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Sr2IrO4/Sr3Ir2O7 superlattice for a model two-dimensional quantum Heisenberg antiferromagnet

Hoon Kim, Joel Bertinshaw, J. Porras, B. Keimer, Jungho Kim, J.-W. Kim, Jimin Kim, Jonghwan Kim, Gahee Noh, Gi-Yeop Kim, Si-Young Choi, and B. J. Kim
Phys. Rev. Research 4, 013229 – Published 28 March 2022
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  • INTRODUCTION
  • METHODS
  • LATTICE STRUCTURE
  • MAGNETIC STRUCTURE
  • PSEUDOSPIN DYNAMICS
  • SUMMARY
  • ACKNOWLEDGMENTS
  • APPENDICES
    • References

    Abstract

    Spin-orbit entangled pseudospins hold promise for a wide array of exotic magnetism ranging from a Heisenberg antiferromagnet to a Kitaev spin liquid depending on the lattice and bonding geometry, but many of the host materials suffer from lattice distortions and deviate from idealized models in part due to inherent strong pseudospin-lattice coupling. Here, we report on the synthesis of a magnetic superlattice comprising the single (n=1) and the double (n=2) layer members of the Ruddlesden-Popper series iridates Srn+1IrnO3n+1 alternating along the c axis, and provide a comprehensive study of its lattice and magnetic structures using scanning transmission electron microscopy, resonant elastic and inelastic x-ray scattering, third harmonic generation measurements, and Raman spectroscopy. The superlattice is free of the structural distortions reported for the parent phases and has a higher point group symmetry, while preserving the magnetic orders and pseudospin dynamics inherited from the parent phases, featuring two magnetic transitions with two symmetry-distinct orders. We infer weaker pseudospin-lattice coupling from the analysis of Raman spectra and attribute it to frustrated magnetic-elastic couplings. Thus, the superlattice expresses a near ideal network of effective spin-one-half moments on a square lattice.

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    • Received 27 September 2021
    • Accepted 18 February 2022

    DOI:https://doi.org/10.1103/PhysRevResearch.4.013229

    Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.

    Published by the American Physical Society

    Physics Subject Headings (PhySH)

    1. Research Areas
    Frustrated magnetismMagnetoelastic effectSpin-orbit coupling
    1. Physical Systems
    IridatesSuperlatticesTwo-dimensional electron system
    1. Techniques
    Light scatteringX-ray resonant magnetic scattering
    Condensed Matter, Materials & Applied Physics

    Authors & Affiliations

    Hoon Kim1,2,*, Joel Bertinshaw3,*, J. Porras3, B. Keimer3, Jungho Kim4, J.-W. Kim4, Jimin Kim1,2, Jonghwan Kim2,5, Gahee Noh5, Gi-Yeop Kim5, Si-Young Choi5, and B. J. Kim1,2,†

    • 1Department of Physics, Pohang University of Science and Technology, Pohang 790-784, Republic of Korea
    • 2Center for Artificial Low Dimensional Electronic Systems, Institute for Basic Science (IBS), 77 Cheongam-Ro, Pohang 37673, South Korea
    • 3Max Planck Institute for Solid State Research, Heisenbergstraße 1, D-70569 Stuttgart, Germany
    • 4Advanced Photon Source, Argonne National Laboratory 9700 Cass Ave, Lemont, Illinois 60439, USA
    • 5Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang 37673, South Korea

    • *These authors contributed equally to this paper.
    • †Author to whom correspondence should be addressed; bjkim6@postech.ac.kr

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    Vol. 4, Iss. 1 — March - May 2022

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    Images

    • Figure 1
      Figure 1

      Stacking pattern of the superlattice as imaged by STEM. (a) Wide field-of-view STEM image along [100] projection. The alternation between single layers (blue) and double layers (orange) is well maintained over the entire field of view. (b) Magnified HAADF-STEM image with single layer (blue) and double layer (orange) indicated. (c) A structural model for the superlattice. The single layer and double layer are shifted by a half unit cell on the SrO planes. IrO6 octahedra are rotated about the c axis as in the parent compounds. (d) [110]- and (e) [100]-projected HAADF (left)- and ABF (right)-STEM images overlaid with the atom positions (Sr, grey; Ir, blue, orange; O, red) from the model.

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    • Figure 2
      Figure 2

      RXD on the superlattice at the Ir L3-edge. Sharp (a) (0 0 L) and (b) (0 2 L) charge reflections are centered at integer-L values (dashed lines), indicating the superlattice is well ordered across the bulk of the sample. Minor impurity peaks that match Sr2IrO4 and Sr3Ir2O7 are marked by cyan and orange sticks, respectively. Miller indices are in the orthorhombic notation; i.e., reciprocal lattice vectors corresponding to the unit cell shown in Fig. 1.

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    • Figure 3
      Figure 3

      RA-THG patterns of the superlattice (open circles) taken under (a) PP, (b) PS, (c) SP, (d) SS geometries. Incident 1200 nm light was used at room temperature. The third harmonic 400 nm light was collected as a function of azimuth angle while the scattering plane rotates about c axis [36, 37]. The THG signals are normalized by the PP trace, overlaid with the best fits to bulk electric dipole induced THG tensor of 4/mmm point group (navy lines).

