  We report on magnetic resonance studies within the magnetically ordered phase
of the quasi-1D antiferromagnet LiCuVO_4. Our studies reveal a spin
reorientational transition at a magnetic field H_c1 ~ 25 kOe applied within the
crystallographical (ab)-plane in addition to the recently observed one at H_c2
\~75 kOe [ M.G. Banks et al., cond-mat/0608554 (2006)]. Spectra of the
antiferromagnetic resonance (AFMR) along low-frequency branches can be
described in the frame of a macroscopic theory of exchange-rigid planar
magnetic structures. These data allow to obtain the anisotropy of the exchange
interaction together with a constant of the uniaxial anisotropy. Spectra of 7Li
nuclear magnetic resonance (NMR) show that, within the magnetically ordered
phase of LiCuVO_4 in the low-field range H < H_c1, a planar spiral spin
structure is realized with the spins lying in the (ab)-plane in agreement with
neutron scattering studies of B.J. Gibson et al. [Physica B Vol. 350, 253
(2004)]. 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 > H_c2 ~ 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.
