Physicists discover ‘stacked pancakes of liquid magnetism’

Newswise — HOUSTON – (May 10, 2023) – Scientists have found "layered cakes of fluid magnetism" which could explain the peculiar electronic properties of certain spiral magnets.

The substances examined exhibit magnetism in low temperatures, but lose their magnetic properties upon thawing. Makariy Tanatar, an experimental physicist from Ames National Laboratory at Iowa State University, observed unusual electronic characteristics in layered helimagnetic crystals and brought this enigma to the notice of Andriy Nevidomskyy, a theoretical physicist from Rice University. Together with former Rice graduate student Matthew Butcher, they constructed a computational model to mimic the quantum states of atoms and electrons in the layered substances.

Magnetic substances experience a phase transition referred to as "thawing" upon heating, causing them to lose their magnetism. To investigate this transition in helimagnets, the scientists conducted numerous Monte Carlo computer simulations and monitored how the atomic magnetic dipoles within the material aligned themselves during the thawing process. Their findings were reported in a recent issue of Physical Review Letters.

At the submicroscopic level, the substances being studied are made up of numerous 2D crystals piled on top of each other in a manner akin to pages in a notebook. The atoms in each crystal layer are arranged in lattices, and the physicists simulated quantum interactions within and between the layers.

Nevidomskyy, an associate professor of physics and astronomy and member of the Rice Quantum Initiative, remarked, "We're accustomed to the notion that if you warm up a solid substance such as a block of ice, it will transform into a liquid and then, at an even higher temperature, it will vaporize and become a gas. The same comparison can be made with magnetic substances, but without actual evaporation taking place."

"The crystal structure remains unchanged," Nevidomskyy explained. "However, if you examine the orientation of the tiny magnetic dipoles - which resemble compass needles - you will notice that they start out in a correlated arrangement. This implies that if you know the orientation of one dipole, you can deduce the orientation of any other dipole, no matter how far apart they are in the lattice. This is the magnetic state or the solid in our analogy. As the temperature increases, the dipoles will eventually become uncorrelated and independent of one another, resulting in a paramagnetic state that is analogous to a gas."

Nevidomskyy said physicists typically think of materials either having magnetic order or lacking it.

"From a classical perspective, a more fitting comparison would be a block of dry ice," Nevidomskyy added. "It essentially skips the liquid phase and transforms straight from a solid into a gas. This is how magnetic transitions are usually depicted in textbooks. We are typically taught that you begin with something that is correlated, such as a ferromagnetic material, and at a certain point, the order parameter vanishes, resulting in a paramagnetic substance."

Tanatar, who works as a research scientist at the Superconductivity and Magnetism Low-Temperature Laboratory at Ames, detected indications that the shift from magnetic order to disorder in helical magnets was accompanied by a transient stage in which electronic characteristics, such as resistance, varied according to direction. For example, there might be differences if the measurements were taken horizontally from side to side versus vertically from top to bottom. This directional behavior, known as anisotropy, is a characteristic feature of many quantum materials, such as high-temperature superconductors, according to physicists.

Nevidomskyy stated, "These stacked materials exhibit dissimilarities in the vertical and horizontal directions, which gives rise to anisotropy. Makariy had a hunch that anisotropy was influencing the melting of magnetism in the material, and our simulations proved it to be correct and also explained the underlying reasons for it."

According to the model, during the transition from magnetic order to disorder, the material goes through an intermediate stage. In this phase, the dipole interactions within the sheets are much stronger compared to those between them. Additionally, the correlations between the dipoles resemble those of a liquid instead of a solid. This leads to the formation of "flattened puddles of magnetic liquids that are stacked up like pancakes," as per Nevidomskyy. Within each pancake-shaped puddle, the dipoles are oriented approximately in the same direction. However, the direction of orientation varies among the neighboring pancakes.

Nevidomskyy explained that "in this scenario, all atoms have their dipoles aligned in the same direction within a given layer. But when you move up to the next layer, the dipoles of the atoms are pointing in different random directions."

The arrangement of atoms in the material causes a "frustration" of the dipoles, preventing them from aligning uniformly throughout the material. Instead, the dipoles in the layers rotate slightly in response to changes in the neighboring pancakes.

Nevidomskyy explained that the frustrations in the material make it difficult for the magnetic dipoles to settle on a single direction, causing them to rotate and shift in each layer to relieve that frustration.

According to Tanatar, the competition between the two magnetic phases in the helimagnetic crystals leads to a lower transition temperature for these phases. The phenomena that lead to magnetic order in this scenario are different from those without competition. This competition between magnetic phases results in the anisotropic behavior of the material during the transition from magnetic order to disorder.

According to Tanatar and Nevidomskyy, even though there is no immediate practical use for their discovery, it could provide insights into the unexplained physics of other anisotropic materials, such as high-temperature superconductors.

Despite the name, high-temperature superconductivity actually occurs at very low temperatures. One theory suggests that materials can become superconductors when they are cooled near a quantum critical point, which is a temperature that is sufficient to suppress long-range magnetic order and give rise to effects caused by strong quantum fluctuations. For example, some magnetic "parent" materials have been shown to have superconductivity close to a quantum critical point where magnetism vanishes.

Tanatar explained that the study suggests an alternative way to suppress long-range magnetic ordering, which could potentially lead to superconductivity. This could be significant in the field of unconventional superconductivity, as it provides additional insight into the mechanisms behind the phenomenon. The idea is that frustration or competing interactions could give rise to weaker effects, such as superconductivity, when the main effect of long-range magnetic ordering is suppressed.

As a graduate student in Nevidomskyy's research group, Butcher conducted the Monte Carlo calculations. He completed his PhD at Rice in 2022 and currently works as an engineering scientist at the Applied Research Laboratories of the University of Texas at Austin.

The research was supported by the Welch Foundation (C-1818), by the Department of Energy’s Basic Energy Sciences program’s Materials Sciences and Engineering Division (DE-AC02-07CH11358) and the National Science Foundation (1917511, 1607611, 1338099). Computational work was supported by Rice University’s Center for Research Computing.

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