High-field NMR of the quasi-one-dimensional antiferromagnet LiCuVO4

We report on NMR studies of the quasi-one-dimensional (1D) frustrated S = 1/2 chain cuprate LiCuVO4 in magnetic fields H up to mu H-0 = 30 T (approximate to 70% of the saturation field H-sat). NMR spectra in fields higher than H-c2 (mu H-0(c2) approximate to 7.5 T) and temperatures T < T-N can be described within the model of a spin-modulated phase in which the magnetic moments are aligned parallel to the applied field H and their values alternate sinusoidally along the magnetic chains. Based on theoretical concepts about frustrated 1D S = 1/2 chains, the field dependence of the modulation strength of the magnetic structure is deduced from our experiments. Relaxation time T-2 measurements of the V-51 nuclei show that T-2 depends on the particular position of the probing V-51 nucleus with respect to the magnetic copper moments within the 1D chains: the largest T-2 value is observed for the vanadium nuclei which are right next to the magnetic Cu2+ ion with the largest ordered magnetic moment. This observation is in agreement with the expectation for the spin-modulated magnetic structure. The (H, T) magnetic phase diagram of LiCuVO4 is discussed.