Orbital Fluctuations Control the Magnetism of a Topological Metal

Tb ions in a triangular layer of TbMn6Sn6
Schematic picture of Tb ions in a triangular layer of TbMn6Sn6 in the uniaxial ferrimagnetic phase. The Tb can exist in two orbital states with either uniaxial (purple) or isotropic (green) magnetic anisotropy.  Collective excitations observed with INS resemble a disordered binary orbital alloy at intermediate temperatures.
Scientific Achievement

Initiation of a spin-reorientation in TbMn6Sn6 by Tb orbital fluctuations revealed by inelastic neutron scattering (INS) experiments.

Significance and Impact

Temperature-dependent occupancy of uniaxial and isotropic orbital states appear quasistatic, resembling a binary orbital alloy.  Their population controls the stability of uniaxial and easy-plane ferrimagnetic order, affecting topological properties that depend on the magnetization direction. 

Research Details
  • TbMn6Sn6 consists of ferromagnetic Mn kagome layers and Tb triangular layers with opposite magnetization, forming a ferrimagnet.
  • INS measurements were performed at the ARCS Spectrometer at the SNS.
  • High and low-energy Tb spin modes represent excitations out of uniaxial and isotropic orbital states. These modes coexist at intermediate temperatures.
  • Random-phase approximation calculations using two-state orbital model best describe the INS data.
  • This can be added to systems that are understood as quantum binary alloys, such as mixed valent metals, low/high-spin magnets, and Ising magnets.

S. X. M. Riberolles, S. X. M. Riberolles, Tyler J. Slade, R. L. Dally, P. M. Sarte, Bing, Tianxiong Han, H. Lane, C. Stock, H. Bhandari, N. J. Ghimire, D. L. Abernathy, P. C. Canfield, J. W. Lynn, B. G. Ueland and R. J. McQueeney, Orbital character of the spin reorientation transition in TbMn6Sn6. Nat. Comm. 14, 2658 (2023) . https://doi.org/10.1038/s41467-023-38174-5