NMR study of the high-field magnetic phase of LiCuVO$_4$

We report on NMR studies of the quasi--1D antiferromagnetic $S=1/2$ chain cuprate LiCuVO$_4$, focusing on the high--field spin--modulated phase observed recently in applied magnetic fields $H > H_{rm c2}$ ($mu_0H_{rm c2} approx 7.5$ T). The NMR spectra of $^7$Li and $^{51}$V around the transition from the ordered to the paramagnetic state were measured. It is shown that the spin--modulated magnetic structure forms with ferromagnetic interactions between spins of neighboring chains within the {bf ab}--plane at low temperatures 0.6 K $ < T < T_{rm N}$. The best fit provides evidence that the mutual orientation between spins of neighboring {bf ab}--planes is random. For elevated temperatures $T_{rm N} < T lesssim 15$ K, short--range magnetic order occurs at least on the characteristic time scale of the NMR experiment.