  Data storage relies on the handling of two states, called bits. The market of
mass storage is currently still dominated by magnetic technology, hard disk
drives for the broad public and tapes for massive archiving. In these devices
each bit is stored in the form of the direction of magnetization of nanosized
magnetic domains, i.e. areas of ferromagnetic materials displaying a uniform
magnetization. While miniaturization is the conventional way to fuel the
continuous increase of device density, disruptive solutions are also sought. To
these pertain in recent years many fundamental studies no longer considering
the magnetic domains themselves, but the manipulation of the domain walls (DWs)
that separate such domains. Concepts of storage and logic based on the
propagation of DWs along lithographically-patterned stripes have been patented,
while many fundamental aspects of DW propagation deeply related to condensed
matter physics are still hotly debated. If one now considers magnetic dots of
submicrometer dimensions, the magnetization has a tendency to curl along the
outer edges of the nanostructure to close its magnetic flux and thereby reduce
its magnetostatic energy. Then both domains and DWs of well-defined geometries
arise, whose combined manipulation has been proposed as a multilevel magnetic
storage scheme...
