Ten years ago, the discovery of quasiparticles called magnetic skyrmions provided important new indications that microscopic spin textures would enable spintronics, a new class of electronics that uses the spin orientation of an electron rather than its charge to encode data.
But although scientists have made great strides in this very young field, they still do not fully understand how to design spintronic materials that would allow for ultra-small, ultra-fast, low-power devices. Skyrmions may look promising, but scientists have long treated skyrmions as just 2D objects. However, recent studies suggest that 2D skyrmions could actually be the genesis of a 3D rotation pattern called hopfions. But no one could experimentally prove that magnetic hopfions exist at nanoscales.
Now a team of researchers co-led by the Berkeley Lab has reported Nature Communications the first demonstration and observation of 3D hopfions emerging from skyrmions at the nanoscale level (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-power magnetic memory devices, and yet ultra-stable memory devices that harness the internal power of electron spins.
“We didn’t just prove that there are complex spin textures like 3D hopfions – we also showed how to study them, and therefore use them,” said co-senior author Peter Fischer, a senior scientist in the Department of Materials Science at Berkeley Laboratory, who also an associate professor of physics at UC Santa Cruz. “To understand how hopfions really work, we need to know how to make and study them. This job was only possible because we have these amazing tools in the Berkeley lab and our collaborative partnership with scientists around the world,” he said.
According to previous studies, hopfions, unlike skyrmions, do not get carried away when moving along the device and are therefore excellent candidates for data technologies. Furthermore, theoretical collaborators in the United Kingdom have predicted that hopfions could be formed from a multilayer 2D magnetic system.
The current study is the first to put the theories to the test, Fischer said.
Use of nanomaking tools in the Berkeley Lab Molecular Foundry, dr. Noah Kent, a physics student at UC Santa Cruz and in Fischer’s group at Berkeley’s lab, worked with Molecular Foundry staff to extract magnetic nanowires from iridium, cobalt and platinum layers.
The multi-layered materials were prepared by UC Berkeley postdoctoral fellow Neal Reynolds under the supervision of co-senior author Frances Hellman, who holds the degrees of senior faculty scientist in the Department of Materials Science at Berkeley Laboratory and professor of physics and materials science and engineering at UC -in Berkeley. She also heads the Department of Energy for Non-Equilibrium Magnetic Materials (NEMM), which supported this study.
Hopfions and skyrmions are known to coexist in magnetic materials, but have a characteristic three-dimensional spin pattern. So, to differentiate them, the researchers used a combination of two advanced magnetic X-ray microscopy techniques – X-PEEM (X-ray photoemission electron microscopy) in the laboratory of Berkeley Lab users, an advanced light source; and X-ray Transmission Magnetic Soft Microscopy (MTXM) at ALBA, a synchrotron light plant in Barcelona, Spain – to show different spin patterns of hopfions and skyrmions.
To confirm their observations, the researchers then performed detailed simulations to mimic how 2D skyrmions within a magnetic device evolve into 3D hopfions in carefully designed multilayer structures and how they will appear when imaged by polarized X-rays.
“Simulations are an extremely important part of this process, allowing us to understand experimental images and design structures that will support hopfions, skyrmions, or other designed 3D spin structures,” Hellman said.
To understand how hopfions will ultimately work in the device, researchers plan to use the unique capabilities of Berkeley Lab and world-class research capabilities – which Fischer describes as “key to conducting such interdisciplinary work” – to further study quixotic quasi-particles’ dynamic behavior.
“We’ve known for a long time that spin textures are almost inevitably three-dimensional, even in relatively thin films, but direct imaging is an experimental challenge,” Hellman said. “The evidence here is exciting and opens the door to finding and exploring even more exotic and potentially significant 3D spin structures.”