Structure and magnetism in the bond-frustrated spinel ZnC r2S e4

The crystal and magnetic structures of stoichiometric ZnC r₂S e₄ have been investigated using synchrotron x-ray and neutron powder diffraction, muon spin relaxation (μ SR ), and inelastic neutron scattering. Synchrotron x-ray diffraction shows a spin-lattice distortion from the cubic F d 3 ¯m spinel to a tetragonal I 4₁/a m d lattice below T_N=21 K , where powder neutron diffraction confirms the formation of a helical magnetic structure with magnetic moment of 3.04 (3 ) μ_B at 1.5 K, close to that expected for high-spin C r^{3 +} . μ SR measurements show prominent local spin correlations that are established at temperatures considerably higher (<100 K ) than the onset of long-range magnetic order. The stretched exponential nature of the relaxation in the local spin-correlation regime suggests a wide distribution of depolarizing fields. Below T_N, unusually fast (>100 μ s^-1) muon relaxation rates are suggestive of rapid site hopping of the muons in static field. Inelastic neutron scattering measurements show a gapless mode at an incommensurate propagation vector of k =[0 0 0.4648 (2 )] in the low-temperature magnetic ordered phase that extends to 0.8 meV. The dispersion is modeled by a two-parameter Hamiltonian, containing ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions with a J_{n n n}/J_{n n}=-0.337 .