How Nanoscale Magnetic Bubbles Might Save Computer Memory

A magnetic skyrmion is like a tiny, nanoscale bubble. It's a "topological" arrangement of a magnetic field, which means that it can be bent and twisted and pulled in any which way, and so long as the structure's basic shape isn't broken or torn, the arrangement is fundamentally preserved. In this case, the structure is a tiny, tiny pocket—the skyrmion—in which the magnetic polarization around the pocket's edge, the field's domain wall (DW), is always pointing inwards, like an inverse porcupine.

This is our bubble. The skyrmion has been long predicted—courtesy of British physicist Tony Skyrme in the 1950s—but only recently produced experimentally. And, now, a group of physicists in the US has successfully produced skyrmions at room temperature, offering hope that these tiny, ultra-stable units might be utilized in future computer memory systems, a field in dire need of new technologies. The promise (or hope) is of memory cells scaled down to just a few nanometers across.

The group's research is published online this week in Science.

A skyrmion is a quasiparticle, which is essentially when some particles within a solid material all start acting in unison as though they were a single particle. More specifically, a skyrmion is a magnetic field that's either oriented outward from a single point in space, or inward toward it. So, the skyrmion quasiparticle constitutes a "magnetic domain," which is a region within some solid material that's all magnetized in a uniform direction—all outwards or inwards, for example. (A skyrmion example is above.)

The bubble are "blown" thanks to a wire that's specially engineered to have a narrow constriction in the middle, like a waistline, and also a particular layered construction that forces the magnetic moments of individual electrons, which would normally be oriented in parallel to the flat material underneath, to all point straight up in an orientation perpendicular to the plane below.

The physicists then take this setup and apply a magnetic field to it, the result of which is "stripes" of upward-pointing magnetic domains surrounded by domains magnetized in the opposite direction. Some current is applied to the wire and the stripes are squeezed through the narrow waistline. The stripes emerge and are stretched and peeled outward, with the result being tiny pockets analogous to the air bubbles that form at the base of a waterfall or kitchen sink. As the current study notes, there's a "spin accumulation" that occurs at the boundary between the skyrmion-bubble and the surrounding material, like pressure.

This is potentially a big deal for the simple reason that engineers are running out of ways to make smaller transistors. When a certain scale is reached in electronics, it becomes impossible to not change the atomic structure of the material itself. (See: quantum tunneling.) The stability of Skyrmions offers a way to to scale down beyond this limit.

A skyrmion, for example, could potentially represent information as simply as the presence of one of the bubbles being a "1" and the absence being a "0," just like using infinitesimal rocks as counters. What's more, the skyrmions required by this scheme are already widely used in the magnetic storage industry, according to Axel Hoffmann, a co-author of the new study and a materials scientist at Argonne National Laboratory.

"Our new method is the simplest way to generate skyrmion bubbles thus far," adds Wanjun Jiang, the study's first author and a postdoctoral researcher at Argonne, in a news release. "We think this method could apply to many more materials. This opens many new opportunities for the future."