Electron spins could become more efficient and easier to manage through a light-based approach using halide perovskite semiconductors. Research teams at Harvard’s Rowland Institute and the University of Cambridge observed ultrafast spin-domain formation in polycrystalline halide perovskite thin films in response to irradiating the films with circularly polarized light at room temperature. Photoinduced spin-charge interconversion in semiconductors, with spin-orbit coupling, could provide a route to spintronics that does not require external magnetic fields, which are challenging to control. An electron can have two spin states, up or down, and these states can be used to store and process information. But manipulating spin states can be tricky, requiring the use of magnetic fields on perfectly ordered materials at extremely low temperatures to work. “People want to talk to spins as the information bit for quite a while now, as the quantum-mechanical property of a spin doesn’t cost as much energy to switch on or to sustain as electrical current,” Rowland fellow Sascha Feldmann said. “But it is harder than it looks. You usually have to apply strong magnetic fields to talk to the tiny magnetic moment a spin has.” Sascha Feldmann, research fellow at Rowland Institute, led researchers at Rowland and the University of Cambridge in a study that used light to optically address spins. Until now, magnetic measurements have been required to study spins. Courtesy of Kris Snibbe, Harvard staff photographer. To observe spins optically, the researchers developed a femtosecond, circular-polarization-resolved, pump-probe microscopy technique based on the information encoded in the spiraling handedness of the light. They used left- or right-handed, circularly polarized light to generate a spin-up or spin-down state. They used information about the spin state to view the spins of the polycrystalline halide perovskite thin films in detail. Feldmann previously used halide perovskite as a semiconductor for solar applications. This versatile material is easy to make, but highly disordered. Under the microscope, the researchers observed photoinduced, ultrafast creation of spin domains on a micrometer scale, formed through lateral spin currents. “Like how charges flow in a wire, we can see how the spins move,” Feldmann said. Spins survived for much longer than the researchers expected, at this scale in each domain. Micrometer-scale variations in the intensity of the optical second-harmonic generation and vertical piezoresponse suggested to the researchers that the picosecond spin-domain formation was driven by local symmetry breaking, leading to spin textures that drove spin-momentum-locked currents, which in turn led to local spin accumulation. “With light we can now optically address spins, which so far one could only learn about using magnetic measurements,” Feldmann said. “With just the polarization of our light, we are able to talk to the spins of the electrons noninvasively.” The ability to observe and manage electron spins optically opens the way for materials with electron spins to be optically manipulated. Once a material is magnetized, even optically, it does not need external power to maintain its state. This makes spintronics an attractive option for storing data efficiently and reliably. The quantum properties of spins could be used by quantum computers to perform tasks that would take a traditional computer thousands of years to complete. Feldmann is excited by the prospect of interfacing photonics, electronics, and spintronics all in one semiconductor material. “A semiconductor is what interacts with light, and a ferromagnet is what is magnetic and does stuff with spins. Here we have found something that combines the best of both worlds,” he said. “It is amazing, because it brings the spintronics people to a place where one can optically control magnetism, and by using spins, it adds another degree of freedom to traditional optical semiconductor devices.” Feldmann is also impressed with how well a material as disordered as halide perovskite behaves when it interacts with light, and he plans to study it further. “The simplicity of the material and making these films and still being able to see spin transport physics is absolutely fantastic for us,” he said. The research was published in Nature Materials (www.doi.org/10.1038/s41563-023-01550-z).
