Structure and magnetism in the bond-frustrated spinel ZnCr2Se4

The crystal and magnetic structures of stoichiometric ZnCr2Se4 have been investigated using synchrotron x-ray and neutron powder diffraction, muon spin relaxation (mu SR), and inelastic neutron scattering. Synchrotron x-ray diffraction shows a spin-lattice distortion from the cubic Fd (3) over barm 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) mu(B) at 1.5 K, close to that expected for high-spin Cr3+. mu 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 mu 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 = [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.