Researchers make a surprising discovery of magnetic interactions in Kagome-class topological magnets

Newswise — A team from Ames National Laboratory conducted an in-depth investigation of TbMn’s magnetism₆snail₆, a Kagome class topology magnet. They were surprised to find that the magnetic spin reorientation in TbMn₆snail₆ occurs by generating increasing numbers of magnetic isotropic ions as the temperature increases.

Rob McQueeney, a scientist at Ames Laboratories and project leader, explains that TbMn₆snail₆There are two different magnetic ions in the material, terbium and manganese. The direction of the manganese moments controls the topological state, “But it is the terbic moment that determines the direction in which the manganese points are,” he said. «The idea is, you have these two types of magnetism, and it’s the combination of their interactions that drives the direction of the moment.»

In this layered material, there is a magnetic phase transition that occurs with increasing temperature. During this phase transition, the magnetic moments shift from the direction perpendicular to the Kagome layer, or uniaxial, to the direction inside the layer or plane. This transition is called spin reorientation.

McQueeney explains that in Kagome metals, the direction of spin controls the properties of topological or Dirac electrons. Dirac electrons occur when the magnetic bands touch each other at a point. However, the magnetic ordering causes a gap at the points where the bands come into contact with each other. This gap helps to stabilize the topological Chern insulator state. “So you can go from semi-Dirac metal to Chern insulator just by turning the direction of the moment,” he said.

Be part of their TbMn₆snail₆ To investigate, the team performed inelastic neutron scattering experiments at the Colliding Neutron Source to understand how magnetic interactions in the material drive spin-redirection transitions. McQueeney says that terbium wants to be uniaxial at low temperatures, while manganese is planar, so they are opposites.

According to McQueeney, behavior at very low or very high temperatures is as expected. At low temperatures, terbium is uniaxial (with elliptical electron orbitals). At high temperatures, terbium is magnetically isotropic (having a spherical orbital shape), allowing flat Mn to determine the overall direction of the moment. The team assumed that each terbium orbital would gradually deform from elliptical to spherical. Instead, they found that both types of terbium exist at intermediate temperatures, but that the spherical terbium density increases with increasing temperature.

“So what we did was determine how the magnetic excitations evolve from this uniaxial state to this easily planar state as a function of temperature. And the longstanding assumption about how it happened is correct,” McQueeney said. “But the problem is that you can’t consider every terbi to be exactly the same over a period of time. Each terbium site can exist in two quantum states, uniaxial or isotropic, and if I look at one, it will be in one or the other at a given time. The probability that it is axial or isotropic depends on the temperature.” We call this an orbital binary quantum alloy.

This study is further discussed in “Orbital characterization of spin orientation transitions in TbMn .”.₆snail₆,” 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 nature communication.

Ames National Laboratory is a National Science Laboratory in the United States Office of Energy operated by Iowa State University. Ames Labs creates innovative materials, technologies and energy solutions. We use our expertise, unique capabilities and interdisciplinary collaboration to solve global problems.

The Ames Laboratory 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 is working to address some of the most pressing challenges of our time. For more information, please visit https://energy.gov/science.