The Future of Computer Memory Is in a Vortex

The magnetic vortex is a classic of backyard science. There are pages and pages of YouTube search results to confirm this fact, and it's easy enough to see the allure. It offers the appearance—and only the appearance—of free energy. Take a circle of magnets aligned such that their "north" or "south" polarizations all point inward, and the result is indeed a magnetic field behaving as a vortex, with some cool effects.

While twirling around an oppositely-polarized magnet is fun and all, the possibilities of these vortices go much deeper—and smaller. One of those possibilities, described in the current edition of Nature Communications, is in super-efficient data storage, where nanoscale versions of magnetic vortices are used to encode information as spin orientations. As the spin orientation of a particle creates a magnetic vortex, in much the same way that macroscale magnets can be arranged for a vortex effect, that vortex can be "read" electrically by measuring its characteristic microwave frequency.

The effect in question occurs on nanoscale disks or pillars just 150 nm across (figure a thousand times thinner than a human hair) made of iron and consisting of two stacked vortices. The setup is highly sensitive to applied currents and fields and is, thus, able to be manipulated in useful ways. But the scale has so far prevented any information that might be stored in the system from being read back.

The discovery here is that this information can be reliably gleaned from the nanopillar setup after all, thanks to the alternating current (AC) microwaves produced when the system is hooked up to a direct current (DC) power source.

"This principle is similar to that of playing a wooden flute: here too, each position of the fingers over the holes clearly corresponds to a particular musical note, a vibration frequency," explains Alina Deac, one of the physicists behind the new study and head of the Helmholtz Young Investigator Group for Spintronics, in a statement.

Each nanopillar has the additional benefit of being able to store two bits of data: one in the actual polarization of the vortex (north or south), and the second from the direction in which the vortex spins. So: up or down, left or right. Note the illustration.

Given that each nanopillar is a stack of two vortices, we're now able to store four bits of data in a single memory cell. The states of these bits are revealed by "discrete" or stepwise frequency patterns carried away by microwaves, which encode the 16 different possible states of those four bits (2^4). "Each mode belongs to a specific combination of vorticities and relative core polarities, which can be reached by applying appropriate sequences of current and field," the paper explains.

Now we're getting somewhere.

Memory is one of the most persistent yet underpublicized problems in future-computing. How can we store more information faster and in one location? There's really no easy answer and the problem is poised to be a critical limiting factor in processing speeds. Computers are getting faster, but they need to be able to handle data faster for it to matter. Spintronics is the usual answer, but, as we can see, it's an open question.

"This [technology] constitutes a reliable electrical read-out method for the state of the device, and thus could pave the way for designing a multi-level, non-volatile memory cell," Deac and her coauthors write. "Indeed, while spin-transfer switching has been identified as a potential write method early-on, the lack of suitable methods of reading the core orientation has been the main cause hampering the development of vortex-based memory devices to-date."