We present inelastic neutron scattering measurements of the Cairo pentagon lattice magnets Bi_2Fe_4O_9 and Bi_4Fe_5O_13F, supported by high field magnetisation measurements of Bi_2Fe_4O_9. Using linear spin wave theory and mean field analyses we determine the spin exchange interactions and single-ion anisotropy in these materials. The Cairo lattice is geometrically frustrated and consists of two inequivalent magnetic sites, both occupied by Fe^3+ ions and connected by two competing nearest neighbour interactions. We found that one of these interactions, coupling nearest neighbour spins on the three-fold symmetric sites, is extremely strong and antiferromagnetic. These strongly coupled dimers are then weakly coupled to a framework formed from spins occupying the other inequivalent site. In addition we found that the Fe^3+ S=5/2 spins have a non-negligible single-ion anisotropy, which manifests as a spin anisotropy gap in the neutron spectrum and a spin-flop transition in high field magnetisation measurements.

2 dimerized antiferromagnetic-ferromagnetic quantum spin-chain BaCu2V2O8 E. S. Klyushina, 2, ∗ A.T.M.N. Islam, J. T. Park, E. A. Goremychkin, † E. Wheeler, B. Klemke, and B. Lake 2 Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany Heinz Maier-Leibnitz Zentrum, TU München, 85747 Garching, Germany Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom Institut Laue-Langevin, Boite Postale 156X, F-38042 Grenoble Cedex 9, France

The magnetic properties of CuO encompass several contemporary themes in condensed matter physics, including quantum magnetism, magnetic frustration, magnetically-induced ferroelectricity and orbital currents. Here we report polarized and unpolarized neutron inelastic scattering measurements which provide a comprehensive map of the cooperative spin dynamics in the low temperature antiferromagnetic (AFM) phase of CuO throughout much of the Brillouin zone. At high energies ($E \gtrsim 100$\,meV) the spectrum displays continuum features consistent with the des Cloizeax--Pearson dispersion for an ideal $S=\frac{1}{2}$ Heisenberg AFM chain. At lower energies the spectrum becomes more three-dimensional, and we find that a linear spin-wave model for a Heisenberg AFM provides a very good description of the data, allowing for an accurate determination of the relevant exchange constants in an effective spin Hamiltonian for CuO. In the high temperature helicoidal phase, there are features in the measured low-energy spectrum that we could not reproduce with a spin-only model. We discuss how these might be associated with the magnetically-induced multiferroic behavior observed in this phase.

The novel quantum magnet BaCu2V2O8 was recently discovered to be a rare physical realization of a one-dimensional antiferromagnetic-ferromagnetic dimerized chain which displays strongly correlated phenomena at elevated temperatures [E. S. Klyushina et al., Phys.Rev.B 93, 241109(R) (2016)]. This paper presents an extended study of the Hamiltonian of BaCu2V2O8 at base temperature. Static susceptibility and inelastic neutron scattering data are compared to several theoretical models. An analytical relation for the dynamic structure factor of the complex unit cell of BaCu(2)V(2)O(8 )is derived and used to identify the intrachain exchange paths. Further analysis using the first moment of the dynamic structure factor was employed to determine the exchange path responsible for the intradimer interaction. This analysis reveals that the dimer chain is formed by a dominant antiferromagnetic exchange interaction J(intra) = 40.92 meV which is realized via the Cu-O-V(II)-O-Cu superexchange path and a weak ferromagnetic coupling J(intra) = -11.97 meV which arises within the copper-oxygen double plaquettes.

Christian Balz, 2, ∗ Bella Lake, 2 A.T.M. Nazmul Islam, Yogesh Singh, Jose A. Rodriguez-Rivera, 5 Tatiana Guidi, Elisa M. Wheeler, Giovanna G. Simeoni, and Hanjo Ryll Helmholtz-Zentrum Berlin für Materialien und Energie, 14109 Berlin, Germany Institut für Festkörperphysik, Technische Universität Berlin, 10623 Berlin, Germany† Indian Institute of Science Education and Research (IISER) Mohali, Knowledge City, Sector 81, Mohali 140306, India NIST Center for Neutron Research, National Institute of Standards and Technology, 20899 Gaithersburg, USA Department of Materials Science, University of Maryland, College Park, 20742 Maryland, USA ISIS Facility, STFC Rutherford Appleton Laboratory, Oxfordshire OX11 0QX, UK Institut Laue-Langevin, 38042 Grenoble, France Heinz Maier-Leibnitz Zentrum, Technische Universitat München, 85748 Garching, Germany (Dated: April 20, 2017)

