With the help of a “playground” they created to observe exotic physics, scientists and colleagues at MIT not only found a new way to manipulate magnetism in a material with light, but also made a rare form of matter. The former could lead to applications such as computer memory storage devices that can read or write information much faster, while the latter introduces new physics.
A solid material is made up of different types of elementary particles, such as protons and neutrons. The “quasi-particles” less familiar to the public are also ubiquitous in these materials. These include excitons, which are composed of an electron and a “hole”, or the space left behind when light shines on a material and the energy of a photon blasts an electron out of it. his usual position. Through the mysteries of quantum mechanics, however, the electron and the hole are always connected and can “communicate” with each other through electrostatic interactions.
“Excitons can be thought of as packets of energy that propagate through a system,” explains Edoardo Baldini, one of the two lead authors of a paper on the Nature Communications work. Baldini, now a professor at the University of Texas at Austin, was a postdoctoral associate at MIT when the work was conducted in the lab of MIT physics professor Nuh Gedik. The other main author is Carina Belvin, a doctoral student from the Gedik group.
“The excitons in this material are rather unique in that they are coupled to the magnetism in the system. It was quite impressive to be able to ‘kick’ the excitons with light and observe the associated changes in magnetism,” says Gedik, who is also associated with MIT’s Materials Research Laboratory.
Current work involves the creation of unusual excitons in the material nickel phosphorus trisulfide (NiPS3). These excitons are “dressed” or affected by the medium which surrounds them. In this case, this environment is magnetism. “So what we’ve found is that by exciting these excitons, we can actually manipulate the magnetism in the material,” Belvin says.
A magnet works because of a property of electrons called spin (another more familiar property of electrons is their charge). Spin can be thought of as an elementary magnet, in which the electrons of an atom are like little needles orienting themselves in a certain way. In your fridge magnets, the spins all point in the same direction and the material is known as ferromagnetic. In the material used by the MIT team, alternating spins point in opposite directions, forming an antiferromagnet.
Physicists have discovered that a pulse of light causes each of the small “needles” of electrons in NiPS3 to start spinning in a circle. The rotating spins are synchronized and form a wave throughout the material, called a spin wave. Spin waves can be used in spin electronics, or spintronics, a field that was introduced in the 1960s.
Spintronics essentially uses the spin of electrons to go beyond electronics, which is based on their charge. The ability to create spin waves in an antiferroelectric material could lead to future computer memory devices that can read or write information much faster than those based solely on electronics. “We’re not there yet. In this article, we’ve demonstrated a process that underlies consistent domain switching: the next step is to actually switch domains,” Baldini says.
Rare form of matter
Thanks to their work, the team has also revealed a rare form of matter. When physicists exposed NiPS3 at intense pulses of light, they found that it transforms into a metallic state that conducts electrons while retaining its magnetism. NiPS3 is usually an insulator (a material that does not conduct electrons). “It’s very rare to have an antiferromagnetic and a metallic state in the same material,” says Belvin.
Physicists believe this happens because intense light causes excitons to collide with each other and separate into their constituents: electrons and holes. “We basically destroy the excitons, so electrons and holes can move like those in a metal,” Baldini says. But these moving particles do not interact with the localized electron spins participating in the spin wave, so the magnetism is retained.
Baldini describes the experimental setup as a “playground for observing many-body physics”, which he defines as “the elegant interaction between different bodies like excitons and spin waves”. He concludes: “what I really liked about this work is that it shows the complexity of the world around us”.
The other authors of the MIT paper are physics professor Senthil Todadri, Ilkem Ozge Ozel (Ph.D. ’18), Dan Mao (Ph.D. ’21, now at Cornell University), Hoi Chun Po (postdoctoral fellow ’18-’21, now at Hong Kong University of Science and Technology), and Clifford Allington (graduate student in chemistry). The other authors are Suhan Son, Inho Hwang and Je-Geun Park from the Institute of Basic Sciences (Korea) and Seoul National University; Beom Hyun Kim of the Korea Institute for Advanced Study; and Jae Hoon Kim and Jongyeon Kim of Yonsei University.
Physicists detect hybrid particle held together by uniquely strong ‘glue’
Carina A. Belvin et al, Excitation-Driven Antiferromagnetic Metal in a van der Waals Correlated Insulator, Nature Communication (2021). DOI: 10.1038/s41467-021-25164-8
Provided by Materials Research Laboratory, Massachusetts Institute of Technology
Quote: Physicists manipulate magnetism with light (February 1, 2022) retrieved April 12, 2022 from https://phys.org/news/2022-02-physicists-magnetism.html
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