Structural phase transition and spontaneous interface reconstruction in La${}_{2/3}$Ca${}_{1/3}$MnO${}_3$/BaTiO${}_3$ superlattices

(La${}_{2/3}$Ca${}_{1/3}$MnO${}_3$)${}_n$/(BaTiO${}_3$)${}_m$ (LCMO${}_n$/BTO${}_m$) superlattices on MgO and SrTiO${}_3$ substrates with different layer thicknesses ($n$ $=$ 10, 38, 40 and $m$ $=$ 5, 18, 20) have been grown by metal organic aerosol deposition (MAD) and have been fully characterized down to the atomic scale to study the interface characteristics. Scanning transmission electron microscopy combined with spatially resolved electron energy-loss spectroscopy provides clear evidence for the existence of atomically sharp interfaces in MAD grown films, which exhibit epitaxial growth conditions, a uniform normal strain, and a fully oxidized state. Below a critical layer thickness the LCMO structure is found to change from the bulk Pnma symmetry to a pseudocubic $R$$\overline{3}$$c$ symmetry. An atomically flat interface reconstruction consisting of a single Ca-rich atomic layer is observed on the compressively strained BTO on LCMO interface, which is thought to partially neutralize the total charge from the alternating polar atomic layers in LCMO as well as relieving strain at the interface. No interface reconstruction is observed at the tensile strained LCMO on BTO interface.

Figure 1

Cross-sectional electron diffraction and high resolution TEM images of (a) a LCMO${}_{10}$/BTO${}_5$ multilayer grown on MgO(100), (b) a LCMO${}_{38}$/BTO${}_{18}$ multilayer grown on STO(100), and (c) a LCMO${}_{40}$/BTO${}_{20}$ multilayer grown on MgO(100), imaged along the [001] zone axis for STO/MgO. The arrowheads indicate the apparent layer boundaries. Below the $d_c$ (LCMO${}_{10}$/BTO${}_5$ and LCMO${}_{38}$/BTO${}_{18}$) the LCMO structure has pseudocubic $R$$\overline{3}$$c$ symmetry on both STO and MgO. Above $d_c$ (LCMO${}_{40}$/BTO${}_{20}$) the LCMO structure has the bulk Pnma symmetry on both STO and MgO, as evidenced by the extra reflections in the electron diffraction pattern. The epitaxial relationship is (100)${}_{\mathrm{BTO}}$‖(100)${}_{\mathrm{LCMO}}$‖(100)${}_{\mathrm{STO}/\mathrm{MgO}}$ and [001]${}_{\mathrm{BTO}}$‖[001]${}_{\mathrm{LCMO}}$‖[001]${}_{\mathrm{STO}/\mathrm{MgO}}$ in the pseudocubic notation.

Figure 2

HRTEM GPA strain analysis maps of the strain component parallel to the interface ($g_{200}$) and perpendicular to the interface ($g_{020}$) for (a) LCMO${}_{10}$/BTO${}_5$ grown on MgO(100) and (b) LCMO${}_{38}$/BTO${}_{18}$ grown on STO(100). In the case of MgO periodical misfit dislocations are present to accommodate the lattice mismatch. LCMO${}_{38}$/BTO${}_{18}$ on STO(100), with $d_{\mathrm{LCMO}}$ < $d_c$, still shows coherently strained LCMO and BTO layers with low residual in-plane strain due to lattice misfit. In both SLs the LCMO has an altered pseudocubic $R$$\overline{3}$$c$ structure.

Figure 3

Temperature dependencies of the resistance for the (a) LCMO${}_{38}$/BTO${}_{18}$ SL grown on STO, and for (b) LCMO${}_{40}$/BTO${}_{20}$ on MgO. The insets demonstrate the corresponding temperature behavior of the capacitance. (c) Hysteretic switching of the capacitance in applied magnetic field for the LCMO${}_{40}$/BTO${}_{20}$ SL grown on MgO. The curves for $T$ $=$ 63, 75, 88, and 100 K are shifted by 2 nF for clarity.

Figure 4

(a) Low magnification HAADF-STEM image of the LCMO${}_{38}$/BTO${}_{18}$ SL on STO. Alternating LCMO and BTO layers can be clearly distinguished. (b) High resolution HAADF image of both interfaces. Both A (green dots) and B cations (red dots) of the perovskite structures are resolved. The LCMO on BTO interface is relatively sharp (arrows), an atomically flat dark band of one unit layer is observed at the BTO on LCMO interface (arrows). (c) Overview STEM-EELS elemental maps of the heterostructure.

Figure 5

High resolution EELS elemental maps of barium, titanium, lanthanum, calcium, and manganese for the (a) compressively strained BTO on LCMO interface and (b) tensile strained LCMO on BTO interface. The elemental maps show a clear offset between the La and Ca lattice and single layer of Ca enrichment at the compressively strained interface (indicated by arrows), while no offset is present in for the tensile strained interface (arrows).

Figure 6

Monochromated EELS spectroscopy of the (a) BTO on LCMO interface with the Ca-rich layer between planes $a$5 and $a$6 and the (b) LCMO on BTO interface. The Ti and Mn maps confirm the sharp interfaces in the material. Line averaged Ti $L_{2,3}$ and Mn $L_{2,3}$ EELS spectra (3 pixels width) from representative B cation planes at the interface and far from the interface. No fine structure changes that could indicate a Ti (4+) or Mn (3.33+) valency change are apparent at the interface.

Figure 7

Helping to reduce the polar catastrophe by interface reconstruction. The top illustration shows a schematic stacking sequence of the observed multilayer system, where the same colors as before have been used (for visibility, the number of stacking layers has been reduced). The graphs show respectively the charge (ρ), generated electric field ($E$), and electrostatic potential ($V$) for the reconstructed (solid line) and nonreconstructed (dashed line) interface at the position of every layer. The ending CaO layer renders the total LCMO layer electrically neutral and therefore reduces the electrostatic potential.