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Competing antiferromagnetic-ferromagnetic states in a d7 Kitaev honeycomb magnet

Hector K. Vivanco, Benjamin A. Trump, Craig M. Brown, and Tyrel M. McQueen
Phys. Rev. B 102, 224411 – Published 14 December 2020
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  • INTRODUCTION
  • EXPERIMENTAL SECTION
  • RESULTS AND DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    The Kitaev model is a rare example of an analytically solvable and physically instantiable Hamiltonian yielding a topological quantum spin liquid ground state. Here we report signatures of Kitaev spin liquid physics in the honeycomb magnet Li3Co2SbO6, built of high-spin d7 (Co2+) ions, in contrast to the more typical low-spin d5 electron configurations in the presence of large spin-orbit coupling. Neutron powder diffraction measurements, heat capacity, and magnetization studies support the development of a long-range antiferromagnetic order space group of CC2/m, below TN=11K at μ0H=0T. The magnetic entropy recovered between T=2 and 50 K is estimated to be 0.6Rln2, in good agreement with the value expected for systems close to a Kitaev quantum spin liquid state. The temperature-dependent magnetic order parameter demonstrates a β value of 0.19(3), consistent with XY anisotropy and in-plane ordering, with Ising-like interactions between layers. Further, we observe a spin-flop-driven crossover to ferromagnetic order with space group of C2/m under an applied magnetic field of μ0H≈0.7T at T=2K. Magnetic structure analysis demonstrates these magnetic states are competing at finite applied magnetic fields even below the spin-flop transition. Both the d7 compass model, a quantitative comparison of the specific heat of Li3Co2SbO6, and related honeycomb cobaltates to the anisotropic Kitaev model further support proximity to a Kitaev spin liquid state. This material demonstrates the rich playground of high-spin d7 systems for spin liquid candidates and complements known d5 Ir- and Ru-based materials.

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    • Received 19 May 2020
    • Revised 17 November 2020
    • Accepted 18 November 2020

    DOI:https://doi.org/10.1103/PhysRevB.102.224411

    ©2020 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    AntiferromagnetismFerromagnetismMagnetic orderOrder parametersPhase diagrams
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Hector K. Vivanco1,2, Benjamin A. Trump3, Craig M. Brown3,4, and Tyrel M. McQueen1,2,5,*

    • 1Department of Chemistry, The Johns Hopkins University, Baltimore, Maryland 21218, USA
    • 2Institute for Quantum Matter, Department of Physics and Astronomy, The Johns Hopkins University, Baltimore, Maryland 21218, USA
    • 3NIST Center for Neutron Research, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA
    • 4Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, USA
    • 5Department of Materials Science and Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, USA

    • *mcqueen@jhu.edu

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    Issue

    Vol. 102, Iss. 22 — 1 December 2020

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    • Figure 1
      Figure 1

      Rietveld refinement (black line) to a high-resolution NPD pattern of Li3Co2SbO6 (a) collected at T=298K (orange points), with difference curve (gray line). The blue ticks correspond to the Bragg reflections for the main phase, and the purple ticks represent the LiOH·H2O impurity with 1.3 wt%. Stacking fault simulations (a, inset) are presented for the two distinguishable stacking variations and a 5% random mixture of both possible stacking fault shifts. The crystal structure (b) is presented, as well as the visualization of the magnetic layer (c) with Co-O-Co bond angles of 90.03∘ and 91.22∘. The distinct possible stacking fault arrangements are presented (d) with given values 1–2 to illustrate the indistinguishable positions.

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    • Figure 2
      Figure 2

      Magnetization isotherms (a) and the first derivative of magnetization [dM/d(μ0H)] (b) for Li3Co2SbO6 displaying curvature of metamagnetic behavior. High field magnetization measurements (a, inset) illustrates the compound approaching the expected saturation value of 3.87μB. A magnetization loop at T=2K (c) demonstrates a small hysteresis (c, inset) indicative of FM interactions. Magnetic susceptibility, estimated as M/μ0H, at μ0H=1T and the inverse susceptibility (d) with the Curie-Weiss fit (purple line) of the inverse susceptibility between T=60 and 300 K. A comparison of the low-temperature susceptibility (d, inset) is provided to visualize the change with an applied field.

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    • Figure 3
      Figure 3

      Temperature-dependent total specific heat over temperature at several applied magnetic fields is presented (a) and the magnetic specific heat is calculated (b) after subtracting phonon contribution measured from Li3Zn2SbO6. The magnetic specific heat demonstrates the transition increasing under applied fields behaving as a ferromagnet. Integrated magnetic entropy (c) plateaus at around 0.6Rln2 at μ0H=0T, approaching 0.5Rln2 under applied fields. This is in agreement with the anisotropic Kitaev model.

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    • Figure 4
      Figure 4

      Field-dependent heat capacity at several temperatures (lines to guide the eye) with no λ anomaly suggesting a spin-crossover transition. The inset presents overlapping isotherms that collapse onto a universal curve when heat capacity is divided by temperature to the power of 2.36 (η).

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    • Figure 5
      Figure 5

      NPD of Li3Co2SbO6 with Rietveld refinements to determine nuclear (Nuc.) and magnetic structures. At μ0H=0T (a), the structure is AFM with alternating FM planes (Cc2/m). This AFM structure persists with reduced correlation length at μ0H=0.1T (b). At higher fields, the magnetic structure is FM (C2/m) (c), (d).

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    • Figure 6
      Figure 6

      Temperature-dependent magnetic order parameter (a) at μ0H=0T indicates an XY-plane ordering with Ising-like ordering between layers with a β value of 0.19(3). Power-law analysis (red line) of integrated intensity and temperature for two-dimensional XY-plane ordering is 0.23 while Ising ordering is 0.125. Field-dependent NPD scans of the (001) peak illustrate the FM peak developing above μ0H=0.2T with a reference scan at T=30K (b). The peak increases in intensity compared to the reference below the spin-flop transition of μ0H=0.7T, which demonstrates the competing AFM-FM magnetic states.

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    • Figure 7
      Figure 7

      Magnetic phase diagram of Li3Co2SbO6 presents the spin crossover of AFM to FM states, with graphical illustrations of the magnetic states, under applied fields implying Kitaev interactions.

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    • Figure 8
      Figure 8

      Normalized specific heat and temperature scaled by exchange interactions (J) with the calculated anisotropic S=3/2 Kitaev model overlaid to display the comparison (a). Several high-spin d7 compound heat capacity data has been extracted to display Kitaev model signatures [27, 28, 41, 42]. Each dataset is offset by 0.3 for clarity. The Kitaev compass model (b) with Li3Co2SbO6 placed in the FM regime demonstrates how the Co-delafossites are proximate to the Kitaev QSL state.

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