A spin liquid is a new state of matter with topological order where the spin moments continue to fluctuate coherently down to the lowest temperatures rather than develop static long-range magnetic order as found in conventional magnets. For spin liquid behavior to arise in a material the magnetic Hamiltonian must be ``frustrated'', where the combination of lattice geometry, interactions, and anisotropies gives rise to competing spin arrangements in the ground state. Theoretical Hamiltonians which produce spin liquids are spin ice, the Kitaev honeycomb model, and the kagome antiferromagnet. Spin liquid behavior, however, in real materials is rare because they can only approximate these Hamiltonians and often have weak higher-order terms that destroy the spin liquid state. ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$ is a new quantum spin liquid candidate with magnetic ${\mathrm{Cr}}^{5+}$ ions that possess quantum spin number $S=\textonehalf{}$. The spins are entirely dynamic in the ground state and the excitation spectrum is broad and diffuse, as is typical of spinons which are the excitations of a spin liquid. In this paper we determine the Hamiltonian of ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$ using inelastic neutron scattering under high magnetic field to induce a field-polarized paramagnetic ground state and spin-wave excitations that can be fitted to extract the interactions. We further explore the phase diagram by using inelastic neutron scattering and heat capacity measurements and establish the boundaries of the spin liquid phase as a function of magnetic field and temperature. Our results show that ${\mathrm{Ca}}_{10}{\mathrm{Cr}}_{7}{\mathrm{O}}_{28}$ consists of distorted kagome bilayers with several isotropic ferromagnetic and antiferromagnetic interactions where, unexpectedly, the ferromagnetic interactions are stronger than the antiferromagnetic ones. This complex Hamiltonian does not correspond to any known spin liquid model and points to new directions in the search for quantum spin liquid behavior.

Unlike conventional magnets where the magnetic moments are partially or completely static in the ground state, in a quantum spin liquid they remain in collective motion down to the lowest temperatures. The importance of this state is that it is coherent and highly entangled without breaking local symmetries. In the case of magnets with isotropic interactions, spin-liquid behaviour is sought in simple lattices with antiferromagnetic interactions that favour antiparallel alignments of the magnetic moments and are incompatible with the lattice geometries. Despite an extensive search, experimental realizations remain very few. Here we investigate the novel, unexplored magnet Ca 10 Cr 7 O 28 , which has a complex Hamiltonian consisting of several different isotropic interactions and where the ferromagnetic couplings are stronger than the antiferromagnetic ones. We show both experimentally and theoretically that it displays all the features expected of a quantum spin liquid. Thus spin-liquid behaviour in isotropic magnets is not restricted to the simple idealized models currently investigated, but can be compatible with complex structures and ferromagnetic interactions.

Ultrasound velocity measurements of the orbital-degenerate frustrated spinel MgV2O4 are performed in the disorder-free high-purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder-introduced single crystal which exhibits spin-glass-like behavior. The measurements reveal coexisting two types of anomalous temperature dependence of the elastic moduli in the cubic paramagnetic phase: Curie-type softening with decreasing temperature, and softening with a characteristic minimum with decreasing temperature. These elastic anomalies should respectively originate from the coexisting orbital fluctuations and spin-cluster excitations.

Effects of interchain couplings and anisotropy on a Haldane chain have been investigated by single-crystal inelastic neutron scattering and density functional theory (DFT) calculations on the model compound SrNi2V2O8. Significant effects on low-energy excitation spectra are found where the Haldane gap (Delta(0) approximate to 0.41J, where J is the intrachain exchange interaction) is replaced by three energy minima at different antiferromagnetic zone centers due to the complex interchain couplings. Further, the triplet states are split into two branches by single-ion anisotropy. Quantitative information on the intrachain and interchain interactions as well as on the single-ion anisotropy is obtained from the analyses of the neutron scattering spectra by the random-phase approximation method. The presence of multiple competing interchain interactions is found from the analysis of the experimental spectra and is also confirmed by the DFT calculations. The interchain interactions are two orders of magnitude weaker than the nearest-neighbor intrachain interaction J = 8.7 meV. The DFT calculations reveal that the dominant intrachain nearest-neighbor interaction occurs via nontrivial extended superexchange pathways Ni-O-V-O-Ni involving the empty d orbital of V ions. The present single-crystal study also allows us to correctly position SrNi2V2O8 in the theoretical D-J(perpendicular to) phase diagram [T. Sakai and M. Takahashi, Phys. Rev. B 42, 4537 (1990)], showing where it lies within the spin-liquid phase.

