Scientists from Ames National Laboratory conducted a comprehensive investigation into the magnetism of TbMn6Sn6, a Kagome layered topological magnet. Surprisingly, they discovered that the magnetic spin reorientation in TbMn6Sn6 involves an increasing number of magnetically isotropic ions as the temperature rises.
Rob McQueeney, the project lead and a scientist at Ames Lab, explained that TbMn6Sn6 contains two different magnetic ions: terbium and manganese. The direction of the manganese moments determines the topological state, while the terbium moment determines the direction in which the manganese points. McQueeney elaborated, “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 material with layered structure, a magnetic phase transition occurs with increasing temperature. During this transition, the magnetic moments shift from being perpendicular to the Kagome layer (uniaxial) to pointing within the layer (planar), resulting in a phenomenon called spin reorientation.
McQueeney further explained that in Kagome metals, the spin direction influences the properties of topological or Dirac electrons. Dirac electrons arise when the magnetic bands touch at a single point. However, the presence of magnetic order creates a gap at the points where the bands touch, stabilizing 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 added.
As part of their investigation on TbMn6Sn6, the team conducted inelastic neutron scattering experiments at the Spallation Neutron Source to gain insights into how the magnetic interactions in the material drive the spin reorientation transition. McQueeney mentioned that the terbium prefers to be uniaxial at low temperatures, while the manganese prefers to be planar, resulting in a conflicting behavior.
According to McQueeney, the behavior at extremely low or high temperatures aligned with expectations. At low temperatures, the terbium exhibits a uniaxial state (with ellipsoid-shaped electronic orbitals), whereas at high temperatures, the terbium becomes magnetically isotropic (with a spherical orbital shape), enabling the planar manganese to dictate the overall direction of the moment.
The team initially presumed that each terbium orbital would gradually transition from ellipsoidal to spherical shape. However, they observed that both types of terbium coexist at intermediate temperatures, with the population of spherical terbium increasing as the temperature rises.
“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,” stated 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.”
The study detailing these findings has been published in the journal Nature Communications.
Source: Ames National Laboratory