"If you were able to visualize it, it would look like a funnel", stated Argonne materials scientist Valentyn Novosad.
Valentyn Novosad and postdoctoral researcher Shikha Jain wanted to find a way to control the magnetic states of pairs or even large groups of these dots that interacted with each other in a lattice.
In the resting state, the cores of the dots are randomly polarized. After applying an oscillating magnetic field to the pairs of dots, the researchers observed that the central cores began to switch back and forth repeatedly between up- and down-magnetizations - which Shikha Jain and Valentyn Novosad characterized as "chaos".
This chaotic system's behaviour is dictated by the fact that the magnetic field, when applied, oscillates at a particular frequency that can be tuned to match the "resonance frequency" - that is, the natural frequency of vibration - for a specific polarity combination in a dot-pair. Each pair of dots has two resonance frequencies, corresponding to parallel (up-up or down-down) or antiparallel (up-down or down-up) magnetization states. In the parallel state, the dots' centres are magnetized in the same direction, while in the antiparallel state they are opposite.
By increasing and then decreasing the strength of the applied field, Argonne's scientists were able to bring the dots' magnetizations into and then out of chaos. When the amplitude of the oscillating field was reduced significantly, the researchers discovered that the new polarizations corresponded to the opposite state of the applied frequency. If the frequency corresponded to the parallel magnetization, an antiparallel arrangement would emerge, and vice versa.
"We were somewhat surprised that the behaviours of multiple dots could be controlled so precisely", Shikha Jain stated. "There had been a lot of work done before on single-dot systems, but no one had really investigated how magnetic dots interact with each other in this kind of environment."
By achieving consistent control of the central polarization's direction, the researchers moved one step closer to creating new magnetic devices, including non-volatile random access memories. These devices are the subject of intense research in academia and industry worldwide as they offer energy efficiency, high operating speed and exceptional reliability. Unlike the silicon-based chips in today's computers, their magnetic counterparts employ spin rather than electric charge to store and process information.
According to Valentyn Novosad, the underlying physics that governs the two-dot interaction should apply as well to a system of many elements. Future studies will seek to explore the collective dynamics in larger three-dimensional crystal-like magnetic structures.
These results were published in the journalNature Communications. The paper is titled "From chaos to selective ordering of vortex cores in interacting mesomagnets". Shikha Jain is a lead author and Valentyn Novosad is the corresponding author of the paper. Co-authoring this paper were Argonne's Distinguished Fellows Frank Fradin and Samuel Bader, senior materials engineer John Pearson, and professors Andrei Slavin and Vasyl Tiberkevich from Oakland University in Rochester, Michigan.