A. V. Prokofiev

Anisotropic exchange inLiCuVO4probed by ESR

We investigated the paramagnetic resonance in single crystals of LiCuVO

Switching the ferroelectric polarization in theS=1∕2chain cuprateLiCuVO4by external magnetic fields

_4

Spin-modulated quasi-one-dimensional antiferromagnet LiCuVO4

with special attention to the angular variation of the absorption spectrum. To explain the large resonance linewidth of the order of 1 kOe, we analyzed the anisotropic exchange interaction in the chains of edge-sharing CuO

Synchrotron-radiation study of the two-leg spin-ladder (VO)2P2O7 at 120 K

_6

Polarized Raman and infrared vibrational analysis of (VO)2P2O7 single crystals

octahedra, taking into account the ring-exchange geometry of the nearest-neighbor coupling via two symmetric rectangular Cu-O bonds. The exchange parameters, which can be estimated from theoretical considerations, nicely agree with the parameters obtained from the angular dependence of the linewidth. The anisotropy of this magnetic ring exchange is found to be much larger than it is usually expected from conventional estimations which neglect the bonding geometry. Hence, the data yield the evidence that in copper oxides with edge-sharing structures the role of the orbital degrees of freedom is strongly enhanced. These findings establish LiCuVO

Magnetic properties of the low dimensional spin system (VO)\(\)P\(\)O\(\):ESR and susceptibility

_4

Oxygen stoichiometry and the problem of the growth of (VO)2P2O7 single crystals

as one-dimensional compound at high temperatures. PACS: 76.30.-v, 76.30.Fc, 75.30.Et

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

We present a detailed study of complex dielectric constant and ferroelectric polarization in multiferroic LiCuVO4 as function of temperature and external magnetic field. In zero external magnetic field, spiral spin order with an ab helix and a propagation vector along the crystallographic b direction is established, which induces ferroelectric order with spontaneous polarization parallel to a. The direction of the helix can be reoriented by an external magnetic field and allows switching of the spontaneous polarization. We find a strong dependence of the absolute value of the polarization for different orientations of the spiral plane. Above 7.5 T, LiCuVO4 reveals collinear spin order and remains paraelectric for all field directions. Thus this system is ideally suited to check the symmetry relations for spiral magnets as predicted theoretically. The strong coupling of ferroelectric and magnetic order is documented and the complex (B,T) phase diagram is fully explored.

Polar phonons in the antiferromagnetic S = 1 2 spin-chain system CuSb 2 O 6

We report on magnetic resonance studies within the magnetically ordered phase of the quasi-onedimensional antiferromagnet LiCuVO4. Our studies reveal a spin reorientational transition at a magnetic field Hc1 25 kOe applied within the crystallographic ab plane in addition to the recently observed one at Hc2 75 kOe M. G. Banks et al., J. Phys.: Condens. Matter 19, 145227 2007. 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 NMR show that, within the magnetically ordered phase of LiCuVO4 in the low-field range H Hc1, 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 2004. Based on NMR spectra simulations, the transition at Hc1 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 HHc2 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.

Incommensurate antiferromagnetic order in the quantum chain compound LiCuVO4

The crystal structure of the ambient-pressure phase of vanadyl pyrophosphate, (VO)2P2O7, has been precisely determined at 120 K from synchrotron X-ray diffraction data measured on a high-quality single crystal. The structure refinement unambiguously establishes the orthorhombic space group Pca2(1) as the true crystallographic symmetry. Moreover, it improves the accuracy of previously published atomic coordinates by one order of magnitude, and provides reliable anisotropic displacement parameters for all atoms. Along the a axis, the structure consists of infinite two-leg ladders of vanadyl cations, (VO)2+, which are separated by pyrophosphate anions, (P2O7)4-. Parallel to the c axis, the unit cell comprises two alternating crystallographically inequivalent chains of edge-sharing VO5 square pyramids bridged by PO4 double tetrahedra. No structural phase transition has been observed in the temperature range between 300 and 120 K.