A. Zheludev

Spin-Vacancy-Induced Long-Range Order in a New Haldane-Gap Antiferromagnet

Magnetic susceptibility, high-field magnetization, and inelastic neutron scattering experiments are used to study the magnetic properties of a new S=1 quasi-one-dimensional antiferromagnet PbNi{sub 2}V {sub 2}O{sub 8} . Interchain interactions are shown to be almost, but not quite, strong enough to destroy the nonmagnetic singlet ground state and the energy gap in the magnetic excitation spectrum. Substituting nonmagnetic Mg{sup 2+} (S=0 ) ions for Ni{sup 2+} (S=1 ) induces a magnetically ordered state at low temperatures. To our knowledge this is the first observation of doping-induced long-range order in a Haldane-gap system. {copyright} {ital 1999} {ital The American Physical Society }

Competition between helimagnetism and commensurate quantum spin correlations in LiCu2O2

Neutron diffraction and bulk measurements are used to determine the nature of the low-temperature ordered state in LiCu2O2, a S=1/2 spin-chain compound with competing interactions. The spin structure is found to be helimagnetic, with a propagation vector (0.5,zeta,0), zeta=0.174. The nearest-neighbor exchange constant and frustration ratio are estimated to be J(1)=5.8 meV and J(2)/J(1)=0.29, respectively. For idealized quantum spin chains, these parameter values would signify a gapped spin-liquid ground state with commensurate spin correlations. The observed temperature dependence of the magnetic propagation vector in LiCu2O2 is attributed to a competition between incommensurate helimagnetism in the classical spin model and commensurability in the quantum case. It is also proposed that long-range ordering in LiCu2O2 is facilitated by intrinsic nonstoichiometry.

Spin Waves and Magnetic Interactions in LiCu2O2

The quasi-one-dimensional helimagnet LiCu{sub 2}O{sub 2} was studied by single crystal inelastic neutron scattering. The dispersion relation of spin wave excitations was measured in the vicinity of the principal magnetic Bragg reflection. A spin wave theoretical analysis of the data yields an estimate of the relevant exchange constants and explains the mechanism of geometric frustration that leads to helimagnetism. It is found that the simple antiferromagnetic J{sub 1}-J{sub 2} model that was previously proposed is inadequate for LiCu{sub 2}O{sub 2}. The experimental findings are generally in a qualitative agreement with first principles calculations of [A. A. Gippius, E. N. Morozova, A. S. Moskvin, A. V. Zalessky, A. A. Bush, M. Baenitz, H. Rosner, and S.-L. Drechsler, Phys. Rev. B 70, 020406 (2004)], though certain important discrepancies remain to be explained.

Experimental spin density in the first purely organic ferromagnet : the β para-nitrophenyl nitronyl nitroxide

Abstract A purely organic ferromagnet, the β form of the para-nitrophenyl nitronyl nitroxide (p-NPNN), was investigated by conventional and polarized neutron diffraction. Several approaches were used to reconstruct the spin density distribution in the radical. Quantitative results in the form of atomic spin populations were obtained, and are discussed in comparison with those previously obtained experimentally for a related phenyl-substituted nitronyl nitroxide. The spin delocalization and/or spin polarization effects are responsible for the transfer of the spin density to the nitrophenyl group. In the framework of the McConnell mechanism these effects play a key role in the intermolecular ferromagnetic interaction, which passes through both the nitro and the phenyl fragments. Hartree-Fock-based ab initio theoretical spin densities found in the literature are inconsistent with the experimental results.

Dynamics of composite haldane spin chains in IPA-CuCl3

Magnetic excitations in the quasi-one-dimensional antiferromagnet IPA-CuCl3 are studied by cold neutron inelastic scattering. Strongly dispersive gap excitations are observed. Contrary to previously proposed models, the system is best described as an asymmetric quantum spin ladder. The observed spectrum is interpreted in terms of composite Haldane spin chains. The key difference from actual S=1 chains is a sharp cutoff of the single-magnon spectrum at a certain critical wave vector.

