51V NMR study of the kagome staircase compound Ni3V2O8

We used V-51 NMR to study magnetic ordering in the Ni3V2O8 single crystal with a Kagome staircase structure of Ni atoms. The NMR spectra were measured in the temperature range T= (3-300) K and magnetic fields H= (2-9.4) T directed along the main a, b, c axes of the orthorhombic (Cmca) crystal. The local magnetic field at the V-51 NMR probe determines position and the shape of the corresponding NMR line. These parameters yield an unique information, respectively, on the uniform and the staggered spin components of the ordered Ni. The NMR data collected at H >= 2 T are considered in line with predictions of the representation theory [A. Harris, Phys. Rev. B 76, 054447 (2007)] with a result that incommensurate amplitude-modulated structure of the spine Ni-s spins acquires in the high-temperature incommensurate (HTI) phase two prominent nearly equal spin components S-a approximate to S-c >> S-b instead of the longitudinal incommensurate spin-density wave (SDW) order with S-a >> S-c, S-b as it was deduced from neutron-diffraction data [M. Kenzelmann et al., Phys. Rev. B 74, 014429 (2006)]. No noticeable variation of SDW polarization in the ab plane was detected below the HTI-low-temperature incommensurate (LTI) transition. In both the HTI and LTI phases two almost equal spin components of the Ni-s spins S-a approximate to S-c >> S-b exist at H < 4.7 T. Their phasing is still not determined. The bulk magnetization in these phases is explained by contribution of the cross-tie Ni-c spins which antiferromagnetic structure in the LTI phase is canted along H.