A team from Ames National Laboratory conducted an in-depth investigation of the magnetism of TbMn6Sn6, a Kagome layered topological magnet. They were surprised to find that the magnetic spin reorientation of TbMn6Sn6 occurs by creating more and more magnetically isotropic ions as the temperature increases.
Rob McQueeney, a scientist at Ames Lab and project leader, explained that TbMn6Sn6There are two different magnetic ions in the material, terbium and manganese. The direction of the manganese moments controls the topological state, “But it’s the terbium moment that determines the direction the manganese is oriented,” he said. “The idea is, you have two magnetic species and it’s the combination of their interactions that controls the direction of the current.”
In this layered material, there is a magnetic phase transition that occurs as the temperature increases. During this transition phase, the magnetic moments move from pointing perpendicular to the Kagome layer, or uniaxial, to pointing within the layer, or planar. This transition is called spin reorientation.
McQueeney explained that in Kagome metals, the direction of spin controls the topological or Dirac properties of the electrons. Dirac electrons occur where the magnetic bands touch at a point. However, the magnetic alignment causes a gap at the points where the bands touch. This gapping stabilizes the topological Chern insulator state. “So you can go from a Dirac semimetal to a Chern insulator just by changing the direction of the current,” he said.
As part of their TbMn6Sn6 investigation, the team conducted inelastic neutron scattering experiments at the Spallation Neutron Source to understand how the magnetic interaction of the material drives the spin reorientation transition. McQueeney said that terbium tends to be uniaxial at low temperatures, while manganese is planar, so they are at odds.
According to McQueeney, the 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 (having a spherical orbital shape), which allows planar Mn to determine the general direction of the moment.
The team hypothesized that each terbium orbital would gradually deform from ellipsoidal to spherical. Instead, they found two types of terbium present at intermediate temperatures, but the population of spherical terbium increases as the temperature increases.
“So, what we did is we determined how the magnetic excitations evolve from this uniaxial state to this easy plane state as a function of temperature. And the long-standing guess how it’s going to be right,” said McQueeney.
“But the nuance is that you can’t treat every terbium the same at some point. Every terbium site can exist in two quantum states, uniaxial or isotropic, and when I look at a site, it’s in one state or the other in an instant. The possibility that it is uniaxial or isotropic depends on the temperature. We call it an orbital binary quantum alloy.”
The study was published in the journal Communication in Nature.
SXM Riberolles et al, Orbital nature of the spin-reorientation transition in TbMn6Sn6, Communication in Nature (2023). DOI: 10.1038/s41467-023-38174-5
Provided by Ames National Laboratory
Citation: A surprising discovery about magnetic interactions in a Kagome layered topological magnet (2023, July 10) retrieved 10 July 2023 from https://phys.org/news/2023-07-discovery-magnetic -interactions-kagome-layered.html
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