High-magnetic-field NMR studies of LiVGe₂O₆: A quasi-one-dimensional spin S=1 system

We report ⁷Li pulsed NMR measurements in polycrystalline and single-crystal samples of the quasi-one-dimensional S=1 antiferromagnet LiVGe₂O₆, whose AF transition temperature is T_N≃24.5 K. The field (B₀) and temperature (T) ranges covered were 9–44.5 T and 1.7–300 K, respectively. The measurements included NMR spectra, the spin-lattice relaxation rate (T₁^-1), and the spin-phase relaxation rate (T₂^-1), often as a function of the orientation of the field relative to the crystal axes. The spectra indicate an AF magnetic structure consistent with that obtained from neutron diffraction measurements, but with the moments aligned parallel to the c axis. The spectra also provide the T-dependence of the AF order parameter and show that the transition is either second-order or weakly first-order. Both the spectra and the T₁^-1 data show that B₀ has at most a small effect on the alignment of the AF moment. There is no spin-flop transition up to 44.5 T. These features indicate a very large magnetic anisotropy energy in LiVGe₂O₆, with orbital degrees of freedom playing an important role. Below 8 K, T₁^-1 varies substantially with the orientation of B₀ in the plane perpendicular to the c axis, suggesting a small energy gap for magnetic fluctuations that is very anisotropic.