76 research outputs found

    Magnetic spectrum of the two-dimensional antiferromagnet La2CoO4 studied by inelastic neutron scattering

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    We report measurements of the magnetic excitation spectrum of the layered antiferromagnet La2CoO4 by time-of-flight neutron inelastic scattering. In the energy range probed in our experiments (0-250 meV) the magnetic spectrum consists of spin-wave modes with strong in-plane dispersion extending up to 60 meV, and a nearly dispersionless peak at 190 meV. The spin-wave modes exhibit a small (~1 meV) dispersion along the magnetic zone boundary. We show that the magnetic spectrum can be described very well by a model of a Heisenberg antiferromagnet that includes the full spin and orbital degrees of freedom of Co2+ in an axially-distorted crystal field. The collective magnetic dynamics are found to be controlled by dominant nearest-neighbour exchange interactions, strong XY-like single-ion anisotropy and a substantial unquenched orbital angular momentum.Comment: 8 pages, 7 figure

    Magnetic structure of Ba(TiO)Cu4_4(PO4_4)4_4 probed using spherical neutron polarimetry

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    The antiferromagnetic compound Ba(TiO)Cu4_4(PO4_4)4_4 contains square cupola of corner-sharing CuO4_4 plaquettes, which were proposed to form effective quadrupolar order. To identify the magnetic structure, we have performed spherical neutron polarimetry measurements. Based on symmetry analysis and careful measurements we conclude that the orientation of the Cu2+^{2+} spins form a non-collinear in-out structure with spins approximately perpendicular to the CuO4_4 motif. Strong Dzyaloshinskii-Moriya interaction naturally lends itself to explain this phenomenon. The identification of the ground state magnetic structure should serve well for future theoretical and experimental studies into this and closely related compounds.Comment: 9 pages, 4 figure

    AA-cation control of magnetoelectric quadrupole order in AA(TiO)Cu4_4(PO4_4)4_4 (AA = Ba, Sr, and Pb)

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    Ferroic magnetic quadrupole order exhibiting macroscopic magnetoelectric activity is discovered in the novel compound AA(TiO)Cu4_4(PO4_4)4_4 with AA = Pb, which is in contrast with antiferroic quadrupole order observed in the isostructural compounds with AA = Ba and Sr. Unlike the famous lone-pair stereochemical activity which often triggers ferroelectricity as in PbTiO3_3, the Pb2+^{2+} cation in Pb(TiO)Cu4_4(PO4_4)4_4 is stereochemically inactive but dramatically alters specific magnetic interactions and consequently switches the quadrupole order from antiferroic to ferroic. Our first-principles calculations uncover a positive correlation between the degree of AA-O bond covalency and a stability of the ferroic quadrupole order.Comment: 7 pages, 4 figure

    Non-equilibrium hysteresis and spin relaxation in the mixed-anisotropy dipolar coupled spin-glass LiHo0.5_{0.5}Er0.5_{0.5}F4_{4}

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    We present a study of the model spin-glass LiHo0.5_{0.5}Er0.5_{0.5}F4_4 using simultaneous AC susceptibility, magnetization and magnetocaloric effect measurements along with small angle neutron scattering (SANS) at sub-Kelvin temperatures. All measured bulk quantities reveal hysteretic behavior when the field is applied along the crystallographic c axis. Furthermore avalanche-like relaxation is observed in a static field after ramping from the zero-field-cooled state up to 200300200 - 300 Oe. SANS measurements are employed to track the microscopic spin reconfiguration throughout both the hysteresis loop and the related relaxation. Comparing the SANS data to inhomogeneous mean-field calculations performed on a box of one million unit cells provides a real-space picture of the spin configuration. We discover that the avalanche is being driven by released Zeeman energy, which heats the sample and creates positive feedback, continuing the avalanche. The combination of SANS and mean-field simulations reveal that the conventional distribution of cluster sizes is replaced by one with a depletion of intermediate cluster sizes for much of the hysteresis loop.Comment: 6 pages, 4 figure

