Scientists from ETH Zurich have discovered a new type of magnetism. Experiments show that an artificially produced material becomes magnetic through a previously unseen mechanism. The most well-known form of magnetism (the kind that sticks things to your refrigerator) is something called ferromagnetism; It occurs when the spins of all electrons in a substance are in the same direction. But there are other forms, such as paramagnetism, a weaker version that occurs when an electron’s spin points in random directions.
In a new study, ETH scientists have discovered a strange new form of magnetism. Researchers investigated the magnetic properties of moiré materials, experimental materials made by stacking two-dimensional layers of molybdenum diselenide and tungsten disulfide. These materials have a cage structure that can contain electrons.
To find out what kind of magnetism these moiré materials had, the team first “poured” electrons into them by applying an electric current and constantly increasing the voltage. Then, to measure its magnetism, they pointed a laser at the material and measured how strongly light was reflected for different polarizations; This can reveal whether the electrons are spinning in one direction (indicating ferromagnetism) or in random directions (indicating paramagnetism).
The material initially exhibited paramagnetism, but when the team added more electrons to the lattice, it exhibited a sudden and unexpected change, becoming ferromagnetic. Interestingly, this change occurred as the lattice was filled with more than one electron per lattice site; this prevented the exchange interaction, the usual mechanism driving ferromagnetism.
“This was striking evidence of a new type of magnetism that cannot be explained by exchange interaction,” said Ataç İmamoğlu, lead author of the study.
The team proposed another mechanism: When multiple electrons enter the lattice nodes, they combine through quantum tunneling into particles called “doubloons” that eventually fill the entire lattice. But when they do this, the electrons minimize their kinetic energy by aligning their spins, thus creating ferromagnetism. This “kinetic magnetism” has been predicted theoretically for decades, but has not been observed in solid materials before.
The researchers plan to investigate this phenomenon more closely, including whether it can be achieved at higher temperatures. After all, for this experiment the material had to be cooled to just above absolute zero.
Source: Port Altele
