Electrons can move without dissipation along a certain direction in altermagnets without a magnetic moment and noncollinearity
4 May 2023
Several years ago, researchers at Johannes Gutenberg University Mainz (JGU) and the Czech Academy of Sciences made a theoretical prediction of an unconventional Hall effect. Now they have been able to substantiate this prediction experimentally, in collaboration with research partners rom Beihang University and Huazhong University of Science and Technology in China, Universidad del Norte in Colombia, and the University of Nottingham in the UK. The research team recently published their findings in Nature Electronics.
The Hall effect
At the end of the 19th century, US physicist Edwin Hall discovered two effects. When you hold a material in a magnetic field and send an electric current through it, the electrons are deflected in a direction perpendicular to both the electric and magnetic field. This is nowadays known as the ordinary Hall effect, which found many applications such as distinguishing N- and P-type semiconductors in the electronics industry. Two years after the first discovery, Hall also found out that if you use a magnetic material like iron in an analogous experiment, the Hall effect is surprisingly strong, even without a magnetic field. This came to be known as the anomalous Hall effect.
About 20 years ago, research established that a large contribution to this anomalous Hall effect in many materials comes from a quantum mechanical field due to an interplay of magnetic order and relativistic coupling among electron spin and orbital motion. Scientists assumed that this effect could not occur without magnetization. However, about a decade later, it was discovered that a complex noncolinear spin ordering can also cause the anomalous Hall effect.
What sounds very theoretical can have a potentially very useful application. While electrons moving through a conventional conductor heat up the atomic lattice and thus lose energy, in materials with anomalous Hall effect the electrons can move without losing their energy – at least in a transverse direction to the applied voltage. Unfortunately, to unlock its full potential more practical magnetic materials without magnetization and complex noncollinearity are required.
Unconventional Hall effect in altermagnets without a magnetic moment and noncollinearity
Researchers at Johannes Gutenberg University Mainz predicted a few years ago that the anomalous Hall effect occurs in materials such as ruthenium oxide with collinear magnetic order even without a net magnetic moment, i.e., in materials in which the magnetization completely cancels out due to alternating spin order. "This was very surprising. Until then it was assumed that the Hall effect gets cancelled by these opposite moments," said Dr. Libor Šmejkal, scientific team leader at the JGU Institute of Physics. "The spin densities in materials like ruthenium dioxide are not spherically distributed around atoms, but they are rather anisotropic, distributed in dumbbell-like shapes which prevent cancellation of the quantum mechanical fields. Therefore, such a material can cause an anomalous Hall effect, which turned out to be surprisingly strong."
Experimental observation
Recently, the research team succeeded in verifying this prediction experimentally. "Our research partners were able to produce about ten nanometers thin films of ruthenium dioxide – with two different crystal lattice orientations. According to the theoretical predictions, one orientation should show no anomalous Hall effect, while the other should show a large anomalous Hall effect," said Šmejkal. This is exactly what was confirmed in the experiment. While one orientation only showed the linear ordinary Hall effect, the other also showed the anomalous Hall effect. "The observation is extremely interesting for two reasons. Firstly, this is the first colinear system in which electrons can be moved without dissipation and without relying on magnetization. Instead, it is a simple magnet with two sublattice structures – a peculiarity that many materials exhibit and which allows for reducing the unwanted stray field from the magnetization. Secondly, these data confirm our recent discovery of a third class of magnets, so-called altermagnets", emphasized Šmejkal. The researchers published their discovery of altermagnetism in Physical Review X in 2022.