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Quadrupolar Fluctuations in a Spin-1 Magnet

October 11, 2021

Scientific Achievement

It is shown that normally invisible quadrupolar spin fluctuations dominate the excitation spectrum of ordered FeI2.

Hybridized quadrupolar and dipolar excitations in spin-1 FeI2. Left: The ground state of the ferromagnetic chain (bottom) has all spins in the Sz=+1 configuration. A single spin flip results in a spin with Sz=0 (top), an excitation with dipolar character. Two spin flips on the same site (middle) yield a bound excited state with quadrupolar character. Right: SU(3) theory shows the excitations are strongly hybridized and quantitatively explains the observed neutron scattering signal.

Significance and Impact

The work shows how the effect of spin-orbit coupling can hybridize dipolar and quadrupolar fluctuations, leading to unexpected quantum excitations despite the classical ground-state, a result that may be applicable to a range of other materials with hitherto unexplained multipolar excitations.

Research Details

  • Successful comparison with SU(3) generalized spin-wave theory explained the observed neutron scattering and allowed extraction of most details of the complex magnetic Hamiltonian of FeI2.
  • Anisotropic exchange interactions are identified as the mechanism behind the dipolar-quadrupolar hybridization, solving a 40+ year old puzzle.

Related Publication: Bai, X. et. al. (2021). Hybridized Quadrupolar Excitations in the Spin-anisotropic Frustrated Magnet FeI2. Nature Physics, 17, 467-472.