Magnetization of the frustrated S = 1/2 chain compound LiCuVO4, focusing on high magnetic field phases, is reported. Besides a spin-flop transition and the transition from a planar spiral to a spin modulated structure observed recently, an additional transition was observed just below the saturation field. This newly observed magnetic phase is considered as a spin nematic phase, which was predicted theoretically but was not observed experimentally. The critical fields of this phase and its dM/dH curve are in good agreement with calculations performed in a microscopic model (M. E. Zhitomirsky and H. Tsunetsugu, preprint, arXiv:1003.4096v2).PACS numbers: 75.50. Ee, 75.10.Jm, 75.10.Pq Unconventional magnetic orders and phases in frustrated quantum spin chains are attractive issues, because they appear under a fine balance of the exchange interactions and are sometimes caused by much weaker interactions or fluctuations. 1-4

We report on magnetic resonance studies within the magnetically ordered phase of the quasi-onedimensional antiferromagnet LiCuVO 4 . Our studies reveal a spin reorientational transition at a magnetic field H c1 Ϸ 25 kOe applied within the crystallographic ab plane in addition to the recently observed one at H c2 Ϸ 75 kOe ͓M. G. Banks et al., J. Phys.: Condens. Matter 19, 145227 ͑2007͔͒. 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 ͑NMR͒ show that, within the magnetically ordered phase of LiCuVO 4 in the low-field range H Ͻ H c1 , 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 ͑2004͔͒. Based on NMR spectra simulations, the transition at H c1 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 H Ͼ H c2 Ϸ 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.

We have performed NMR experiments on the quasi one-dimensional frustrated spin-1/2 system LiCuVO4 in magnetic fields H applied along the c-axis up to field values near the saturation field Hsat. For the field range Hc2 < H < Hc3 (µ0Hc2 ≈ 7.5 T and µ0Hc3 = [40.5 ± 0.2] T) the 51 V NMR spectra at T = 380 mK exhibit a characteristic double-horn pattern, as expected for a spinmodulated phase in which the magnetic moments of Cu 2+ ions are aligned parallel to the applied field H and their magnitudes change sinusoidally along the magnetic chains. For higher fields, the 51 V NMR spectral shape changes from the double-horn pattern into a single Lorentzian line. For this Lorentzian line, the internal field at the 51 V nuclei stays constant for µ0H > 41.4 T, indicating that the majority of magnetic moments in LiCuVO4 are already saturated in this field range. This result is inconsistent with the previously observed linear field dependence of the magnetization M (H) for Hc3 < H < Hsat with µ0Hsat = 45 T [L. E. Svistov et al., JETP Letters 93, 21 (2011)]. We argue that the discrepancy is due to non-magnetic defects in the samples. The results of the spin-lattice relaxation rate of 7 Li nuclei indicate an energy gap which grows with field twice as fast as the Zeeman energy of a single spin, therefore, suggesting that the two-magnon bound state is the lowest energy excitation. The energy gap tends to close at µ0H ≈ 41 T. Our results suggest that the theoretically predicted spin-nematic phase, if it exists in LiCuVO4, can be established only within the narrow field range 40.5 < µ0H < 41.4 T .