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 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 in Zurich a team of researchers led by Ataç Imamoğlu at the Institute for Quantum Electronics and Eugene Demler at the Institute for Theoretical Physics have now 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 Imamoğlu’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 (thirty times larger than those of the two semiconductors), which can be filled with electrons by applying an electric voltage. “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”, says Imamoğlu. “However, so far very little was known about their magnetic properties.”
To investigate these magnetic properties, Imamoğlu and his coworkers measured whether for a certain electron filling the moiré 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 in 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.