Pioneering research co-led by Berkeley Lab has implications for next-generation information technologies.
A decade ago, the discovery of quasi-particles called magnetic skyrmions provided important new clues as to how microscopic spin textures would enable spin electronics, a new class of electronics that uses the orientation of an electron spin rather than its charge to encode data.
But even though scientists have made great strides in this very young field, they still do not fully understand how to design spintronics materials that allow for ultra-light, ultra-fast devices with low power. Skyrmions may seem promising, but scientists have long treated skyrmions as just 2D objects. Recent studies, however, have suggested that 2D skyrmions could actually be the origin of a 3D spin pattern called hopfions. But no one had been able to experimentally prove that magnetic hop fions exist on the nanoscale.
Now, a team of researchers co-led by Berkeley Lab has reported in Nature communication the first demonstration and observation of 3D hop fions coming from nanoscale skyrmions (billionths of a meter) in a magnetic system. The researchers say their discovery heralds a major step forward in realizing high-density, high-speed, low-efficiency, yet ultra-stable magnetic memory device that harnesses the inherent power of electron spin.
“We have not only proven the existence of complex spin textures such as 3D hopfions ̵
According to previous studies, hopfions, unlike skyrmions, do not drift when moving along a device and are therefore excellent candidates for computer technologies. In addition, theoretical partners in the United Kingdom had predicted that hopfions could emerge from a multilayer 2D magnetic system.
The current study is the first to put these theories to the test, Fischer said.
Use of Nanofabrication Tools at Berkeley Labs Molecular Foundry, Noah Kent, Ph.D. students in physics at UC Santa Cruz and in Fischer’s group at the Berkeley Lab, worked with Molecular Foundry staff to carve magnetic nanopills from layers of iridium, cobalt and platinum.
The multi-layer materials were produced by UC Berkeley postdoctoral fellow Neal Reynolds under the supervision of co-author Frances Hellman, who holds titles as a senior researcher in the Berkeley Labs Materials Sciences Division and professor of physics and materials science and engineering at UC Berkeley. She also heads the Department of Energy’s Non-Equilibrium Magnetic Materials (NEMM) program, which supported this study.
Hopfions and skyrmions are known to exist in magnetic materials, but they have a characteristic spin pattern in three dimensions. So to distinguish them from each other, the researchers used a combination of two advanced magnetic X-ray microscopy techniques – X-PEEM (X-ray photoemission electron microscopy) at Berkeley Lab’s synchrotron user facility, the advanced light source; and magnetic soft X-ray transfer microscopy (MTXM) at ALBA, a synchrotron lighting system in Barcelona, Spain – to image the different spin patterns of hopfions and skyrmions.
To confirm their observations, the researchers then performed detailed simulations to mimic how 2D skyrmions inside a magnetic device evolve into 3D hop fions in carefully designed multilayer structures, and how these appear when imaged by polarized X-ray light.
“Simulations are a very important part of this process, enabling us to understand the experimental images and to design structures that support hopfions, skyrmions, or other designed 3D spin structures,” Hellman said.
To understand how hopfions will ultimately work in a unit, researchers plan to use Berkeley Lab’s unique world-class capabilities and research facilities – which Fischer describes as “essential to performing such interdisciplinary work” to further investigate the dynamic behavior of quixotic quasi-particles. .
“We have long known that spin textures are almost inevitably three-dimensional, even in relatively thin films, but direct image processing has been experimentally challenging,” Hellman said. “The evidence here is intriguing, and it opens doors to find and explore even more exotic and potentially significant 3D spin structures.”
Reference: “Creating and Observing Hopfions in Multilayer Magnetic Systems” by Noah Kent, Neal Reynolds, David Raftrey, Ian TG Campbell, Virasawmy, Scott Dhuey, Rajesh V. Chopdekar, Aurelio Hierro-Rodriguez, Andrea Sorrentino, Eva Pereiro, Salvador Ferrer , Frances Hellman, Paul Sutcliffe and Peter Fischer, 10 March 2021, Nature communication.
DOI: 10.1038 / s41467-021-21846-5
Co-authors with Fischer and Hellman include David Raftrey, Ian TG Campbell, Selven Virasawmy, Scott Dhuey and Rajesh V. Chopdekar of Berkeley Lab; Aurelio Hierro-Rodriguez of the University of Oviedo and Andrea Sorrentino, Eva Pereiro and Salvador Ferrer of ALBA Synchrotron, Spain.
The Advanced Light Source and Molecular Foundry are DOE Office of Science user facilities at Berkeley Lab.
This work was supported by the US Department of Energy Office of Science.