Large Orbital Currents Induced in Light Metals
Physicists at Uppsala University have shown in an international collaboration that very large orbital currents can be induced in the light metal chromium.
Previous research has predicted that electric fields can generate large currents of orbital angular momentum, but it has been a great challenge to demonstrate that this is indeed the case.
Magneto-optic method to identify large orbital currents. The orbital Hall effect leads to accumulation of orbital momentum on the surface of a chromium layer which is detected through magneto-optic spectroscopy. Image: Sanaz Alikhah.
Electrons in metals have a spin momentum as well as an orbital angular momentum. The spin momentum is an intrinsic property of electrons while orbital angular momentum is created by the electrons’ motion around the atomic nucleus. When electrons with opposite spin momentum move through a metal with strong enough spin-orbit coupling, they begin to move in opposite directions, which is an effect called the spin Hall effect.
The spin Hall effect converts an electric current into a spin current in metals. Twelve years ago it was discovered that this spin current could be used to efficiently control magnetization in magnetic devices with short electric pulses. This discovery opened up for the ongoing development of a whole new technology for digital information processing. But to obtain large enough spin currents, materials consisting of heavier atoms are needed, such as platinum which are very expensive.
Previous careful calculations have shown that there exist as well an orbital Hall effect which is significantly greater than the spin Hall effect, and which is moreover not dependent of the spin-orbit coupling. But until now it has not been possible to demonstrate existence of the orbital Hall effect. One of the reasons that the orbital Hall effect could not be demonstrated, is that the two Hall effects always appear together and are hard to tell apart.
In the new study the researchers used magneto-optic detection to carefully detect the accumulation of orbital magnetization on the surface of a 30 nanometers thin chromium film. Thereby they could show that the magneto-optic signal corresponded conclusively with all the properties expected for an orbital current.
The researchers could also through precise calculations predict how the magneto-optic signal changed with the thickness of the chromium film. And this turned out to precisely correspond to the measurements and therefore, confirmed that the signal came from the orbital Hall effect.
“The discovery of the orbital Hall effect can have great significance for magnetic random-access memories of the future. The heavy metals used now, such as platinum, are both expensive, environmentally harmful and could be replaced with cheap green metals, such as chromium. Then, one can accomplish at the same time even greater effects which could mean that the elements can be operated more energy-efficiently,” says Peter Oppeneer.
The study has been carried out in an international collaboration between Sanaz Alikhah, Marco Berritta and Peter Oppeneer from Uppsala University and Igor Lyalin and Roland Kawakami from Ohio State University.
Article Reference
Magneto-Optical Detection of the Orbital Hall Effect in Chromium. Igor Lyalin, Sanaz Alikhah, Marco Berritta, Peter M. Oppeneer, and Roland K. Kawakami, Phys. Rev. Lett. 131, 156702; Published October 11, 2023. DOI:https://doi.org/10.1103/PhysRevLett.131.156702
Previous Study
Miron, I., Garello, K., Gaudin, G. et al. Perpendicular switching of a single ferromagnetic layer induced by in-plane current injection. Nature 476, 189–193 (2011). DOI:https://doi.org/10.1038/nature10309
Contact
Sanaz Alikhah, PhD student at the Department of Physics and Astronomy, sanaz.alikha@physics.uu.se
Marco Berritta, guest researcher at the Department of Physics and Astronomy, m.berritta@exeter.ac.uk
Peter Oppeneer, professor at the Department of Physics and Astronomy, peter.oppeneer@physics.uu.se
Camilla Thulin