Spin-modulated quasi-one-dimensional antiferromagnet LiCuVO4

We report on magnetic resonance studies within the magnetically ordered phase of the quasi-onedimensional antiferromagnet LiCuVO4. Our studies reveal a spin reorientational transition at a magnetic field Hc1 25 kOe applied within the crystallographic ab plane in addition to the recently observed one at Hc2 75 kOe M. G. Banks et al., J. Phys.: Condens. Matter 19, 145227 2007. Spectra of the antiferromagnetic resonance along low-frequency branches can be described in the framework of a macroscopic theory of exchange-rigid planar magnetic structures. These data allow us to obtain the parameter of the anisotropy of the exchange susceptibility together with a constant of the uniaxial anisotropy. Spectra of 7 Li nuclear magnetic resonance NMR show that, within the magnetically ordered phase of LiCuVO4 in the low-field range H Hc1, a planar spiral spin structure is realized with the spins lying in the ab plane, in agreement with neutron-scattering studies of Gibson et al. Physica B 350, 253 2004. Based on NMR spectra simulations, the transition at Hc1 can well be described as a spin-flop transition, where the spin plane of the magnetically ordered structure rotates to be perpendicular to the direction of the applied magnetic field. For HHc2 75 kOe, our NMR spectra simulations show that the magnetically ordered structure exhibits a modulation of the spin projections along the direction of the applied magnetic field H.