Competing phases in a frustrated magnet: a study with neutron scattering.

Zero-field magnetic neutron diffraction on the pyrochlore magnet ZnCr$_{2}$S$_{4}$ reveals successive phase transitions from a paramagnet into a helical (T$_{N1}$ = 13 K) then collinear (T$_{N2}$ = 8 K) spin ordered state as the system is cooled. This occurs as the system crosses a critical line in J$_{1}$-J$_{2}$ space, where J$_{1}$ and J$_{2}$ are the first and second nearest neighbor exchange constants respectively. We report the magnetic field dependence of the dc susceptibility and neutron scattering of ZnCr$_{2}$S$_{4}$ in the form of a temperature-field phase diagram. We also study the low energy excitation spectrum of ZnCr$_{2}$S$_{4}$ which was found to be consistent with the J$_{1}$-J$_{2}$ model.