Structural distortions at polar manganite interfaces

Electronic, lattice, and spin interactions at the interfaces between crystalline complex transition-metal oxides can give rise to a wide range of functional electronic and magnetic phenomena not found in bulk. At heterointerfaces, these interactions may be enhanced by combining oxides where the polarity changes at the interface. The physical structure between nonpolar $\mathrm{SrTi}{\mathrm{O}}_3$ and polar $\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Mn}{\mathrm{O}}_3$ ($x=0.2$) is investigated using high-resolution synchrotron x-ray diffraction to directly determine the role of structural distortions in compensating the polar discontinuity. At both the oxide-oxide interface and vacuum-oxide interfaces, the lattice is found to expand and rumple along the growth direction. The $\mathrm{SrTi}{\mathrm{O}}_3/\mathrm{L}{\mathrm{a}}_{1-x}\mathrm{S}{\mathrm{r}}_x\mathrm{Mn}{\mathrm{O}}_3$ interface also exhibits intermixing of La and Sr over a few unit cells. The results thus demonstrate that polar distortions and ionic intermixing coexist and both pathways play a significant role at interfaces with polar discontinuities.

Figure 1

(a) Crystal truncation rods (CTRs) for a nominal 10-uc $\mathrm{L}{\mathrm{a}}_{0.8}\mathrm{S}{\mathrm{r}}_{0.2}\mathrm{Mn}{\mathrm{O}}_3$ film grown on (001)-oriented $\mathrm{SrTi}{\mathrm{O}}_3$. Measured data (blue circles) and best fits (red) are shown. (b) Half-order superstructure rods for a nominal 10-uc $\mathrm{L}{\mathrm{a}}_{0.8}\mathrm{S}{\mathrm{r}}_{0.2}\mathrm{Mn}{\mathrm{O}}_3$ film grown on $\mathrm{SrTi}{\mathrm{O}}_3$. The layer-resolved octahedral rotations are determined by fits (red lines) to the measured diffraction intensities (blue circles).

Figure 2

Structural results for a nominal 10-uc $\mathrm{L}{\mathrm{a}}_{0.8}\mathrm{S}{\mathrm{r}}_{0.2}\mathrm{Mn}{\mathrm{O}}_3/\mathrm{SrTi}{\mathrm{O}}_3$ interface as a function of layer distance from the nominal film/substrate interface. The nominal film/substrate interface is at $z=0\AA$. (a) Profile of out-of-plane lattice parameter $c$ determined from the vertical distance between $A$-site (La/Sr) ions and (b) vertical displacements of oxygen ions relative to cations [$A$ site (La/Sr), $B$ site(Ti/Mn)] in the same plane. (c) Fractional ionic composition as a function of layer distance from the nominal interface.

Figure 3

Layer-resolved in-plane $B$-O-$B$ ($B=\mathrm{Ti}$, Mn) bond parameters. (a) In-plane averaged $B$-O-$B$ bond angles ${\theta }_{B-\text{O}-B}$. The inset illustrates the effect of the polar displacements on the $B$-O-$B$ bond angles in neighboring unit cells along the $x$ direction. (b) In-plane averaged bond distances $d_{B-\text{O}}$ along a $B$-O-$B$ chain. (c) The resulting one-electron bandwidth $W=\cos \left(\frac{180-{\theta }_{B-\mathrm{O}-B}}{2}\right)/d_{B\text{-O}}^{3.5}$. The dashed line indicates $W$ for bulk $\mathrm{L}{\mathrm{a}}_{0.8}\mathrm{S}{\mathrm{r}}_{0.2}\mathrm{Mn}{\mathrm{O}}_3$. (d) Bandwidth difference δ$W$ for neighboring O ions along the [010] direction.