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Weird science: New kind of magnetism discovered

Researchers in Switzerland have detected a new type of magnetism in an artificially produced material. The material becomes ferromagnetic through minimization of the kinetic energy of its electrons.

By Oliver Morsch, ETH Zurich

For a magnet to stick to a fridge door, inside of it several physical effects need to work together perfectly. The magnetic moments of its electrons all point in the same direction, even if no external magnetic field forces them to do so. This happens because of the so-called exchange interaction, a combination of electrostatic repulsion between electrons and quantum mechanical effects of the electron spins, which, in turn, are responsible for the magnetic moments. This is a common explanation for the fact that certain materials like iron or nickel are ferromagnetic, or permanently magnetic, as long as one does not heat them above a particular temperature.

At ETH, a technical university in Zurich, a team of researchers led by Atac Imamoglu at the Institute for Quantum Electronics and Eugene Demler at the Institute for Theoretical Physics have detected a new type of ferromagnetism in an artificially produced material, in which the alignment of the magnetic moments comes about in a completely different way. They recently published their results in the scientific journal Nature.

Artificial material with electron filling
In Imamoglu's laboratory, PhD student Livio Ciorciaro, post-doc Tomasz Smolenski, and colleagues produced a special material by putting atomically thin layers of two different semiconductor materials (molybdenum diselenide and tungsten disulfide) on top of each other. In the contact plane, the different lattice constants of the two materials -- the separation between their atoms -- leads to the formation of a two-dimensional periodic potential with a large lattice constant (30 times larger than those of the two semiconductors), which can be filled with electrons by applying an electric voltage.

"Such moire materials have attracted great interest in recent years, as they can be used to investigate quantum effects of strongly interacting electrons very well," says Imamoglu. "However, so far very little was known about their magnetic properties."

To investigate these magnetic properties, Imamoglu and his coworkers measured whether for a certain electron filling the moire material was paramagnetic, with its magnetic moments randomly oriented, or ferromagnetic. They illuminated the material with laser light and measured how strongly the light was reflected for different polarizations. The polarization indicates which direction the electromagnetic field of the laser light oscillates, and depending on the orientation of the magnetic moments -- and hence the electron spins -- the material will reflect one polarization more strongly than the other. From this difference, one can then calculate whether the spins point in the same direction or in different directions, from which the magnetization can be determined.

In the moire material produced at ETH, the electron spins are disordered if there is exactly one electron per lattice site (top). As soon as there are more electrons than lattice sites (bottom) and pairs of electrons can form doublons (red), the spins align ferromagnetically, as this minimizes the kinetic energy. [Credit: Illustration by ETH Zurich]

 

 

 

 

Striking evidence
By steadily increasing the voltage, the physicists filled the material with electrons and measured the corresponding magnetization. Up to a filling of exactly one electron per site of the moire lattice (also known as a Mott insulator), the material remained paramagnetic. As the researchers kept adding electrons to the lattice, something unexpected happened: The material suddenly behaved very much like a ferromagnet.

"That was striking evidence for a new type of magnetism that cannot be explained by the exchange interaction," Imamoglu says. In fact, if the exchange interaction were responsible for the magnetism, that should have shown up also with fewer electrons in the lattice. The sudden onset, therefore, pointed toward a different effect.

Kinetic magnetism
Eugene Demler, in collaboration with post-doc Ivan Morera, finally had the crucial breakthrough realization: The team could be looking at a mechanism that the Japanese physicist Yosuke Nagaoka had theoretically predicted as early as 1966. In that mechanism, by making their spins point in the same direction the electrons minimize their kinetic energy (energy of motion), which is much larger than the exchange energy.

The ETH researchers measured the magnetic susceptibility (which depends on the alignment of the spins) as the electron filling in the moire lattice was varied. When the lattice is occupied by more than one electron per site, ferromagnetic interactions lead to a sharp increase in magnetic susceptibility if the temperature is low enough. [Credit: Illustration by ETH Zurich]

 

 

 

 

In the experiment performed by the ETH researchers, this happens as soon as there is more than one electron per lattice site inside the moire material. As a consequence, pairs of electrons can team up to form so-called doublons. The kinetic energy is minimized when the doublons can spread out over the entire lattice through quantum mechanical tunnelling. This, however, is only possible if the single electrons in the lattice align their spins ferromagnetically, as otherwise quantum mechanical superposition effects that enable the free expansion of the doublons are disturbed.

"Up to now, such mechanisms for kinetic magnetism have only been detected in model systems, for example in four coupled quantum dots," says Imamoglu, "but never in extended solid-state systems like the one we use."

As a next step, Imamoglu wants to change the parameters of the moire lattice in order to investigate whether the ferromagnetism is preserved for higher temperatures; in the current experiment, that material still had to be cooled down to a tenth of a degree above absolute zero.

Published November 2023

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