In so-called kinetic magnetism, entirely different mechanisms are at play compared to conventional magnets. Until now, this particular form of magnetism had only been discussed theoretically. However, a Swiss research team from the Eidgenössische Technische Hochschule (ETH) Zurich has, for the first time, successfully provided experimental evidence. The basis of this discovery was a specially engineered “moire material.”
In a regular magnet, several physical effects must align perfectly within its core for it to adhere to iron, nickel, or its magnetic counterpart. Even without the influence of an external magnetic field, its electrons’ magnetic moments align in the same direction.
Magnetic Moments With a Different Orientation
This process occurs through the so-called exchange interaction, a combination of electrostatic repulsion between electrons and the quantum mechanical effects of electron spins, which are responsible for the magnetic moments. This is the commonly accepted explanation for why certain materials like iron or nickel exhibit ferromagnetism, remaining permanently magnetic until heated beyond a specific temperature.
Researchers at ETH Zurich, led by Ataç Imamoğlu from the Institute of Quantum Electronics and Eugene Demler from the Institute of Theoretical Physics, have now demonstrated a new form of ferromagnetism in an artificially produced material. In this case, the alignment of magnetic moments is achieved through a different mechanism. The findings have been presented in the scientific journal Nature.
Imamoğlu’s group created a unique material for their experiments by stacking thin layers of two different semiconductor materials (molybdenum diselenide and tungsten sulfide). The distinct lattice constants of these materials, representing the atomic spacing, result in a two-dimensional periodic potential with a large lattice constant in the contact plane. This potential can be filled with electrons by applying an electrical voltage.
Moiré Materials
Such moiré materials have attracted great interest in recent years, as they can be used to investigate quantum effects of strongly interacting electrons very well,” said Imamoğlu. “However, so far very little was known about their magnetic properties,” In order to analyze these magnetic properties, Imamoğlu and his colleagues investigated whether the moiré material was paramagnetic, i.e., the magnetic moments were disordered, or ferromagnetic at a certain electron filling.
They illuminated the material with laser light and determined the extent to which the light was reflected for various polarizations. Polarization indicates the direction in which the electromagnetic field of the laser light oscillates. Depending on the alignment of the magnetic moments—and thus the electron spins—the material reflects one polarization more strongly than the other. This difference allows for the calculation of whether the spins all point in the same or different directions, leading to the determination of magnetization.
Surprising Behavior
As the physicists gradually increased the voltage, they filled the material with electrons and measured the magnetization at each step. Up to a filling of exactly one electron per Moiré lattice site (also known as a Mott insulator), the material remained paramagnetic. However, when the researchers filled more electrons into the lattice, something unexpected happened: the material suddenly behaved similar to a ferromagnet.
“That was striking evidence for a new type of magnetism that cannot be explained by the exchange interaction,” stated Imamoğlu. If exchange interactions were responsible for magnetism, it would occur with fewer electrons in the lattice. The abrupt onset pointed to a different effect.
The study’s co-author, Eugene Demler, eventually came up with the crucial insight that it might be a mechanism that Japanese physicist Yosuke Nagaoka had predicted theoretically in 1966. In this scenario, electrons minimize their kinetic energy, which is much larger than their exchange energy, by aligning their spins in parallel. In the experiment conducted by ETH researchers, this occurs once there is more than one electron per lattice site in the Moiré material.
Further Experiments
This passage discusses the formation of doublons, where pairs of electrons combine in a manner that minimizes kinetic energy. The kinetic energy is minimized by allowing the doublons to spread throughout the lattice via quantum mechanical tunneling. This propagation is facilitated when individual electrons in the lattice align their spins ferromagnetically. Failure to achieve this alignment disrupts quantum mechanical superposition effects, impeding the free spread of doublons.
The statement mentions that, so far, such mechanisms for kinetic magnetism have only been observed in model systems consisting of, for example, four quantum dots. The speaker, Imamoğlu, expresses the intention to modify the parameters of the Moiré lattice to investigate whether ferromagnetism persists at higher temperatures. In the current experiment, the material had to be cooled to a temperature slightly above absolute zero.