Ultrasound velocity measurements of the orbitally degenerate frustrated spinel MgV2O4 are performed in a high-purity single crystal which exhibits successive structural and antiferromagnetic phase transitions, and in the disorder-introduced single crystal which exhibits spin-glass-like behavior. The measurements reveal that two types of unusual temperature dependence of the elastic moduli coexist in the cubic paramagnetic phase, which are resolved by magnetic-field and disorder sensitivities: huge Curie-type softening with decreasing temperature (convex temperature dependence), and concave temperature dependence with a characteristic minimum. These elastic anomalies suggest the coupling of the lattice to coexisting orbital fluctuations and spin-cluster excitations.

Magnetically induced ferroelectric multiferroics present an exciting new paradigm in the design of multifunctional materials, by intimately coupling magnetic and polar order. Magnetoelectricity creates a novel quasiparticle excitation--the electromagnon--at terahertz frequencies, with spectral signatures that unveil important spin interactions. To date, electromagnons have been discovered at low temperature (<70 K) and predominantly in rare-earth compounds such as RMnO3. Here we demonstrate using terahertz time-domain spectroscopy that intersublattice exchange in the improper multiferroic cupric oxide (CuO) creates electromagnons at substantially elevated temperatures (213-230 K). Dynamic magnetoelectric coupling can therefore be achieved in materials, such as CuO, that exhibit minimal static cross-coupling. The electromagnon strength and energy track the static polarization, highlighting the importance of the underlying cycloidal spin structure. Polarized neutron scattering and terahertz spectroscopy identify a magnon in the antiferromagnetic ground state, with a temperature dependence that suggests a significant role for biquadratic exchange.

E. Wawrzyńska, R. Coldea, E.M. Wheeler I.I. Mazin, M.D. Johannes, T. Sörgel, M. Jansen, R.M. Ibberson, P.G. Radaelli H.H. Wills Physics Laboratory, University of Bristol, Tyndall Avenue, Bristol, BS8 1TL, United Kingdom Clarendon Laboratory, University of Oxford, Parks Road, Oxford OX1 3PU, United Kingdom Institute Laue-Langevin, BP 156, 38042 Grenoble Cedex 9, France Code 6393, Naval Research Laboratory, Washington, D.C. 20375 Max-Planck Institut für Festkörperforschung, Heisenbergstrasse 1, D-70569 Stuttgart, Germany ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot OX11 0QX, United Kingdom (Dated: June 10, 2013)

Polarized inelastic neutron scattering on magnetically ordered Fe1.02Te0.95Se0.05 and superconducting Fe1.02Te0.60Se0.40 has been used to elucidate the anisotropy of the magnetic dynamical susceptibility at or near the (pi, pi) momentum space position. By investigating Fe1.02Te0.60Se0.40 in its superconducting state at 2 K, where a spin resonance at about 6 meV has been reported, we show that the susceptibility in the direction perpendicular to the c axis is by about 35% larger than for the direction along the c axis suggesting dominant singlet pairing of s(+/-) type. At higher temperatures the same type of anisotropy is present in the spin dynamics of the normal state. On the other hand, for the Fe1.02Te0.95Se0.05 sample we find an almost isotropic response in the paramagnetic state. When the compound is magnetically ordered at (pi, 0), the response near (pi, pi) persists, but with reduced intensity. The c-axis component is only slightly reduced whereas the in-plane component appears to be strongly reduced, leading to reverted anisotropy with respect to the superconducting compound.

We have grown several single crystals of the highly frustrated S = 1 spinel MgV2O4 using different starting compositions and growth conditions. From our study of their physical properties by magnetic susceptibility, heat capacity, and single-crystal neutron and powder-diffraction measurements, we observe that a minute amount of disorder suppresses the structural and magnetic phase transitions. As little as 3% disorder in the octahedral site introduced random strain, which was enough to completely suppress the transitions and induce a spin-glass phase at low temperatures. We believe that the reason is spin-exchange disorder rather than disorder in the weakly diluted magnetic lattice. Our results also show that the MgO-V2O3 system is a solid solution that melts slightly incongruently, and we demonstrate that by using an optimized solvent in the traveling-solvent floating-zone configuration a large single crystal (l = 25 mm, d = 6 mm) with a homogeneous composition, free of site disorders can be grown.