Structure and magnetism in the bond-frustrated spinel \(ZnCr_2Se_4\)

The crystal and magnetic structures of stoichiometric \(ZnCr_2Se_4\) have been investigated using synchrotron x-ray and neutron powder diffraction, muon spin relaxation (\(μSR\)), and inelastic neutron scattering. Synchrotron x-ray diffraction shows a spin-lattice distortion from the cubic \(Fd\bar3m\) spinel to a tetragonal \(I4_1/amd\) 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 \(Cr^{3+}\). \(μSR\) 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 (\(>100 μs^{-1}\)) 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 = [000.4648(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 \(J_{nnn}/J_{nn} = -0.337\).