Low-energy spin excitations in any long-range ordered magnetic system in the absence of magnetocrystalline anisotropy are gapless Goldstone modes emanating from the ordering wave vectors. In helimagnets, these modes hybridize into the so-called helimagnon excitations. Here we employ neutron spectroscopy supported by theoretical calculations to investigate the magnetic excitation spectrum of the isotropic Heisenberg helimagnet ZnCr 2 Se 4 with a cubic spinel structure, in which spin-3/2 magnetic Cr 3+ ions are arranged in a geometrically frustrated pyrochlore sublattice. Apart from the conventional Goldstone mode emanating from the (0 0 q h ) ordering vector, low-energy magnetic excitations in the single-domain proper-screw spiral phase show soft helimagnon modes with a small energy gap of ∼ 0.17 meV, emerging from two orthogonal wave vectors (q h 0 0) and (0 q h 0) where no magnetic Bragg peaks are present. We term them pseudo-Goldstone magnons, as they appear gapless within linear spin-wave theory and only acquire a finite gap due to higher-order quantum-fluctuation corrections. Our results are likely universal for a broad class of symmetric helimagnets, opening up a new way of studying weak magnon-magnon interactions with accessible spectroscopic methods.is justified by the negligibly small magneto-crystalline anisotropy [8-10]. Thus we consider throughout the paper J i j ≡ J n if sites i and j are n th neighbors [see Fig. 1 (a)]. Depending on the chemical composition, chromium spinels exhibit different mechanisms of frustration, such as geometric frustration that occurs if dominant NN interactions are antiferromagnetic, or bond frustration which originates from competition between ferromagnetic NN and antiferromagnetic further-neighbor exchange.To estimate the range and relative strengths of coupling constants J n in chromium spinels, Yaresko [7] performed ab initio calculations to extract exchange parameters up to the fourth nearest neighbor for various compounds of this family. Calculations showed that the NN interaction J 1 changes gradually from antiferromagnetic in some oxides to ferromagnetic in sulfides and selenides, while the next-nearestneighbor (NNN) interaction J 2 is noticeably weaker than the antiferromagnetic J 3 exchange parameter (see Table I). For the HgCr 2 O 4 system, J 1 can be even weaker than J 2 (or comparable, depending on the effective Coulomb repulsion U), so that the third-nearest-neighbor interaction J 3 may become dominant. Therefore, the existing theoretical phase diagram restricted to only NN and NNN interactions [3] appears insufficient for a realistic description of these materials. The importance of the two 3 rd -nearest-neighbor exchange paths on the pyrochlore lattice has been also emphasized for the spin-1 2 molybdate Heisenberg antiferromagnet Lu 2 Mo 2 O 5 N 2 [11], where J 3 and J 3 have opposite signs and dominate over J 2 . It was recently conjectured that this may lead to an unusual "gearwheel" type of a quantum spin liquid [12]. arXiv:1705.04642v3 [cond-mat.str-el]

The multiferroic ferrimagnet Cu 2 OSeO 3 with a chiral crystal structure attracted a lot of recent attention due to the emergence of magnetic skyrmion order in this material. Here, the topological properties of its magnon excitations are systematically investigated by linear spin-wave theory and inelastic neutron scattering. When considering Heisenberg exchange interactions only, two degenerate Weyl magnon nodes with topological charges ±2 are observed at high-symmetry points. Each Weyl point splits into two as the symmetry of the system is further reduced by including into consideration the nearest-neighbor Dzyaloshinsky-Moriya interaction, crucial for obtaining an accurate fit to the experimental spin-wave spectrum. The predicted topological properties are verified by surface state and Chern number analysis. Additionally, we predict that a measurable thermal Hall conductivity can be associated with the emergence of the Weyl points, the position of which can be tuned by changing the crystal symmetry of the material.

Abstract. We performed small-angle neutron scattering (SANS) measurements on the helimagnetic spinel compound ZnCr 2 Se 4 . The ground state of this material is a multi-domain spin-spiral phase, which undergoes domain selection in a magnetic field and reportedly exhibits a transition to a proposed spin-nematic phase at higher fields. We observed a continuous change in the magnetic structure as a function of field and temperature, as well as a weak discontinuous jump in the spiral pitch across the domain-selection transition upon increasing field. From our SANS results we have established the absence of any long-range magnetic order in the high-field (spin-nematic) phase. We also found that all the observed phase transitions are surprisingly isotropic with respect to the field direction.arXiv:1601.03005v1 [cond-mat.str-el]

We observe a hysteretic reorientation of the magnetic vortex lattice in the noncentrosymmetric superconductor Ru7B3, with the change in orientation driven by altering magnetic field below Tc. Normally a vortex lattice chooses either a single or degenerate set of orientations with respect to a crystal lattice at any given field or temperature, a behavior well described by prevailing phenomenological and microscopic theories. Here, in the absence of any typical VL structural transition, we observe a continuous rotation of the vortex lattice which exhibits a pronounced hysteresis and is driven by a change in magnetic field. We propose that this rotation is related to the spontaneous magnetic fields present in the superconducting phase, which are evidenced by the observation of time-reversal symmetry breaking, and the physics of broken inversion symmetry. Finally, we develop a model from the Ginzburg-Landau approach which shows that the coupling of these to the vortex lattice orientation can result in the rotation we observe.

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