Diffusive excitonic bands from frustrated triangular sublattice in a singlet-ground-state system

Abstract

Magnetic order in most materials occurs when magnetic ions with finite moments arrange in a particular pattern below the ordering temperature. Intriguingly, if the crystal electric field (CEF) effect results in a spin-singlet ground state, a magnetic order can still occur due to the exchange interactions between neighboring ions admixing the excited CEF levels. The magnetic excitations in such a state are spin excitons generally dispersionless in reciprocal space. Here we use neutron scattering to study stoichiometric Ni2Mo3O8, where Ni2+ ions form a bipartite honeycomb lattice comprised of two triangular lattices, with ions subject to the tetrahedral and octahedral crystalline environment, respectively. We find that in both types of ions, the CEF excitations have nonmagnetic singlet ground states, yet the material has magnetic order. Furthermore, CEF spin excitons from the tetrahedral sites form a dispersive diffusive pattern around the Brillouin zone boundary, likely due to spin entanglement and geometric frustrations.

Direct observation of topological magnon polarons in a multiferroic material

“…1 b), we consider these two magnon modes to be doubly degenerate in the collinear antiferromagnetic state, as also found in its sister compound Co 2 Mo 3 O 8 39 . We note that the energy scales of the excitation spectra in the other isostructural compound Ni 2 Mo 3 O 8 38 are rather different from those in Fe 2 Mo 3 O 8 (Fig. 1 e) and Co 2 Mo 3 O 8 39 .…”

“…Above T N , the peaks disappear. This suggests that the two modes correspond to the spin-wave excitations originating from long-range magnetic order, rather than the crystal-electric-field excitations 38 . Given that there are four sublattices of Fe 2+ ions in the magnetic unit cell of Fe 2 Mo 3 O 8 (Fig.…”

“…1 e) and Co 2 Mo 3 O 8 39 . In Ni 2 Mo 3 O 8 , the ground state of the crystal-electric-field levels of Ni 2+ ions is a nonmagnetic singlet, while the interplay between crystal-electric-field effect and magnetic exchange interactions gives rise to a magnetic order with a relatively low transition temperature of T N = 5.5 K. As a consequence, spin waves are observed below 1.5 meV, and higher-energy excitations are believed to arise from the crystal-electric-field spin excitons with a robust temperature dependence 38 .…”

Magnetization reversal through an antiferromagnetic state

“…Recently, the honeycomb antiferromagnets A 2 Mo 3 O 8 ( A =Mn,Fe,Co,Ni,Zn) have emerged as a versatile material class with a hexagonal structure in the polar space group P 6 3 m c 7 – 17 . These compounds exhibit optical magnetoelectric effects such as directional dichroism 18 , 19 and giant thermal Hall effects 20 , 21 , and the control of magnetization and polarization can be achieved in magnetic fields of a few Tesla 22 , 23 or even by ultrafast modulation via laser pulses 24 .…”