Incommensurate magnetic ordering in Cu2Te2O5X2 (X=Cl, Br) studied by single crystal neutron diffraction

Polarized and unpolarized neutron diffraction studies have been carried out on single crystals of the coupled spin tetrahedra systems Cu2Te2O5X2 (X=Cl, Br). A model of the magnetic structure associated with the propagation vectors k'Cl ~ -0.150,0.422,1/2 and k'Br ~ -0.172,0.356,1/2 and stable below TN=18 K for X=Cl and TN=11 K for X=Br is proposed. A feature of the model, common to both the bromide and chloride, is a canted coplanar motif for the 4 Cu2+ spins on each tetrahedron which rotates on a helix from cell to cell following the propagation vector. The Cu2+magnetic moment determined for X=Br, 0.395(5)muB, is significantly less than for X=Cl, 0.88(1)muB at 2K. The magnetic structure of the chloride associated with the wave-vector k' differs from that determined previously for the wave vector k~0.150,0.422,1/2 [O. Zaharko et.al. Phys. Rev. Lett. 93, 217206 (2004)]

Thermal effects versus spin nematicity in a frustrated spin-1/2 chain

The spin-nematic phase is an intriguing state of matter that lacks usual long-range dipolar order, yet it exhibits higher multipolar order. This makes its detection extremely difficult and controversial. Recently, nuclear magnetic resonance (NMR) has been proposed as one of the most suitable techniques to confirm its existence. We report a $^{17}$O NMR observation of the reduction of the local magnetization in the polarized state of the frustrated spin-1/2 chain $\beta$-TeVO$_4$, which was previously proposed to be a fingerprint of the spin-nematic behavior. However, our detailed study shows that the detected missing fraction of the magnetization, probed by NMR frequency shift, is thermally activated, thus undermining the presence of the spin-nematic phase in the investigated compound. This highlights the importance of careful considerations of temperature-dependent NMR shift that has been overlooked in previous studies of spin nematicity.Comment: accepted for publication in PRB(R), with supplementar

Multiferroic FeTe2_2O5_5Br: Alternating spin chains with frustrated interchain interactions

A combination of density functional theory calculations, many-body model considerations, magnetization and electron spin resonance measurements shows that the multiferroic FeTe$_2$O$_5$Br should be described as a system of alternating antiferromagnetic $S=5/2$ chains with strong Fe-O-Te-O-Fe bridges weakly coupled by two-dimensional frustrated interactions, rather than the previously reported tetramer models. The peculiar temperature dependence of the incommensurate magnetic vector can be explained in terms of interchain exchange striction being responsible for the emergent net electric polarization.Comment: 7 pages, 6 figure

Magnetoelectric effect due to local noncentrosymmetry

Magnetoelectrics often possess ions located in noncentrosymmetric surroundings. Based on this fact we suggest a microscopic model of magnetoelectric interaction and show that the spin-orbit coupling leads to spin-dependent electric dipole moments of the electron orbitals of these ions, which results in non-vanishing polarization for certain spin configurations. The approach accounts for the macroscopic symmetry of the unit cell and is valid both for commensurate and complex incommensurate magnetic structures. The model is illustrated by the examples of MnWO4, MnPS3 and LiNiPO4. Application to other magnetoelectrics is discussed.Comment: 11 pages, 2 figures, 2 table

Spiral spin-liquid and the emergence of a vortex-like state in MnSc2_2S4_4

Spirals and helices are common motifs of long-range order in magnetic solids, and they may also be organized into more complex emergent structures such as magnetic skyrmions and vortices. A new type of spiral state, the spiral spin-liquid, in which spins fluctuate collectively as spirals, has recently been predicted to exist. Here, using neutron scattering techniques, we experimentally prove the existence of a spiral spin-liquid in MnSc$_2$S$_4$ by directly observing the 'spiral surface' - a continuous surface of spiral propagation vectors in reciprocal space. We elucidate the multi-step ordering behavior of the spiral spin-liquid, and discover a vortex-like triple-q phase on application of a magnetic field. Our results prove the effectiveness of the $J_1$-$J_2$ Hamiltonian on the diamond lattice as a model for the spiral spin-liquid state in MnSc$_2$S$_4$, and also demonstrate a new way to realize a magnetic vortex lattice.Comment: 10 pages, 11 figure

New structural and magnetic aspects of the nanotube system Na2V3O7

We present new experimental results of low temperature x-ray synchrotron diffraction, neutron scattering and very low temperature (mK-range) bulk measurements on the nanotube system {\tube}. The crystal structure determined from our data is similar to the previously proposed model (P. Millet {\it et al.} J. Solid State Chem. $\bf{147}$, 676 (1999)), but also deviates from it in significant details. The structure comprises nanotubes along the c-axis formed by stacking units of two V-rings buckled in the $ab$-plane. The space group is P$\bar{3}$ and the composition is nonstoichiometric, Na(2-x)V3O7, x=0.17. The thermal evolution of the lattice parameters reveals anisotropic lattice compression on cooling. Neutron scattering experiments monitor a very weak magnetic signal at energies from -20 to 9 meV. New magnetic susceptibility, specific heat measurements and decay of remanent magnetization in the 30 mK - 300 mK range reveal that the previously observed transition at ~76 mK is spin-glass like with no long-range order. Presented experimental observations do not support models of isolated clusters, but are compatible with a model of odd-legged S=1/2 spin tubes possibly segmented into fragments with different lengths