    Spin anisotropy of the resonance in superconducting FeSe0.5Te0.5

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    We have used polarized-neutron inelastic scattering to resolve the spin fluctuations in superconducting FeSe0.5Te0.5 into components parallel and perpendicular to the layers. A spin resonance at an energy of 6.5 meV is observed to develop below T_c in both fluctuation components. The resonance peak is anisotropic, with the in-plane component slightly larger than the out-of-plane component. Away from the resonance peak the magnetic fluctuations are isotropic in the energy range studied. The results are consistent with a dominant singlet pairing state with s^{\pm} symmetry, with a possible minority component of different symmetry.Comment: 5 pages, 4 figure

    Tuning the superconducting and magnetic properties in Fe_ySe_0.25Te_0.75 by varying the Fe-content

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    The superconducting and magnetic properties of Fey_{y}Se0.25_{0.25}Te0.75_{0.75} single crystals (0.9y1.10.9\leq y \leq1.1) were studied by means of x-ray diffraction, SQUID magnetometry, muon spin rotation, and elastic neutron diffraction. The samples with y<1y<1 exhibit coexistence of bulk superconductivity and incommensurate magnetism. The magnetic order remains incommensurate for y1y\geq 1, but with increasing Fe content superconductivity is suppressed and the magnetic correlation length increases. The results show that the superconducting and the magnetic properties of the Fey_{y}Se1x_{1-x}Tex_{x} can be tuned not only by varying the Se/Te ratio but also by changing the Fe content

    Spin excitations in the skymion host Cu2OSeO3

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    We have used inelastic neutron scattering to measure the magnetic excitation spectrum along the high-symmetry directions of the first Brillouin zone of the magnetic skyrmion hosting compound Cu2_2OSeO3_3. The majority of our scattering data are consistent with the expectations of a recently proposed model for the magnetic excitations in Cu2_2OSeO3_3, and we report best-fit parameters for the dominant exchange interactions. Important differences exist, however, between our experimental findings and the model expectations. These include the identification of two energy scales that likely arise due to neglected anisotropic interactions. This feature of our work suggests that anisotropy should be considered in future theoretical work aimed at the full microscopic understanding of the emergence of the skyrmion state in this material.Comment: 5 pages, 6 figure

    Hour-glass magnetic spectrum in an insulating, hole-doped antiferromagnet

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    Superconductivity in layered copper-oxide compounds emerges when charge carriers are added to antiferromagnetically-ordered CuO2 layers. The carriers destroy the antiferromagnetic order, but strong spin fluctuations persist throughout the superconducting phase and are intimately linked to super-conductivity. Neutron scattering measurements of spin fluctuations in hole-doped copper oxides have revealed an unusual `hour-glass' feature in the momentum-resolved magnetic spectrum, present in a wide range of superconducting and non-superconducting materials. There is no widely-accepted explanation for this feature. One possibility is that it derives from a pattern of alternating spin and charge stripes, an idea supported by measurements on stripe-ordered La1.875Ba0.125CuO4. However, many copper oxides without stripe order also exhibit an hour-glass spectrum$. Here we report the observation of an hour-glass magnetic spectrum in a hole-doped antiferromagnet from outside the family of superconducting copper oxides. Our system has stripe correlations and is an insulator, which means its magnetic dynamics can conclusively be ascribed to stripes. The results provide compelling evidence that the hour-glass spectrum in the copper-oxide superconductors arises from fluctuating stripes.Comment: 13 pages, 4 figures, to appear in Natur

    Magnetic excitations and electronic interactions in Sr2_2CuTeO6_6: a spin-1/2 square lattice Heisenberg antiferromagnet

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    Sr2_2CuTeO6_6 presents an opportunity for exploring low-dimensional magnetism on a square lattice of S=1/2S=1/2 Cu2+^{2+} ions. We employ ab initio multi-reference configuration interaction calculations to unravel the Cu2+^{2+} electronic structure and to evaluate exchange interactions in Sr2_2CuTeO6_6. The latter results are validated by inelastic neutron scattering using linear spin-wave theory and series-expansion corrections for quantum effects to extract true coupling parameters. Using this methodology, which is quite general, we demonstrate that Sr2_2CuTeO6_6 is an almost realization of a nearest-neighbor Heisenberg antiferromagnet but with relatively weak coupling of 7.18(5) meV.Comment: 10 pages, 7 figure
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