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 $J_{nnn}/J_{nn} = -0.337$.