Magnetic charge crystals imaged in artificial spin ice

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A team of scientists has reported direct visualization of magnetic charge crystallization in an artificial spin ice material, a first in the study of a relatively new class of frustrated artificial magnetic materials-by-design known as “Artificial Spin Ice.”

These charges are analogs to electrical charges with possible applications in magnetic memories and devices; in describing this class of materials, the new work demonstrates their utility.

Los Alamos National Laboratory staff scientist Cristiano Nisoli explained, “Magnetic technology generally concerns itself with manipulation of localized dipolar degrees of freedom,” he said. “The ability of building materials containing delocalized monopolar charges is very exciting with possible technological implications in data storage and computation.”

Honeycomb configuration helps disassemble magnetic islands

“The emergence of magnetic monopoles in spin ice systems is a particular case of what physicists call fractionalization, or deconfinement of quasi-particles that together are seen as comprising the fundamental unit of the system, in this case the north and south poles of a nanomagnet,” Nisoli said.

“We have seen how arranging magnets in a honeycomb configuration allows for these charges to be sort of ‘stripped’ from the magnetic islands to which they belong and become relevant degrees of freedom.”

Nanoscale magnets prevent freezing

The unique properties of spin ice materials have fascinated scientists since they were first discovered in the late 1990s in naturally occurring rare earth titanites.

The material is aptly named: the highly complex ordering of nanoscale magnets in spin ice obey the same rules that determine the positional ordering of hydrogen and oxygen atoms in frozen water ice.

Both have “spin”—degrees of freedom—with frustrated interactions that prevent complete freezing, even at absolute zero.

In 2006, an interdisciplinary team of physicists and materials scientists designed the first artificial spin ice, a two-dimensional array of magnetic nanoislands that are fabricated to interact in complex ways, depending on the chosen design of
the array.

The islands were lithographically printed onto a substrate, arranged in a square-lattice pattern, with the north and south poles of each nanomagnet meeting and interacting at their four-pronged vertices.