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Frustrated Magnetism, Confused Physicists

13 March 2015

             Dr. Michel Gingras has returned to TRIUMF where he was once a postdoctoral fellow to rejuvenate his research studies in condensed matter. Currently faculty at the University of Waterloo, over the five-month long visit he will be splitting his time between TRIUMF’s Theory Department and UBC as he wrestles with the theoretical understanding of frustrated magnetism.

While his recent talk at the TRIUMF colloquium may have had a perplexing title - “Spin Ice - A Magnetic Analogue of Common Water Ice with Emergent Electrodynamics and Deconfined Excitations” - a quick chat with Michel will show you that while it is still a complex concept, the frustrated magnetism of spin ice can be rather straightforward to understand!

A frustrated magnet is a magnetic material that has a very specific atomic structure that make the formation of magnetic order difficult. The way that the atoms within spin ice are arranged can be represented with tetrahedra, or triangular-based pyramids, joined at each of the four corners – the so-called pyrochlore lattice. In spin ice, at each corner where two tetrahedra touch, there is a magnetic moment. It is this structure of corner-shared tetrahedra that makes spin ice a frustrated magnet.

“The amazing thing is that this sort of structure appears in various other places all throughout nature. One of the most unlikely of places that we observe it is when water freezes to form common ice. Indeed, a very similar structure forms when everyday water freezes, as a result of the Bernal-Fowler ice rules that every first-year undergraduate chemistry student learns. At the joints in water however, there is not a magnetic moment, but rather a pair of hydrogen atoms,” Michel explains. Spin ice and water ice are unrelated; there is no a priori reason why the behaviour of hydrogen in water ice and magnetic moment in the pyrochlore lattice should behave similarly according to the laws of statistical mechanics – but they do.

The problem with spin ices – and frustrated magnets in general – is that as they are cooled down towards absolute zero their entropy does not go to zero as well; this violates the third law of thermodynamics. Thermodynamics is the study of heat and energy transfer, and its laws are considered some of the best established in the whole of science. Entropy is used widely in chemistry and physics as a measure of disorder in a system. The fact that both water ice and spin ice seemingly violate the third law of thermodynamics by not having zero entropy at zero temperature is what has confused physicists and chemists for decades.

Until very recently, most theoretical and experimental work on spin ice materials has focused on systems where the orientation of the magnetic moments is classical. Michel’s current work is focused on a new variant of spin ices, quantum spin ice, in which the magnetic moments are able to fluctuate, rotate or flip via the laws of quantum mechanics.

While frustrated magnetism and spin ices have been keeping Michel busy, he has still found time to enjoy his new environment. “It is nice to get away from Waterloo sometimes,” he admits, which (at the time of this article) had experinces a week of -20oC weather (it has significantly warmed up since then!) “I’ve done much less skiing here than I would’ve liked to, but it’s still very exciting to be here at TRIUMF and UBC working with some old friends.”

Dr. Gingras is scheduled to return to the University of Waterloo on May 31st. Feel free to drop by his office and say hi, or offer him a cup of ice water!

–Christopher Zaworski, Outreach & Communications Assistant