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

Zero-field magnetic neutron diffraction on the pyrochlore magnet ZnCr₂S₄ 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 J₁-J₂ space, where J₁ and J₂ are the first and second nearest neighbor exchange constants respectively. We report the magnetic field dependence of the dc susceptibility and neutron scattering of ZnCr₂S₄ in the form of a temperature-field phase diagram. We also study the low energy excitation spectrum of ZnCr₂S₄ which was found to be consistent with the J₁-J₂ model.