MSGID: 1:317/3 64acdb4b   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    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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   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   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/07/230710133051.htm   
      
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