Pseudo-Goldstone Magnons in the Frustrated S=3/2 Heisenberg Helimagnet ZnCr2Se

Abstract

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]

Breathing chromium spinels: a showcase for a variety of pyrochlore Heisenberg Hamiltonians

“…The corresponding direct space spin configuration is illustrated in Figure 4C. It is worth emphasizing that the shift of the spectral weight away from q = 0 is the combined effect of both J 3a and J 3b couplings of a comparable magnitude (40). Above the ordering temperature, and within the cooperative paramagnetic regime, we observe that the spectral weight is distributed rather uniformly over the spiral surface suggesting the presence of an approximate spiral spin liquid on the pyrochlore lattice stabilized concomitantly by thermal and quantum fluctuations.…”

Doping-induced redistribution of magnetic spectral weight in the substituted hexaborides Ce1−xLaxB6 and Nikitin1, Portnichenko2, Dukhnenko3 et al. 2018Phys. Rev. BSelf Cite123160View full textAdd to dashboardCite

“…This is why frustrated magnets usually exhibit complex magnetic phase diagrams in which multiple thermodynamic phases are found in close proximity [99][100][101][102][103]. Our present results indicate that the similarly rich phase diagram of Ce 1−x Nd x B 6 and Ce 1−x La x B 6 , which comprises at least two different multipolar phases and a number of AFM phases, could be the result of a similar competition among different Fermi-surface nesting vectors, where the charge-carrier doping and magnetic-moment concentration play the role of tuning parameters.…”

Inelastic Neutron Scattering Intensities of Ferromagnetic Cluster Spin Waves

“…[29] . The Mn 10 system exhibits ten spin wave excitations (k = 0, ..., 9) which result in four INS transitions at energies of 3.5, 7, 16.3, and 22.4 meV. In Figure 7 the functions W u (Q) are plotted for Mn 10 , where the atomic form factors were already taken into account.…”

“…4D INS is applied quite frequently to inorganic magnetic compounds or metals, where mostly antiferromagnetic (AFM), extended systems are studied , . Impressively complete plots of the magnon dispersion branches in ordered systems can be recorded relatively quickly , , .…”

“…[6][7][8] 4D INS is applied quite frequently to inorganic magnetic compounds or metals, where mostly antiferromagnetic (AFM), extended systems are studied. [4,[7][8][9][10][11][12] Impressively complete plots of the magnon dispersion branches in ordered systems can be recorded relatively quickly. [7,8,[13][14][15] The technique is also ideally suited for studying the magnetic correlations in frustrated, disordered systems, where features are typically broad both in energy E and Q space, and are thus difficult if not impossible to identify from measurements at few points or in a limited (E,Q) range.…”