Scientists have recently discovered a new type of magnetism


“The reason we see magnetism in our everyday lives is the strength of electron exchange interactions,” said Atac Imamoglu, a physicist at the Institute of Quantum Electronics and co-author of the study.

However, as Nagaoka theorized in the 1960s, exchange interactions cannot be the only way to magnetize a material. Nagaoka imagined a square, two-dimensional lattice, where there was just one electron at each site on the lattice. Then they discovered what would happen if you removed one of those electrons under certain conditions. As the remaining electrons of the lattice interact, the hole where the missing electron was will scatter around the lattice.

In Nagaoka's scenario, the overall energy of the lattice will be lowest when all of its electron spins are aligned. Every electron configuration will look the same—as if the electrons in the world's most boring sliding tile puzzle were identical tiles. These parallel spins, in turn, will make the material ferromagnetic.

When two grids create a pattern with a twist into existence

Imamoglu and his colleagues had the inkling that they could create Nagaoka magnetism by experimenting with single-layer sheets of atoms that could be put together to form a complex moiré pattern (pronounced mwah-re, In atomically thin, layered materials, Moiré patterns can fundamentally change how electrons—and thus the material—behave. For example, in 2018 physicist Pablo Jarillo-Herrero and colleagues demonstrated that two-layer stacks of graphene gained the ability to superconduct when they offset the two layers with a twist.

Atak Imamoglu

Atak Imamoglu and his colleagues suspected that their newly synthesized material might exhibit some strange magnetic properties, but they didn't know exactly what they would find.

Courtesy of Atak Imamoglu

Moiré materials have since emerged as a compelling new system in which magnetism can be studied, colocated with clouds of supercooled atoms and complex materials such as cuprates. “Moiré materials basically provide us with a playground to synthesize and study many-body states of electrons,” Imamoglu said.

The researchers set out to synthesize a material from monolayers of semiconductor molybdenum diselenide and tungsten disulfide, which belongs to a class of materials that previous simulations had implied could exhibit Nagaoka-style magnetism. They then applied weak magnetic fields of varying strengths to the Moiré material, while tracking how many of the material's electrons aligned with the field.

The researchers repeated these measurements while applying different voltages to the material, which changed how many electrons were in the Moiré lattice. He felt something strange. The material is more prone to align with an external magnetic field – that is, to behave more ferromagnetically – only when it has 50 percent more electrons than lattice sites. And when there were fewer electrons in the lattice than in the lattice sites, the researchers saw no sign of ferromagnetism. This was the opposite of what they would expect if standard-issue Nagaoka ferromagnetism was at work.

Although the material was magnetized, exchange interactions did not appear to be driving it. But even the simplest versions of Nagaoka's theory could not fully explain its magnetic properties.

When your stuff got magnetized and you were somewhat surprised

Finally the talk came to the movement. Electrons reduce their kinetic energy by spreading out in space, allowing the wave function describing an electron's quantum state to overlap with that of its neighbors, tying their fates together. In the team's material, once the Moiré lattice had more electrons than lattice sites, the material's energy dropped when extra electrons were pumped in like fog onto a Broadway stage. They then combine transiently with electrons in the lattice to form two-electron combinations called doubloons.

These wandering extra electrons, and the doubloons they created, could not move and diffuse within the lattice until all the electrons in the surrounding lattice sites had aligned spins. As the material continued to pursue its lowest-energy state, the end result was that the doublon created small, localized ferromagnetic fields. Up to a certain limit, the more doubloons flow through the lattice, the more ferromagnetic the material becomes.