Discovery of “stacked pancakes of liquid magnetism” in layered helimagnets

Scientists have made a fascinating discovery of “liquid magnetism stacked like pancakes” that could potentially explain the unusual electronic behavior of certain helical magnets with layers.

The anomalous electronic behavior of layered helimagnetic crystals caught the attention of experimental physicist Makariy Tanatar from Ames National Laboratory at Iowa State University. Tanatar collaborated with Rice theoretical physicist Andriy Nevidomskyy and former Rice graduate student Matthew Butcher to develop a computational model that could simulate the quantum states of atoms and electrons in these layered materials.

When magnetic materials are heated, they undergo a transition from a magnetic to non-magnetic state. Using Monte Carlo computer simulations, the researchers studied this transition in helimagnets and observed how the magnetic dipoles of atoms within the material organized themselves during the heating process. Their findings were recently published in Physical Review Letters.

At a submicroscopic level, the materials being studied comprise thousands of 2D crystals stacked on top of each other, similar to pages in a notebook. In each crystal sheet, atoms form lattices, and the scientists simulated quantum interactions both within and between the sheets.

According to Andriy Nevidomskyy, an associate professor of physics and astronomy at Rice University and a member of the Rice Quantum Initiative, our understanding of the phase transition of matter is often limited to the transformation of solids, liquids, and gases. However, a similar analogy can be made with magnetic materials, even though nothing evaporates in the true sense of the word.

In the case of magnetic materials, the crystal structure remains intact. However, the arrangement of the tiny magnetic dipoles, which are similar to compass needles, undergoes a transformation as they are heated. Initially, the dipoles are arranged in a correlated manner, such that the orientation of one dipole determines the orientation of all others, regardless of their position in the lattice. This is the magnetic state, analogous to a solid in the traditional phase transition analogy. As the temperature rises, the dipoles become increasingly independent, or random, with respect to one another. This state is known as a paramagnet, similar to a gas in the traditional phase transition analogy.

Andriy Nevidomskyy, a member of the Rice Quantum Initiative and an associate professor of physics and astronomy, explained that conventional physics considers materials to either have magnetic order or not have it. He likened this to the phase transition of dry ice, which skips the liquid phase and goes directly from solid to gas. This is similar to how magnetic transitions are typically described in textbooks, where a material starts with correlated magnetic order and then loses it as it transitions to a disordered state.

Makariy Tanatar, a research scientist at Ames’ Superconductivity and Magnetism Low-Temperature Laboratory, discovered that helical magnets undergoing the transition from magnetic order to disorder have a transitory phase where electronic properties, such as resistance, exhibit directional behavior or anisotropy. This anisotropy is a characteristic of many quantum materials, including high-temperature superconductors.

Nevidomskyy explained that the anisotropy observed in these layered materials is due to how magnetism melts in the material, as demonstrated by their computational modeling. The modeling revealed that anisotropy affects how magnetism melts in the material and provided an explanation for why it occurs.

The researchers’ model revealed that the material goes through an intermediate phase during its transition from magnetic order to disorder. This phase is characterized by stronger dipole interactions within sheets of the material than between them. The correlations between the dipoles in this phase are similar to those of a liquid rather than a solid, resulting in “flattened puddles of magnetic liquids that are stacked up like pancakes,” according to Nevidomskyy. Each pancake consists of atoms with dipoles pointing roughly in the same direction, but this direction varies between neighboring pancakes.

The atomic arrangement of the material creates frustration that prevents the dipoles from aligning uniformly throughout the material. Instead, the dipoles in each layer shift and rotate slightly in response to changes in neighboring pancakes. “Frustrations make it difficult for the arrows, these magnetic dipoles, to decide where they want to point, at one angle or another,” said Nevidomskyy. “And to relieve that frustration, they tend to rotate and shift in each layer.”

The researchers noted that this discovery may not have immediate applications, but it could provide insights into the physics of other anisotropic materials like high-temperature superconductors. High-temperature superconductivity occurs at very cold temperatures, and one theory suggests that materials may become superconductors when cooled near a quantum critical point. This temperature is sufficient to suppress long-range magnetic order and give rise to effects brought about by strong quantum fluctuations. The researchers’ findings suggest that frustration or competing interactions could also lead to the suppression of the main effect, long-range magnetic ordering, and give way to weaker effects such as superconductivity.

Source: Rice University

Leave a Comment