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    • Figure 4
      Figure 4

      Magnetometry of the superlattice. (a) The superlattice magnetic order consistent with our data. (b) Field-cooled M-T curves of the superlattice (navy), Sr2IrO4 (cyan) and Sr3Ir2O7 (orange). (c) M-H hysteresis at 5 K comparing the superlattice (navy) and the bulk Sr2IrO4 (cyan). For a direct comparison, Sr2IrO4 curves are multiplied by the mass proportion (≈0.36) of SL in the superlattice. (d) M-T curves measured with fields applied along [100] and [110].

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    • Figure 5
      Figure 5

      Magnetic RXD study on the superlattice. (a) Magnetic (1 0 L) reflections appear at every integer L value, contributed by signals from both SL and DL. (b) (1 0 L) scans measured at every 5 K upon heating from 200 K to 245 K. (1 0 21) reflection dominated by SL disappears around T=220 K. (c) At 250 K, the intensity modulation along (1 0 L) coincides with DL structure factor squared (dotted line), indicating that the magnetic intensities are dominated by DL. (d) Temperature dependence of (1 0 8) and (1 0 10) reveal two magnetic transitions at TNA = 220 K and TNB = 280 K. (e) Polarization analysis to separate AFM signals from in-plane (blue) and out-of-plane (orange) moments.

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    • Figure 6
      Figure 6

      Magnetic excitations in the superlattice. (a) RIXS map measured at T=10 K along high-symmetry directions indicate that the SL and DL modes follow the dispersions of the parent systems (plotted with markers). (b) Spectra at select q points. The fitted peaks (solid lines) are compared with those of the parent systems (markers). (c) The superlattice spectrum at the zone corner (π,0) (navy line) is well reproduced by a linear sum of Sr2IrO4 and Sr3Ir2O7 spectra (black line). (d) The two-magnon Raman spectrum (navy dots), measured in the B2g channel at T=15 K, is also well approximated by summing the Sr2IrO4 and Sr3Ir2O7 spectral intensities (black line).

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    • Figure 7
      Figure 7

      Pseudospin-lattice coupling estimated by Raman spectroscopy. (a) The magnetic mode at the zone center is observed in B2g scattering geometry (superlattice, navy; Sr2IrO4, blue gray). (b) The temperature-dependent component of the spin-wave gap Δ(T) extracted from Raman spectra in (a). The trend of the superlattice and Sr2IrO4 share the same functional form A1−T/TN+B, where A is the offset in the log-log plot and proportional to the strength of PLC. Temperature dependence of A1g phonons in (c) Sr3Ir2O7 and (d) the superlattice. (e) Integrated intensity of the lower energy A1g phonon. The spin-dependent component scales with the ordered moment squared MAFM2, and dashed lines are fits to functional form of 1−(T/TN), whose slopes are proportional to the PLC strength Λ. All Raman spectra are corrected for laser heating and Bose factors.

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    • Figure 8
      Figure 8

      Crystal structure of Sr2IrO4. (a) Crystal structure along [110] projection. Each layer is shifted by a half unit cell on the SrO planes. (b) HAADF-STEM image (left) and ABF-STEM image (right) in the [110] projection show the highly ordered stacking pattern. (c) [110]- and (d) [100]-projected HAADF (left)- and ABF (right)-STEM images overlaid with the atom positions (Sr, grey; Ir, blue; O, red).

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    • Figure 9
      Figure 9

      Crystal structure of Sr3Ir2O7. (a) Crystal structure along [110] projection. Each layer is shifted by a half unit cell on the SrO planes. (b) HAADF-STEM image (left) and ABF-STEM image (right) in the [110] projection show the highly ordered stacking pattern. (c) [110]- and (d) [100]-projected HAADF (left)- and ABF (right)-STEM images overlaid with the atom positions (Sr, grey; Ir, orange; O, red).

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    • Figure 10
      Figure 10

      Superlattice RA-THG patterns (open circles) in (a) PP, (b) PS, (c) SP, and (d) SS geometries, and their best fits to different point groups. The third harmonic light was collected on a 2D detector (left images) following the experimental schemes in Refs. [36, 37]. The THG signals are extracted from the 2D images and normalized by the maximum intensity of the PP trace (right plots), overlaid with the best fits to bulk electric dipole induced THG tensors of 4/mmm (green), mmm (yellow) and 4/m (red) point groups.

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    • Figure 11
      Figure 11

      Short range AFM order in SL above TNA = 220 K. (a) Temperature dependence of (H 0 21) scan, where double-layer contribution nearly vanishes. Each scan is acquired at every 5 K upon heating. (b) RIXS spectra below (blue) and above (red) TNA. (c) High resolution RIXS spectra near (π,π) magnetic zone center. Inset: Measured q points are marked by circles and enclosed in an ellipse in panels (b) and (c), respectively.

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    • Figure 12
      Figure 12

      (a) Superlattice magnetic structure with induced magnetic moments (green) based on the representation analysis. The net ferromagnetic moments in single layers (navy) can incline the c-axis AFM in double layers (orange), giving rise to small net in-plane moments in them (green). Grey spheres are Sr ions, and oxygens are indicated by octahedral cages. (b) The Wyckoff positions of the iridium ions in the single-layer (navy) and double-layer (orange) viewed from above. (c) The induced magnetic structure viewed from above. Plus (or minus) symbols in the double-layer indicate c-axis moments pointing upward (or downward).

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