Research ArticleMATERIALS SCIENCE

Nanoscale ferroelastic twins formed in strained LaCoO3 films

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Science Advances  29 Mar 2019:
Vol. 5, no. 3, eaav5050
DOI: 10.1126/sciadv.aav5050
  • Fig. 1 1D ferroelastic twinning domain is observed in an LCO thin film.

    (A) Schematic of 1D periodic twin domains. Twinning domains constitute a spatially unidirectional structural modulation along the [100] orientation. The domains are parallel to the step-edge direction of substrate. α and β are the miscut direction and miscut angle for a vicinal substrate, respectively. The inset shows a sketch of the monoclinically distorted LCO lattice at the ferroelastic domain wall. Notably, the tilt angle between two twin domains is γ = 2.2 ± 0.1°, derived from x-ray diffraction measurements (extended data fig. S1). (B) Top view of the stripe-like ferroelastic twin domains. Two different colors represent differently oriented ferroelastic domains with an average periodicity ζ. (C) Reciprocal space map (RSM) of an LCO film around the substrate’s 103 reflection. RSMs are recorded by azimuthally rotating the sample with a step size of 90° with respect to the surface’s normal. The LCO films have a monoclinically distorted lattice structure evidenced by the different qz spacing between the film’s peak and substrate’s peak. Two splitting satellite reflections at the same qz are shown for h03 reflections but are absent for 0k3 reflections. (D) Rocking curve scans around the LCO 002 reflections as a function of the in-plane rotation angle ϕ in a step of 10°. The real-space reflection angles are transformed into the reciprocal space wave vectors, from which we calculated ζ = 1/Δqx ~ 10 nm. A cosine modulation of the satellite peak position indicates that the domain structure is strictly aligned perpendicular to the [100] orientation.

  • Fig. 2 Orbital polarization modulated by unidirectional structural distortions.

    (A). Schematic of the scattering geometry for XAS and XLD measurements with the x-ray beam aligned parallel to the (100) and (010) scattering planes. (B). XAS of an LCO film for the Co L-edge measured by the out-of-plane (Ioop, solid lines, Eoop//[001]) and in-plane (Iip, dashed lines, Eip//[100] or [010]) linearly polarized x-ray beams. (C) XLD of an LCO film for the Co L-edges. The XLD spectra indicate that the hole occupancy in the d3z2r2 orbital is larger than that of the dx2y2 orbital for both measuring configurations. The degree of orbital polarization in the (010) plane is about two times larger than that in the (100) plane, indicating a clear anisotropic orbital occupancy induced by 1D twin domains. All spectra are collected and repeated more than four times with bulk-sensitive FY detection mode at 10 K.

  • Fig. 3 In-plane magnetic anisotropy.

    (A) M(H) hysteresis loops and (B) M(T) curves measured from an LCO film with H applied along the [010], [100], and [001] directions. For the M(T) curves, the cooling field was set at 0.1 T, and the measurements were carried out during the sample warm up under a magnetic field of 0.1 T. The inset of (B) shows the angular dependence of the in-plane magnetization at 10 and 70 K under a magnetic field of 1 kOe. ϕ is the in-plane rotation angle with ϕ = 0° is H // [100] and ϕ = 90° is H // [010]. The in-plane magnetization is strongly modulated by the twin domain arrangement. (C to E) Schematic energy-level diagrams of Co3+ low-spin (LS), intermediate-spin (IS), and high-spin (HS) state configurations, respectively.

  • Fig. 4 In-plane magnetic anisotropy enforced by the orthorhombic substrate.

    (A) The top view lattice structure of orthorhombic (110)-oriented NGO. The in-plane lattice parameter along the [1¯10] orientation is larger than that along the [001] orientation, leading to in-plane anisotropic shear strain in LCO films. (B) XMCD spectra for Co L-edge at 10 K with a magnetic field of 5 T applied along the [1¯10] and [001] orientations. The XMCD signals were calculated from the difference between the μ+ and μ divided by their sum, as described by (μ+ − μ)/(μ+ + μ), where μ+ and μ denote XAS obtained from the right- and left-hand circular polarized photons, respectively. (C) Nuclear scattering length density (nSLD) and (D) magnetic moment (derived from the magnetic scattering length density, mSLD) depth profiles of an LCO film. The inset of (C) shows the schematic of the sample geometry. The LCO film was grown on a NGO substrate and then capped with a STO thin layer to prevent loss of oxygen at the LCO surface. PNR measurements were performed at 10 K after field cooling in 3 T. The magnetic field was applied along the [1¯10] and [001] orientations, respectively.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/5/3/eaav5050/DC1

    Fig. S1. Structural properties of an LCO film.

    Fig. S2. Topography of an LCO film capped with an STO ultrathin layer.

    Fig. S3. Evolution of the 1D twin domain in LCO films grown on STO substrates with different miscut angles.

    Fig. S4. Thickness-dependent twin domain periodicity in LCO films.

    Fig. S5. 1D twin domain in LCO films grown on NGO substrates.

    Fig. S6. In-plane magnetic anisotropy in LCO films grown on NGO substrate.

    Fig. S7. Checkerboard-like twin domains observed in LCO films on LSAT substrates.

    Fig. S8. Magnetic properties of LCO film on LSAT substrate with checkerboard-like twin domains.

    Fig. S9. Anisotropic electronic states in LCO films on LSAT substrates.

    Fig. S10. Comparison of the in-plane magnetic anisotropy in LCO films with and without (w.o.) 1D twin domains.

  • Supplementary Materials

    This PDF file includes:

    • Fig. S1. Structural properties of an LCO film.
    • Fig. S2. Topography of an LCO film capped with an STO ultrathin layer.
    • Fig. S3. Evolution of the 1D twin domain in LCO films grown on STO substrates with different miscut angles.
    • Fig. S4. Thickness-dependent twin domain periodicity in LCO films.
    • Fig. S5. 1D twin domain in LCO films grown on NGO substrates.
    • Fig. S6. In-plane magnetic anisotropy in LCO films grown on NGO substrate.
    • Fig. S7. Checkerboard-like twin domains observed in LCO films on LSAT substrates.
    • Fig. S8. Magnetic properties of LCO film on LSAT substrate with checkerboard-like twin domains.
    • Fig. S9. Anisotropic electronic states in LCO films on LSAT substrates.
    • Fig. S10. Comparison of the in-plane magnetic anisotropy in LCO films with and without (w.o.) 1D twin domains.

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