a naturally formed
antenna, they use the center of a magnetic
vortex which is produced in a small, ultra-thin ferromagnetic disk.
Due to the disk's limited size, the spins do not all line up in parallel as
normal but lie along concentric circles in the plane of the disk. This, in
turn, forces the spins from a small area in the center of the disk, which measures
just a few nanometers in diameter, to straighten up and, thus, to point away
from the surface of the disk. If this central region is subjected to an
alternating magnetic field then a spin wave is produced.
A few more tricks are needed, however,
in order to shorten the wavelength as required. Consequently, a second tiny
disk is placed onto the first, separated by a thin, non-magnetic layer. When
this separating layer is fabricated with a specific thickness, then the two
disks interact in such a way as to elicit an antiferromagnetic coupling between
the disks - the spins try to point in opposite directions - which reduces the
wavelength of the emitted spin waves many times over. "Only in this way do
we arrive at a result wh
Attractive properties
for applications
The scientists not only demonstrated
the short wavelengths of the spin waves generated in this way but were also
able to reveal other wave properties which could be very useful for future
applications. With the help of high-speed movies taken with an X-ray microscope
belonging to the Max Planck Institute for Intelligent Systems in Stuttgart
(which is installed at the Helmholtz-Zentrum Berlin) they showed that the
wavelength can be adjusted precisely by the selection of the excitation
frequency. Similar measurements were also carried out at the Paul Scherrer
Institute in Switzerland. The results are consistent with a theoretical model
which was developed specifically for this study at Oakland University in the
USA. What is more, a remarkable phenomenon was predicted, which so far has not
been seen directly in the experiments: The speed at which the spin waves
travel was calculated to be heavily dependent on their propagation direction
(forwards or backwards) - another point which could enable a large number of
applications in signal processing.
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