Magnetic skyrmions at room temperature: New digital memory?

An exotic, swirling object with the sci-fi name of a “magnetic skyrmion” could be the future of nanoelectronics and memory storage. Physicists at UC Davis and the National Institute of Standards and Technology (NIST) have now succeeded in making magnetic skyrmions, formerly found at temperatures close to absolute zero, at room temperature.

“This is a potentially new way to store information, and the energy costs are expected to be extremely low,” said Kai Liu, professor of physics at UC Davis and corresponding author of a paper on the work, published in the journal Nature Communications Oct. 8.

Skyrmions were originally described over 50 years ago as a type of hypothetical particle in nuclear physics. Actual magnetic skyrmions were discovered only in 2009, as chiral patterns of magnetic moments — think of a moment as a tiny compass needle — in materials close to absolute zero temperature, in the presence of a strong magnetic field.

Magnetic skyrmions fall into two types, Liu said: “Bloch skyrmions,” with a hurricane-like spiral pattern of magnetic moments around a perpendicular center, surrounded by magnetic moments oriented in the opposite direction to the center; and “hedgehogs,” where the magnetic moments orient like spikes on a  hedgehog or sea urchin.

The interesting thing about magnetic skyrmions, Liu said, is that they are “topologically protected:” they can be continuously deformed, in the same way that a coffee mug shape can be deformed into a bagel shape, but they do not readily go back into a state where all the magnetic moments are aligned. That means they can potentially store information at an energy cost much lower than current technology, Liu said.

Together with graduate student Dustin Gilbert, now a postdoctoral fellow at NIST, Liu and colleagues designed a nanosynthesis approach to achieve artificial “Bloch” magnetic skyrmions at room temperature. They created a pattern of magnetic nanodots, each about half a micron across, on a multilayered film where the magnetic moments are aligned normal to the plane. They used ion beam irradiation to modify the interface between the dots and the film to allow “imprinting” of the magnetic moments of the dots into the film.

Using neutron-scattering at NIST Center for Neutron Research, they were able to resolve the magnetic profiles along the depth of the hybrid structure. Combined with magnetic imaging studies at NIST and Lawrence Berkeley Laboratory, they were able to find the first direct evidence of arrays of stable spiral magnetic skyrmions beneath the nanodots at room temperature, even without an external magnetic field.

The availability of stable magnetic skyrmions at room temperature opens up new studies on their properties and potential development in electronic devices, such as nonvolatile magnetic memory storage.

Coauthors on the paper are Brian Maranville, Andrew Balk, Brian Kirby, Daniel Pierce, John Unguris and Julie Borchers at NIST, and Peter Fischer, LBL and UC Santa Cruz. Nanofabrication work and other characterizations were carried out in Liu’s laboratory and at the Center for Nano and Micro Manufacturing at UC Davis. The work was funded by the National Science Foundation.

UC Davis

Nanotechnology World Association

Simple detection of magnetic skyrmions

At present, tiny magnetic whirls – so called skyrmions – are discussed as promising candidates for bits in future robust and compact data storage devices. At the University of Hamburg these exotic magnetic structures were recently found to exist in ultrathin magnetic layers and multilayers, similar to the ones used in current hard-disk drives and magnetic sensors. However, up to now an additional magnet was necessary for a read-out of skyrmions.

Now researchers from the University of Hamburg and the Christian-Albrechts-Universität in Kiel have demonstrated that skyrmions can be detected much more easily because of a drastic change of the electrical resistance in these magnetic whirls. For future data storage concepts this promises a significant simplification in terms of fabrication and operation.

Stable whirls in magnetic materials were predicted over 25 years ago, but the experimental realization was achieved only recently. The discovery of such skyrmions in thin magnetic films and multilayers, already used in today’s technology, and the possibility to move these skyrmions at very low electrical current densities, has opened the perspective to use them as bits in novel data storage devices.

Up to now individual magnetic whirls were detected either by electron microscopy or by the resistance change in a tunnel contact with a magnetic probe. Employing a scanning tunneling microscope researchers of the University of Hamburg were now able to demonstrate that the resistance changes also when a non-magnetic metal is used in such a measurement. ‘In our experiment we can move a metallic tip over a surface with atomic-scale precision, and in this way we can measure the resistance at different positions in a skyrmion’ says Christian Hanneken, a PhD student in the group of Prof. Roland Wiesendanger. This enables the proof for the locally varying resistance within the magnetic whirl. ‘We found a resistance change of up to 100%, allowing a simple detection scheme for skyrmions’, as Dr. Kirsten von Bergmann explains.

In collaboration with theoretical physicists from the University of Kiel the researchers were able to identify the origin of the resistance change in the magnetic whirl: it is due to the canting between the atomic magnets from one atom to the next (see figure). The larger the angle between the adjacent atomic magnets, the stronger is the change in electrical resistance. ‘Electrons have a spin, and thus they interact with magnetic structures’, says Prof. Stefan Heinze from the University of Kiel. When the electrons are travelling through a magnetic whirl, they feel the canting between the atomic magnets, leading to a local resistance change of the material. ‘We were able to understand this effect by performing extensive numerical computer simulations of the electronic properties and developed a simple model for this effect’, as the PhD student Fabian Otte explains.

University of Hamburg

Nanotechnology World Association