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Helical bunching and symmetry lowering inducing multiferroicity in Fe langasites

L. Chaix, R. Ballou, A. Cano, S. Petit, S. de Brion, J. Ollivier, L.-P. Regnault, E. Ressouche, E. Constable, C. V. Colin, A. Zorko, V. Scagnoli, J. Balay, P. Lejay, and V. Simonet
Phys. Rev. B 93, 214419 – Published 16 June 2016
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    Abstract

    The chiral Fe-based langasites represent model systems of triangle-based frustrated magnets with a strong potential for multiferroicity. We report neutron-scattering measurements for the multichiral Ba3MFe3Si2O14 (M=Nb,Ta) langasites revealing new important features of the magnetic order of these systems: the bunching of the helical modulation along the c axis and the in-plane distortion of the 120∘ Fe-spin arrangement. We discuss these subtle features in terms of the microscopic spin Hamiltonian and provide the link to the magnetically induced electric polarization observed in these systems. Thus, our findings put the multiferroicity of this attractive family of materials on solid ground.

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    • Received 10 June 2015
    • Revised 25 May 2016

    DOI:https://doi.org/10.1103/PhysRevB.93.214419

    ©2016 American Physical Society

    Physics Subject Headings (PhySH)

    1. Physical Systems
    Hamiltonian systemsMultiferroics
    1. Techniques
    Neutron scattering
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    L. Chaix1,2,3,4, R. Ballou2,3, A. Cano5, S. Petit6, S. de Brion2,3, J. Ollivier1, L.-P. Regnault3,7, E. Ressouche3,7, E. Constable2,3, C. V. Colin2,3, A. Zorko8, V. Scagnoli9,10, J. Balay2,3, P. Lejay2,3, and V. Simonet2,3

    • 1Institut Laue-Langevin, 6 rue Jules Horowitz, 38042 Grenoble, France
    • 2Institut Néel, Centre National de la Recherche Scientifique, 38042 Grenoble, France
    • 3Université Grenoble Alpes, 38042 Grenoble, France
    • 4Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, Menlo Park, California 94025, USA
    • 5Centre National de la Recherche Scientifique, Université Bordeaux, ICMCB, UPR 9048, F-33600 Pessac, France
    • 6Commissariat à l'Énergie Atomique et aux Énergies Alternatives, Centre de Saclay/DSM/IRAMIS/Laboratoire Léon Brillouin, 91191 Gif-sur-Yvette, France
    • 7MEM-MDN, INAC, 38054 Grenoble, France
    • 8Jožef Stefan Institute, Jamova c. 39, SI-1000 Ljubljana, Slovenia
    • 9Laboratory for Mesoscopic Systems, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland
    • 10Paul Scherrer Institute, 5232 Villigen PSI, Switzerland

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    Vol. 93, Iss. 21 — 1 June 2016

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    • Figure 1
      Figure 1

      (a) Ba3NbFe3Si2O14 crystallographic structure in the ab plane and along the c axis, with five magnetic exchange paths J1 to J5. (b) Schematic representation of the revisited magnetic structure projected along the c axis. The threefold axis is broken leading to different J1 interactions (case of an isosceles triangle represented). The bunching of the helices in the Nb compound is reported through the successive angles between consecutive magnetic moments along c, deviating from the regular angle 2πτ=51.4∘.

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    • Figure 2
      Figure 2

      Zero-energy cut (integrated from −0.1 to 0.1 meV) from IN5 measurements at 1.5 K, on the Nb (a) and Ta (b) compounds. The blue arrows on the energy maps indicated the [0±1ℓ]* directions of the 1D−Q cuts shown below in vertical log scale. (c),(d) summarize the measurements performed on the Nb compound at 2 K on IN22 with polarized neutrons. (c) shows the magnetic (red), nuclear (green), and chiral (blue) elastic contributions from scans obtained by rotating the crystal (ω scans) with the H-H configuration. The solid lines are Gaussian fits. The magnetic and chiral contributions measured in the H-H (circles) and G-H (triangles) configurations for the main (0, 1, ℓ±τ) and extra (0, 1, ℓ±3τ) satellites, vs the calculated contributions using the model given in the caption of Fig. 3, are displayed in (d). Note that the G-H configuration cannot separate the magnetic from nuclear scattering, but we checked that σM+σN≈σM for these satellites.

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    • Figure 3
      Figure 3

      Spin-waves spectra of Nb (a) and Ta (b) measured along the [0−1ℓ]* and [01ℓ]* directions, respectively, at 1.5 K on IN5. Below: Spin-waves calculations with the following parameters for the Nb(c)/Ta(d): J1=0.85/0.85±0.1 meV, J2=0.24/0.24±0.05meV,J3=0.053/0.22±0.03 meV, J4=0.017/0.014±0.05meV,J5=0.24/0.053±0.05 meV, D=−0.028/−0.028±0.005meV, easy-plane anisotropy K=0.045/0.053±0.005 meV. The red and orange arrows show the spectral weight extinctions.

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    • Figure 4
      Figure 4

      (a) Neutron-scattering intensity vs the energy at the magnetic satellite position for Ta and Nb, obtained from 1D−Q cut (integration ranges Δh=Δk=Δl=0.05) measured on IN5 with a wavelength of 8 Å. Below: Calculated scattering intensity of the c branch only showing the c gap of the Nb (b) and Ta (c) compounds using the revised model with the parameters of the caption of Fig. 3.

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