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Magnetic properties of the triangular lattice magnets A4B′B2O12 (A=Ba, Sr, La; B′=Co, Ni, Mn; B=W, Re)

R. Rawl, M. Lee, E. S. Choi, G. Li, K. W. Chen, R. Baumbach, C. R. dela Cruz, J. Ma, and H. D. Zhou
Phys. Rev. B 95, 174438 – Published 25 May 2017
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  • INTRODUCTION
  • EXPERIMENT
  • RESULTS
  • DISCUSSION
  • CONCLUSION
  • ACKNOWLEDGMENTS
    • References

    Abstract

    The geometrically frustrated two-dimensional triangular lattice magnets A4B′B2O12 (A=Ba, Sr, La; B′=Co, Ni, Mn; B=W, Re) have been studied by x-ray diffraction, ac and dc susceptibilities, powder neutron diffraction, and specific-heat measurements. The results reveal the following: (i) The samples containing Co2+ (effective spin-1/2) and Ni2+ (spin-1) ions with small spin numbers exhibit ferromagnetic (FM) behavior and ordering, respectively, while the sample containing Mn2+ (spin-5/2) ions with a large spin number exhibits antiferromagnetic (AFM) ordering. We ascribe these spin-number-manipulated ground states to the competition between the AFM B′-O-O-B′ and FM B′-O-B-O-B′ superexchange interactions. (ii) The chemical pressure introduced into the Co-containing samples through the replacement of different-size ions on the A site finely tunes the FM behavior temperature of the system. This effect is not simply governed by the lattice parameters and requires more precise structural measurements to elucidate.

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    • Received 21 February 2017
    • Revised 1 May 2017

    DOI:https://doi.org/10.1103/PhysRevB.95.174438

    ©2017 American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    AntiferromagnetismFerromagnetismInsulatorsMagnetic susceptibility
    1. Physical Systems
    Magnetic systemsPolycrystalline materials
    1. Techniques
    Neutron diffraction
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    R. Rawl1, M. Lee2,3, E. S. Choi3, G. Li4, K. W. Chen3, R. Baumbach3, C. R. dela Cruz5, J. Ma1,6, and H. D. Zhou1,3

    • 1Department of Physics and Astronomy, University of Tennessee, Knoxville, Tennessee 37996-1200, USA
    • 2Department of Physics, Florida State University, Tallahassee, Florida 32306-3016, USA
    • 3National High Magnetic Field Laboratory, Florida State University, Tallahassee, Florida 32310-3706, USA
    • 4School of Physics and Materials Science, Anhui University, Hefei, Anhui 230601, People's Republic of China
    • 5Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA
    • 6Department of Physics and Astronomy, Shanghai Jiao Tong University, Shanghai 200240, People's Republic of China

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    Vol. 95, Iss. 17 — 1 May 2017

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    Images

    • Figure 1
      Figure 1

      (a) Crystalline structure of A4CoB2O12 with the space group R-3mH. (b) Staggered pattern of triangular Co2+ planes along the c axis.

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    • Figure 2
      Figure 2

      (a)–(e) Powder x-ray diffraction patterns (red crosses) and Rietveld refinements (black line) for five A4CoB2O12 compounds. The blue line at the bottom of each panel is the difference curve. The black ticks are the reflection positions.

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    • Figure 3
      Figure 3

      (a)–(e) The inverse dc susceptibility of the A4CoB2O12 compounds. The solid and dashed lines are Curie-Weiss fittings of the low-temperature and high-temperature regimes, respectively. Insets: dc magnetization taken at 1.8 K. The saturation magnetization of the Co2+ ion is extrapolated using a linear fit to account for the Van Vleck paramagnetic contribution.

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    • Figure 4
      Figure 4

      (a)–(e) The real part of the ac susceptibility χAC for cobalt containing A4B′B2O12 compounds measured from 0.3 to 2.0 K under different dc magnetic fields. Excitation fields of 2 Oe at low ac frequencies were used.

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    • Figure 5
      Figure 5

      For Ba2La2NiW2O12, (a) the Rietveld refinement of the NPD pattern measured at room temperature using a neutron wavelength of λ=1.5405Å. (b) The temperature dependence of CP measured at different dc fields. Inset: the field dependence of TC. (c) The inverse dc susceptibility. The solid line is the linear fitting. Inset: the temperature dependence of dc susceptibility measured under different fields. (d) The dc magnetization curve measured at 2 K.

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    • Figure 6
      Figure 6

      NPD of Ba2La2NiW2O12 taken at 2 and 20 K. The overlap of magnetic Bragg peaks with lattice Bragg peaks indicates FM long-range ordering.

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    • Figure 7
      Figure 7

      For Ba2La2MnW2O12, (a) the Rietveld refinement of the NPD pattern taken at room temperature. (b) The temperature dependence of CP measured at zero field. (c) The inverse dc susceptibility. The solid line is the linear fitting.

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    • Figure 8
      Figure 8

      (a) The temperature dependence of χAC for Ba2La2MnW2O12 under applied dc fields. (b) The dc field dependence of χAC at various temperatures. Inset: the enlargement of the data around 4 T. (c) The derivative of the field dependence of χAC at different temperatures.

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    • Figure 9
      Figure 9

      Magnetic phase diagram of Ba2La2MnW2O12. Transition temperatures were found from the temperature derivative (red solid squares), the field derivative (blue open squares) of χAC, and zero-field CP measurements (green triangle).

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    • Figure 10
      Figure 10

      (a) Pathways for FM B′-O-B-O-B′ (black dashed line) and AFM B′-O-O-B′ (green solid line) superexchange interactions. (b) Energy levels of 4f orbitals in cubic symmetry, with the highest energy levels having the same symmetry as p orbitals. (c) Orbital diagram of FM B′-O-B-O-B′ interaction with f orbitals represented by symmetrically similar p orbitals.

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    • Figure 11
      Figure 11

      Temperature dependence of lattice parameters a and c for Ba2La2CoW2O24. No structural distortion is seen upon the transition from high spin (S=3/2) to low spin (S=1/2) of the Co2+ ions.

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