Exploring Materials for ‘Racetrack’ Memory

Dalhousie University scientists access the CNBC to identify materials with the magnetic properties required for a major breakthrough in computer memory.

Image: Everspin Technologies

“Racetrack memory is a form of spintronics, meaning that it harnesses an electron’s spin as well as its charge. One type of spintronic memory chip, the MRAM chip (pictured above), is already in use in aerospace circuits because of its reliability.”

When a computer shuts down, all the data in its fast short-term memory is lost. When it is turned back on, it takes time to reload its short-term memory using the information stored in its slower long-term memory.

Eliminating this wait is just one of the benefits that could be realized in future computers using a concept that scientists at IBM have dubbed ‘racetrack memory.’ Racetrack memory would combine the advantages of both types of memory: the volume and stability of the long-term memory (the hard disk) and the speed and tiny size of the short-term memory (the RAM). If realized, it would allow computers and devices to access enormous amounts of information in a billionth of a second, radically changing how computers work.

In general terms, racetrack memory would store information in a series of incredibly small wires. Each nanowire would contain about 100 tiny magnetic regions that would speed along the wire like a series of race cars. Every ‘car’ would store one bit of information based on the direction of its magnetic field, and the information would either be read or written as the cars passed by a device beside the wire.

A nanowire ‘racetrack’ would store information as a series of movable magnetic states, illustrated here as red and blue cars. A device would read out the information as these ‘cars’ sped along the wire. (Illustration: CNBC, adapted from IBM)

Scientists, including Professor Ted Monchesky of Dalhousie University, are now on the hunt to identify materials that have the right properties to turn racetrack memory into a reality. So far, one challenge in implementing this technology has been the amount of energy needed to push the cars along the track, which currently creates too much heat. To overcome this problem, Monchesky is looking for materials that have tiny magnetic regions that can be moved quickly and with very little energy.

For help in this search, Monchesky’s research team collaborated with the Canadian Neutron Beam Centre in a series of experiments to characterize thin films of manganese silicide (MnSi). Since one of the principles of nanotechnology is that the properties of materials are different in very small structures, the researchers were particularly curious about the nanoscopic magnetic regions of MnSi.

Using a combination of experimental techniques, including one called ‘polarized neutron reflectometry’ available at the CNBC, Monchesky’s research team discovered intriguing magnetic properties in thin films of MnSi less than 40 nanometres thick.

The findings resulted in a series of exciting discoveries. First, in research published in 2011, Monchesky’s team showed that the magnetic order in these MnSi thin films takes the shape of a helix. Using neutrons, they could measure the pitch of the helices in the film with nanometre precision.

Illustration of magnetic helices between the top and bottom edges of a MnSi thin film. The small arrows indicate the direction of local magnetic fields in the films, while the big red arrows show the direction and strength of an external magnetic field applied to the film. Monchesky observed that as the helices unwound due to an increasing external magnetic field, the number of turns took specific values, as illustrated. (Illustration: Monchesky)

The team followed up this observation with further measurements and theoretical calculations to explain their results, published in 2012. They found that, under specific conditions, these one-dimensional helices would combine to form two-dimensional magnetic structures known as skyrmions.

Magnetic materials often contain many magnetic domains, within which all the magnetic moments are aligned. There’s a wall around each domain that is the interface between one domain and the next. “If you could take a magnetic domain and shrink it, eventually you get to the point where the domain is gone and only the wall is left,” explains Monchesky. “These leftover walls are the smallest possible magnetic structures, and we call them skyrmions.”

Illustration of skyrmions in a narrow strip of a thin film. The small arrows indicate the direction of the magnetic field. A key finding is the ‘twisted’ magnetic fields at the outer edges, which would serve to protect the type of skyrmions illustrated here and keep them in the centre of the strip.

The discovery of skyrmions in MnSi thin films has exciting implications for the evolution of computers, namely because skyrmions have two properties necessary for overcoming the heat problems related to racetrack memory: they have high mobility and are extremely small. In fact, the diameter of the skyrmions in MnSi was found to be approximately 20 nanometres, which is roughly the width of a computer transistor (although skyrmions can be even smaller in some materials).

Further exploration of these MnSi thin films using polarized neutron reflectometry at the CNBC led to two additional findings, published in 2013 and 2014. First, the magnetism at the interface of the film is effectively locked in a twisted orientation. Second, the number of turns in the helices that make up the skyrmions must take specific values. These constraints on the magnetic states effectively lock the skyrmions in place in the middle of the film, stopping them from diffusing to the edge, where they could disappear. In other words, if skyrmions were used in a memory device, these edge states would prevent memory loss even when turned off.

Conceptual illustration of a type of racetrack memory device that encodes data as the presence or absence of a skyrmion. Coloured arrows represent the direction of local magnetic fields (red = up, blue = down, yellow = horizontal). The electrical current (A) or voltage (V) in a tunnel junction read-out device (the blue and orange boxes) changes as the skyrmions pass underneath. (Illustration: Nature Nanotechnology)

Monchesky and his team were able to explain their observations using theoretical calculations based on fundamental physics principles for thin films of the class of materials to which MnSi belongs—a group known as ‘chiral magnetic materials.’ That means the magnetic properties discovered by Monchesky’s team are expected to be found in other chiral magnetic materials as well, which is important because the skyrmions in MnSi require extremely cold temperatures, making MnSi itself an unlikely choice for use in a device.

“Although using MnSi may not be practical, we observed new fundamental behaviour that will be important in the design of future devices based on chiral magnetic materials,” says Monchesky. Indeed, it might only be a matter of time before the right material is identified for a major breakthrough in computer memory. “There’s a lot of excitement right now in the scientific community about skyrmions,” he adds, pointing to an example of a bulk material where skyrmions have been found at room temperature.

DOI: 10.1103/PhysRevB.90.014406

This research story was republished with the permission of the Canadian Institute for Neutron Scattering.

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