Taming tornadoes at the nanoscaleJune 12, 2018
By Gene Stowe and Steve Koppes
Superconductors contain arrays of tiny tornadoes of supercurrent, called vortex filaments, each carrying a single quantum of magnetic field. The motion of these tiny tornadoes leads to resistance and furthermore determines the electrical response of all applied superconducting materials.
But a magnet-controlled “switch” that can reconfigure the array of vortex filaments in a superconductor could provide unprecedented flexibility in managing the superconductor’s electrical properties.
“It’s a nice, versatile platform that can conceivably be used to tweak and to tune other electronic or magnetic systems.” — Zhili Xiao, professor at Northern Illinois University.
The new dynamic system, developed by scientists at the University of Notre Dame and the U.S. Department of Energy’s (DOE) Argonne National Laboratory, enables in situ adjustments of the vortex filaments, thereby altering the material’s properties directly. The new system may lead to applications in superconducting microelectronics and computing with vortex bits. The scientists announced their findings in a paper published on June 11 in Nature Nanotechnology.
“We work on fundamental aspects of superconductivity with an eye towards novel and better applications,” said Boldizsár Jankó, a Notre Dame professor of physics. “One of the major problems in superconductor technology is that most of them have these filaments, these tiny tornadoes of supercurrent. When these move, then you have resistance.”
Researchers have been trying to design new devices and technologies to “pin,” or fasten, these filaments to a specified position. Previous efforts to pin the filaments, such as irradiating or drilling holes in the superconductor, resulted in static, unchangeable arrays, or fixed, ordered arrangements of filaments.
“In a paper published in the May 20, 2016, issue of Science, we introduced a newly designed array of nanomagnets that can not only mimic the magnetic charge distribution of an artificial square spin ice structure, but also allow unprecedented control over the magnetic charge locations via local and external magnetic fields,” said the article’s lead author, Yonglei Wang, a resident associate in Argonne’s Materials Science division, who is affiliated with Notre Dame and Nanjing University.
(Spin is a particle’s natural angular momentum. The structures are called “ice” because they involve patterned atomic spins, two pointing in and two pointing out at the vertex of a square spin lattice, analogous to the “two-in, two-out” Pauling’s ice rule that determines the proton positional ordering in water ice.)
Building upon this work, a bilayer structure consisting of an artificial spin ice nanomagnet array atop a superconducting film was fabricated using capabilities at the Center for Nanoscale Materials (CNM), a DOE Office of Science User Facility at Argonne, in collaboration with CNM scientists.
The stray magnetic fields emanating from the ends of the bar-shaped nanomagnets can attract and/or repel the underlying vortex filaments in the superconductor. Reconfiguring the magnetic orientations of these nanobar magnets with an external in-plane magnetic field results in a real-time rearrangement of the “magnetically pinned” vortex filaments in the superconductor. This makes possible multiple, reversible spin cycle configurations for the vortices.
The research team showed that such control over magnetic changes can be exploited in controlling the array of vortices in the superconductor spin ice material, ranging from ordered to disordered and frustrated lattices. (In this context, frustration refers to an important phenomenon in magnetism related to the arrangement of spins.)
“We can imagine there are many different patterns of the nanomagnets that one can create. Each of those different patterns will lead to a different behavior,” said Argonne Distinguished Fellow Wai-Kwong Kwok, who is a co-author of the Nature Nanotechnology paper.
“It’s a nice, versatile platform that can conceivably be used to tweak and to tune other electronic or magnetic systems,” said Zhili Xiao, who is also a co-author of the paper and a professor at Northern Illinois University.
Wang attributed the project’s success to the close collaboration between experimentalists and theorists. The experimentalists could not directly “see” the moving vortices responsible for the measured electron behavior of the superconductor in the laboratory. To overcome this challenge, co-author Xiaoyu Ma, a doctoral student in physics at Notre Dame, carried out computer simulations that created “movies” elucidating the vortex motion influenced by the nanomagnet arrays. The simulations successfully reproduced Argonne’s experimental results with high consistency.
The research is expected to provide a new setting at the nanoscale for the design and manipulation of geometric order and frustration in a wide variety of material systems. These include magnetic skyrmions, two-dimensional materials, topological insulators/semimetals and colloids in soft materials.
The DOE Office of Science, Basic Energy Sciences, along with the National Science Foundation provided funding for this project.
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