Field Dependence of Magnetic Ordering in Kagome-staircase Compound Ni3V2O8

We present powder and single-crystal neutron diffraction measurements of the Kagome-staircase compound Ni3V2O8 in fields up to 8. 5 T applied along the buckled plane direction. Our zero-field measurements show that at TPH=9.1 K NVO undergoes a transition to an incommensurate order which is dominated by a longitudinally- modulated structure. Upon further cooling a transition is induced at THL=6.3K to an incommensurate spiral structure with moments in the Kagome plane. At TLC=4K the order becomes commensurate in a first-order phase transition, with the ordered moment pointing along the a-axis. A magnetic field along the buckled plane direction reduces the amount of incommensurate order by inducing antiferromagnetic order on nonspine Ni sites pointing perpendicular to the Kagome plane. The temperature-field phase diagram is mostly consistent with that obtained from specific heat and magnetization measurements, with the exception of a transition at 2.3K which was not seen with neutron diffraction. Our neutron measurements can be described with space group Cmca for T15K.