Researchers make a surprising discovery about the magnetic interactions
in a Kagome layered topological magnet

Date:
July 10, 2023
Source:
DOE/Ames National Laboratory
Summary:
A team 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 in TbMn6Sn6 occurs
by generating increasing numbers of magnetically isotropic ions
as the temperature increases.


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FULL STORY ==========================================================================
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 in TbMn6Sn6
occurs by generating increasing numbers of magnetically isotropic ions
as the temperature increases.

Rob McQueeney, a scientist at Ames Lab and project lead, explained
that TbMn6Sn6has 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 that the
manganese points," he said. "The idea is, you have these two magnetic
species and it is the combination of their interactions which 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 within the layer, or
planar. This transition is called a spin reorientation.

McQueeney explained that in Kagome metals, the spin direction controls
the properties of topological or Dirac electrons. Dirac electrons occur
where the magnetic bands touch at one point. However, magnetic order
causes gapping at the points where the bands are touching. This gapping stabilizes the topological Chern insulator state. "So you can go from
a Dirac semimetal to a Chern insulator just by turning the direction of
the moment," he said.

As part of their TbMn6Sn6 investigation, the team performed inelastic
neutron scattering experiments at the Spallation Neutron Source to
understand how the magnetic interactions in the material drive the spin reorientation transition.

McQueeney said that the terbium wants to be uniaxial at low temperatures,
while the 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, the terbium is uniaxial (with
electronic orbitals shaped like an ellipsoid). At high temperatures,
the terbium is magnetically isotropic (with a spherical orbital shape),
which allows the planar Mn to determine the overall moment direction. The
team assumed that each terbium orbital would gradually deform from
ellipsoidal to spherical. Instead, they found both types of terbium
exist at intermediate temperatures, however the population of spherical
terbium increases as the temperature increases.

"So, what we did was we determined how the magnetic excitations evolve
from this uniaxial state into this easy plane state as a function of temperature.

And the long-standing assumption of how it happens is correct," said
McQueeney.

"But the nuance is that you can't treat every terbium as being exactly
the same on some timescale. 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 some instant time. The probability that it's
uniaxial or isotropic depends on temperature." We call this an orbital
binary quantum alloy.

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Story Source: Materials provided by DOE/Ames_National_Laboratory. Note:
Content may be edited for style and length.


========================================================================== Journal Reference:
1. S. X. M. Riberolles, Tyler J. Slade, R. L. Dally, P. M. Sarte,
Bing Li,
Tianxiong Han, H. Lane, C. Stock, H. Bhandari, N. J. Ghimire, D. L.

Abernathy, P. C. Canfield, J. W. Lynn, B. G. Ueland,
R. J. McQueeney.

Orbital character of the spin-reorientation transition in TbMn6Sn6.

Nature Communications, 2023; 14 (1) DOI: 10.1038/s41467-023-38174-5 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2023/07/230710133051.htm

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