Scientists Finally Observe Long-Predicted Form of Magnetism
Quantum physicists have finally observed Naraoka ferromagnetism in the lab.
Magnetism is critical to how many electronics and machines work, and this particular kind has implications for quantum computing.
An arrangement of electrons behaves like a sliding-tile puzzle, while the magnetism stays constant.
Scientists believe they’ve made a concrete example of an unusual, theoretical form of ferromagnetism first described by a researcher more than 50 years ago.
Nagaoka ferromagnetism, named for the scientist who discovered it, Yosuke Nagaoka, is a special case of the same magnetic forces that make regular, refrigerator-type magnets work—ferro meaning iron, plus a few other metals that are naturally receptive to magnetism. Identifying it in real life—in this case using a quantum system of electrons—can help scientists understand how spontaneous ferromagnetism works.
The analogy for Nagaoka ferromagnetism has been the “15 puzzle,” the sliding-squares puzzle where you must make a picture or put the numbers in order by sliding one tile at a time. Nagaoka theorized that when all the electrons were pointed in the same direction—part of the underpinning of quantum mechanics in solid-state physics is how electrons are arranged in relation to each other—the entire system would be magnetized.
Moreover, the “perfect case” Nagaoka envisioned would be impervious to arrangement, hence the sliding tile angle. No matter how the electrons shuffled within the system, they’d stay magnetized. Their order doesn’t make a difference, so if the electrons were a “set,” say, their arrangements would be like combinations rather than permutations.
To create a real example of the Nagaoka case, quantum physicists at Delft University of Technology made a two-by-two lattice of quantum dots, which are microscopic arrangements of particles that can carry current and light and show quantum behaviors. They supercooled the lattice and put electrons on three of the four “squares” in the lattice. (Supercooling in general allows scientists to observe subatomic particles interacting because it slows the particles way down, relatively speaking.)
The scientists used a special sensor and electrical system that could measure how the electrons were pointing, and indeed, they found that the electrons kept the same homogeneous spin pattern as they were moved around the lattice. They say this is because the electrons want to stay in the easiest, lowest energy state—but being the first to observe and document the Nagaoka effect firsthand is still a big deal.
The secret of ferromagnetism is one of the biggest mysteries in science. “Itinerant ferromagnetism is actually one of the hardest problems in theoretical condensed matter physics,” a physicist told Quanta magazine in 2019. Nagaoka ferromagnetism in particular “has been rigorously studied theoretically but has remained unattainable in experiments,” the research team explained in its paper.
In this case, itinerant ferromagnetism is a specific part of the larger idea of ferromagnetism, “which originates purely from long-range interactions of free electrons and whose existence in real systems has been debated for decades.” In other words, far-apart electrons interact in such a way that they generate a ferromagnetic force, but scientists haven't been able to observe this phenomenon. So its existence has remained a contentious hypothesis.
The researchers suggest their results have implications both in the study of ferromagnetism and the future of quantum computers. If the lattice can reliably retain its magnetic charge despite whatever manipulations, that’s useful for any system where manipulation serves a purpose like storing information or indicating a position. Sounds pretty computer-y already.
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