Ion irradiation creates an asymmetric potential or ‘ratchet’ for the main walls (visualised as light-yellow spheres). The bit with a magnetic coating is shifted one position to the left by sequentially positioning a field upwards and downwards. |
Researchers
from Eindhoven University of Technology in The Netherlands and the FOM
Foundation have successfully made “magnetic domain-wall ratchet” memory,
a computer memory that is built up from moving bits of magnetized
areas. This memory potentially offers many advantages compared to
standard hard disks, such as a higher speed, lower electricity
consumption and much longer life. Using concentrated ion bundles the
researchers have influenced the magnetic wires the bits move through,
and they have successfully controlled bits at the nanometre scale and
subsequently constructed a new memory. The research results were
published online by Nature Nanotechnology on Sunday 15 July.
The
bits in a nanowire can be conceptualized as areas that can have two
possible magnetic directions, a 0 or a 1. Usually all of the bits are
simultaneously set at either 0 or 1 during the construction as they
reverse like compass needles. The researchers have now demonstrated that
bits can be coherently transferred without the information they contain
being lost. This method of magnetic data transport is radically
different from that in current computers, where rotating magnetic disks
are mechanically moved to address data.
Saw tooth
By
cleverly varying how the ions are fired across a nanowire, a repeating,
saw-tooth-shaped energy landscape is created. This asymmetric saw tooth
is crucial: it forces a domain wall, the boundary between bits, to move
in a single direction under a variable magnetic field. Due to the
variable magnetic field the force on the domain wall continually
reverses and this is alternately pushed over the incline and then
subsequently pushed back against the sharp edge (see Figure). After one
cycle of the magnetic field two domain walls are pushed up by exactly
one position. This net transfer of a bit would be impossible without
saw-tooth potential.
In a 0.5 nm-thick cobalt ring with a diameter of 12 µm, a bit (black) is coherently transferred in a variable magnetic field (~16 mT) in an anti-clockwise direction. |
The
researchers used a circular magnetic wire. By using this circle the
domain walls can always rotate and the bits are retained. This one-way
traffic of the domains is a movement comparable to that of a rattle or
‘ratchet’.
Unique opportunities
The
discovery offers unique opportunities for the development of
alternative memory concepts. After the proof-of-principle experiments
the scientists will focus on a next generation ratchet. This will be
based on radical new effects such as the use of spin currents generated
in the neighbouring non-magnetic layers or the use of electric fields to
influence the domain or movement.