Temperature-dependent spin gap and singlet ground state in BaCuSi{sub 2}O{sub 6}

Bulk magnetic measurements and inelastic neutron scattering were used to investigate the magnetic properties of BaCuSi{sub 2}O{sub 6}, a quasi-two-dimensional antiferromagnet with a square-lattice bilayer structure. The compound was shown to have a spin-singlet dimerized ground state with a gap {Delta}{approx}4.5 meV in the magnetic excitation spectrum. The dispersion of the gap modes was measured and found to be temperature dependent and totally suppressed at temperatures higher than the characteristic gap energy. The observed excitations are analogous to magnetoexcitons in light rare-earth compounds, but are an intrinsic property of a simple Heisenberg Hamiltonian for the S=1/2 magnetic bilayer. The behavior is discussed in comparison to that of related systems, in particular, Cs{sub 3}Cr{sub 2}Br{sub 9}. {copyright} {ital 1997} {ital The American Physical Society}

Uniaxial stress dependence of the [ζ ζ 0]-TA2 anomalous phonon branch in Ni2MnGa

Abstract Results of neutron scattering experiments on single crystals of Ni2MnGa as a function of uniaxial stress and temperature are reported. Emphasis is placed on the behavior of the low-energy part of the [ζ ζ 0] TA2 phonon branch and the associated premartensitic phase transformation. The dip in the phonon dispersion curve is stress-dependent and shifts from ζ = 0.33 for zero stress to ζ = 0.36 for an applied stress of 95 MPa. The energy of the soft phonon decreases with increasing stress. The premartensitic transition temperature is increased from 265 K in the unstrained sample to ≈ 300 K at 95 MPa.

FIELD-INDUCED COMMENSURATE-INCOMMENSURATE PHASE TRANSITION IN A DZYALOSHINSKII-MORIYA SPIRAL ANTIFERROMAGNET

We report an observation of a commensurate-incommensurate phase transition in a Dzyaloshinskii-Moriya spiral magnet Ba_2CuGe_2O_7. The transition is induced by applying a magnetic field in the plane of spin rotation. In this experiment we have direct control over the strength of the commensurate potential, while the preferred incommensurate period of the spin system remains unchanged. Experimental results for the period of the soliton lattice and bulk magnetization as a function of external magnetic field are in quantitative agreement with theory.

Magnetic anisotropy and low-energy spin waves in the Dzyaloshinskii-Moriya spiral magnet Ba{sub 2}CuGe{sub 2}O{sub 7}

Neutron-diffraction and inelastic-scattering experiments are used to investigate in detail the field dependence of the magnetic structure and low-energy spin-wave spectrum of the Dzyaloshinskii-Moriya helimagnet Ba{sub 2}CuGe{sub 2}O{sub 7}. The results suggest that the previously proposed model for the magnetism of this compound (an ideal sinusoidal spin spiral, stabilized by isotropic exchange and Dzyaloshinskii-Moriya interactions) needs to be refined. Both recent and previously published data can be quantitatively explained by taking into account the Kaplan-Shekhtman-Entin-Wohlman-Aharony term, a special magnetic anisotropy term that was predicted to always accompany Dzyaloshinskii-Moriya interactions in insulators. {copyright} {ital 1999} {ital The American Physical Society}

COEXISTENCE OF HALDANE-GAP EXCITATIONS AND LONG-RANGE ANTIFERROMAGNETIC ORDER IN MIXED-SPIN NICKELATES R2BANIO5

The spin dynamics of the S=1 Ni-chains in mixed-spin antiferromagnets Pr_2 Ba Ni O_5 and Nd_x Y_2-x Ba Ni O_5 is described in terms of a simple Ginzburg-Landau Lagrangian coupled to the sublattice of rare-earth ions. Within this framework we obtain a theoretical explanation for the experimentally observed coexistence of Haldane gap excitations and long-range magnetic order, as well as for the increase of the Haldane gap energy below the Neel point. We also predict that the degeneracy of the Haldane triplet is lifted in the magnetically ordered phase. The theoretical results are consistent with the available experimental data.