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Formation of collective spins in frustrated clusters

J. Robert, V. Simonet, B. Canals, R. Ballou, E. Lhotel, C. Darie, P. Bordet, B. Ouladdiaf, M. Johnson, J. Ollivier, D. Braithwaite, H. Rakoto, and S. de Brion
Phys. Rev. B 77, 054421 – Published 19 February 2008
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  • INTRODUCTION
  • EXPERIMENT
  • ANALYSIS AND DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGEMENTS
    • References

    Abstract

    Using magnetization, specific heat, and neutron scattering measurements, as well as exact calculations on realistic models, the magnetic properties of the La3Cu2VO9 compound are characterized on a wide temperature range. At high temperature, this oxide is well described by strongly correlated atomic S=1∕2 spins while decreasing the temperature it switches to a set of weakly interacting and randomly distributed entangled pseudospins S̃=1∕2 and S̃=0. These pseudospins are built over frustrated clusters, similar to the kagomé building block, at the vertices of a triangular superlattice, the geometrical frustration intervening then at different scales.

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    • Received 16 October 2007

    DOI:https://doi.org/10.1103/PhysRevB.77.054421

    ©2008 American Physical Society

    Authors & Affiliations

    J. Robert1, V. Simonet1, B. Canals1, R. Ballou1, E. Lhotel1, C. Darie1, P. Bordet1, B. Ouladdiaf2, M. Johnson2, J. Ollivier2, D. Braithwaite3, H. Rakoto4, and S. de Brion5

    • 1Institut NEEL, CNRS and Université Joseph Fourier, Boîte Postale 166, F-38042 Grenoble, Cedex 9, France
    • 2Institut Laue-Langevin, Boîte Postale 154, F-38042 Grenoble, Cedex, France
    • 3CEA-Grenoble, DRFMC/SPSMS/IMAPEC, 17 rue des Martyrs, F-38054 Grenoble, Cedex, France
    • 4Laboratoire National des Champs Magnétiques Pulsés, 143 Avenue de Rangueil, F-31400 Toulouse, France
    • 5Grenoble High Magnetic Field Laboratory, CNRS, Boîte Postale 166, F-38042 Grenoble, France

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    Issue

    Vol. 77, Iss. 5 — 1 February 2008

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    • Figure 1
      Figure 1
      (Color online) (a) La3Cu2VO9 structure projected along the b axis: LaO6∕3 layers alternating with (Cu∕V)O3∕3 ones with the O polyhedron drawn around Cu and V atoms. (b) La3Cu2VO9 structure projected along the c axis: (Cu∕V)O3∕3 layers showing the nine Cu2+ clusters at the vertices of a triangular lattice. La, V, Cu, and O atoms are represented by large green, yellow, blue, and small red circles, respectively.Reuse & Permissions
    • Figure 2
      Figure 2
      La3Cu2VO9 powder neutron diffractogram recorded at 3K on D2B, the corresponding refined pattern, and their difference. The refined structural parameters are presented in Table .Reuse & Permissions
    • Figure 3
      Figure 3
      (Color online) DFT calculated positions of the Cu (blue circles), V (large yellow circles), and O (small red circles) atoms in the nine-spin clusters (a) and eight-spin clusters (b). The labeling of the different superexchange interactions is used in Sec. 3A.Reuse & Permissions
    • Figure 4
      Figure 4
      La3Cu2VO9 magnetic susceptibility from M∕H measurements: in the high temperature range at 6T using the BS magnetometer and at 0.1T using the Quantum Design MPMS magnetometer; in the low temperature range using the purpose built SQUID magnetometer in 0.01, 0.1, and 0.2T (open circles), in good agreement with the higher temperatures measurements (plain circles). A discontinuity around 2K is pointed out by the vertical line.Reuse & Permissions
    • Figure 5
      Figure 5
      La3Cu2VO9 inverse linear magnetic susceptibility measured in 0.1 and 6T below and above 350K, respectively (open circles). Three regions of distinct magnetic behaviors are materialized in gray. A low temperature zoom is shown in the inset. The straight lines in regions I and III are calculated in a Curie-Weiss model (see text).Reuse & Permissions
    • Figure 6
      Figure 6
      Magnetization isotherm measured around T=1.4K in increasing and decreasing pulsed fields up to 55T, and its field derivative. The dotted line indicates the middle of the magnetization plateau, which is related to the Zeeman crossing of the two first levels of the system with ΔS=+1 (see inset of Fig. 7).Reuse & Permissions
    • Figure 7
      Figure 7
      High field magnetization isotherms in meaningful units, measured by extraction at 3K, and compared to the BS magnetization measurements. The solid and dotted horizontal lines point out the calculated saturation value of the ground state and of the first excited state magnetizations (see Sec. 3). The Zeeman diagram corresponding to the field-induced crossing of these two levels is shown in the inset.Reuse & Permissions
    • Figure 8
      Figure 8
      (Color online) Specific heat measurements at low temperature (circles) analyzed with two models: (i) The phonon contribution (dashed line) is obtained from a fit of Cp∕T as a function of T2 using the low temperature approximation Cp=β1T3+β2T5 in the range 12–28K. The resulting magnetic signal is represented with square symbols. (ii) The second model is based on the multi-J spin cluster description reported in Table of Sec. 3. The lattice contribution of the form Cp=β1T3+β2T5 (dotted lines) is adjusted such that the remaining signal (triangles) coincides with the calculated magnetic specific heat (solid line) above 5K.Reuse & Permissions
    • Figure 9
      Figure 9
      (Color online) Inelastic neutron scattering measurements. Upper frame: energy spectra at 2, 150, and 300K, integrated on the Q interval [0.1,2]Å−1. Lower frame: 2 and 150K spectra corrected from the phonon contribution using the 300K spectrum (see text).Reuse & Permissions
    • Figure 10
      Figure 10
      Comparison of the inverse of the measured susceptibility (circles) with calculated ones (line) from a single-J model (gray) and from the multi-J model of Table taking into account eight-spin (dotted) and nine-spin clusters (dashed).Reuse & Permissions
    • Figure 11
      Figure 11
      Calculated total M(H) in the multi-J model of Table , with the eight-spin (dotted) and nine-spin (dashed) contributions, at 5K. The gray line is the field derivative of the total M(H).Reuse & Permissions
    • Figure 12
      Figure 12
      Eight-spin and nine-spin cluster degeneracies as a function of the energy difference with respect to the fundamental level (E0) in the low energy region, for the multi-J model of Table .Reuse & Permissions
    • Figure 13
      Figure 13
      Calculated total ⟨S̃2⟩ as a function of temperature for the eight-spin cluster (dotted lines) and nine-spin cluster (dashed lines) in the multi-J model of Table . Zoom of the low temperature range in the inset.Reuse & Permissions
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