Many of us
may still be familiar with the simple physical principles of magnetism from
school. However, this general knowledge about north and south poles quickly
becomes very complex when looking at what happens down to the atomic level. The
magnetic interactions between atoms at such minute scales can create unique
states, such as skyrmions.
Skyrmions
have very special properties and can exist in certain material systems, such as
a "stack" of different sub-nanometer-thick metal layers. Modern
computer technology based on skyrmions -- which are only a few nanometers in
size -- promises to enable an extremely compact and ultrafast way of storing
and processing data. As an example, one concept for data storage with skyrmions
could be that the bits "1" and "0" are represented by the
presence and absence of a given skyrmion. This concept could thus be used in
"racetrack" memories (see info box). However, it is a prerequisite
that the distance between the skyrmion for the value "1" and the
skyrmion gap for the value "0" remains constant when moving during
the data transport, otherwise large errors could occur.
As a
better alternative, skyrmions having different sizes can be used for the
representation of "0" and "1." These could then be
transported like pearls on a string without the distances between the pearls
playing a big role. The existence of two different types of skyrmions (skyrmion
and skyrmion bobber) has so far only been predicted theoretically and has only
be shown experimentally in a specially-grown monocrystalline material. In these
experiments, however, the skyrmions exist only at extremely low temperatures.
These limitations make this material unsuitable for practical applications.
Experience
with ferromagnetic multilayer systems and magnetic force microscopy
The
research group led by Hans Josef Hug at Empa has now succeeded in solving this
problem: "We have produced a multilayer system consisting of various
sub-nanometer-thick ferromagnetic, noble metal and rare-earth metal layers, in
which two different skyrmion states can coexist at room temperature," says
Hug. His team had been studying skyrmion properties in ultra-thin ferromagnetic
multilayer systems using the magnetic force microscope that they developed at
Empa. For their latest experiments, they fabricated material layers made from
the following metals: iridium (Ir), iron (Fe), cobalt (Co), platinum (Pt) and
the rare-earth metals terbium (Tb) and gadolinium (Gd).
Between
the two ferromagnetic multilayers that generate skyrmions -- in which the
combination of Ir/Fe/Co/Pt layers is overlaid five times -- the researchers
inserted a ferrimagnetic multilayer consisting of a TbGd alloy layer and a Co
layer. The special feature of this layer is that it cannot generate skyrmions
on its own. The outer two layers, on the other hand, generate skyrmions in
large numbers.
The
researchers adjusted the mixing ratio of the two metals Tb and Gd and the
thicknesses of the TbGd and Co layers in the central layer in such a way that
its magnetic properties can be influenced by the outer layers: the
ferromagnetic layers "force" skyrmions into the central ferrimagnetic
layer. This results in a multilayer system where two different types of
skyrmions exist.
Experimental
and theoretical evidence
The two
types of skyrmions can easily be distinguished from each other with the
magnetic force microscope due to their different sizes and intensities. The
larger skyrmion, which also creates a stronger magnetic field, penetrates the
entire multilayer system, i.e. also the middle ferrimagnetic multilayer. The
smaller, weaker skyrmion, on the other hand only exists in the two outer
multilayers. This is the great significance of the latest results with regard
to a possible use of skyrmions in data processing: if binary data -- 0 and 1 --
are to be stored and read, they must be clearly distinguishable, which would be
possible here by means of the two different types of skyrmions.
Using the
magnetic force microscope, individual parts of these multilayers were compared
with each other. This allowed Hug's team to determine in which layers the
different skyrmions occur. Furthermore, micromagnetic computer simulations
confirmed the experimental results. These simulations were carried out in
collaboration with theoreticians from the universities of Vienna and Messina.
Empa
researcher Andrada-Oana Mandru, the first author of the study, is hopeful that
a major challenge towards practical applications has been overcome: "The
multilayers we have developed using sputtering technology can in principle also
be produced on an industrial scale," she said. In addition, similar
systems could possibly be used in the future to build three-dimensional data
storage devices with even greater storage density. The team recently published
their work in the renowned journal Nature Communications.
Racetrack
Memory
The
concept of such a memory was designed in 2004 at IBM. It consists of writing
information in one place by means of magnetic domains -- i.e. magnetically
aligned areas -- and then moving them quickly within the device by means of
currents. One bit corresponds to such a magnetic domain. This task could be
performed by a skyrmion, for example. The carrier material of these magnetic
information units are nanowires, which are more than a thousand times thinner than
a human hair and thus promise an extremely compact form of data storage. The
transport of data along the wires also works extremely fast, about 100,000
times faster than in a conventional flash memory and with a much lower energy
consumption.