Researchers made surprising discoveries about magnetic interactions in

The team from Ames National Laboratory conducted an in-depth investigation of the magnetism of TbMn₆sn₆, Kagome’s layered topology magnet. They were surprised to find that the magnetic spin reorientation in TbMn₆sn₆ occurs by producing an amount of magnetically isotropic ions that increases with increasing temperature.

Rob McQueeney, a scientist at Ames Lab and project leader, explained that TbMn₆sn₆has two different magnetic ions in the material, terbium and manganese. The direction of the manganese moment controls the state of the topology, “But it is the rising moment that determines the direction of the manganese point,” he said. “The idea is, you have these two magnetic species and it’s their combination of interactions that controls the direction of the moment.”

In this layered material, there is a magnetic phase transition that occurs as the temperature increases. During this phase transition, the magnetic moments switch from pointing perpendicular to the Kagome layer, or uniaxial, to pointing inward to the layer, or planar. This transition is called spin reorientation.

McQueeney explains that in Kagome’s metal, the direction of spin controls the topological or Dirac properties of electrons. The electron dirac occurs where the magnetic tapes touch at a single point. However, the magnetic arrangement causes gaps at the points where the bands touch. This gap stabilizes the state of the topological Chern insulator. “So you can go from a Dirac semimetal to a Chern insulator by simply rotating the direction of the moment,” he says.

As part of their TbMn₆sn₆ investigation, the team performed inelastic neutron scattering experiments at the Spallation Neutron Source to understand how magnetic interactions in materials drive spin reorientation transitions. McQueeney said that terbium wants to be uniaxial at low temperatures, whereas manganese is planar, so the two are at odds.

According to McQueeney, behavior at very low or very high temperatures is as expected. At low temperatures, terbium is uniaxial (with electronic orbitals shaped like ellipsoids). At high temperatures, terbium is magnetically isotropic (with a spherical orbit shape), which allows the planar Mn to determine the direction of the overall moment. The team assumed that each terbium orbital would gradually change shape from ellipsoidal to spherical. In contrast, they found both types of terbium were present at intermediate temperatures, but the population of spherical terbium increased with increasing temperature.

“So what we did was determine how the magnetic excitation evolves from this uniaxial state to this easy plane state as a function of temperature. And the old assumptions about how that happened are true,” McQueeney said. “But the nuance is that you can’t assume every terbium is exactly the same on a certain time scale. Every terbium site can exist in two quantum states, uniaxial or isotropic, and if I look at a site, it’s either in one state or the other at any instant of time. It may be uniaxial or isotropic depending on temperature.” We call this binary quantum alloy orbital.

This research is discussed further in “Character of the spin-reorientation transition orbit in TbMn₆sn₆,” written by SXM Riberolles, Tyler J. Slade, RL Dally, PM Sarte, Bing Li, Tianxiong Han, H. Lane, C. Stock, H. Bhandari, NJ Ghimire, DL Abernathy, PC Canfield, JW Lynn, BG Ueland , and RJ McQueeney, and published in natural communication.

Ames National Laboratory is the US Department of Energy Office of Science National Laboratory operated by Iowa State University. Ames Laboratory creates innovative materials, technologies and energy solutions. We use our expertise, unique capabilities and cross-disciplinary collaboration to solve global problems.

The Ames Lab is supported by the US Department of Energy’s Office of Science. The Office of Science is the largest supporter of basic research in the physical sciences in the United States and works to address some of the most pressing challenges of our time. For more information, please visit https://energy.gov/science.