Electron Accumulation and Emergent Magnetism in ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ Heterostructures

Emergent phenomena at polar-nonpolar oxide interfaces have been studied intensely in pursuit of next-generation oxide electronics and spintronics. Here we report the disentanglement of critical thicknesses for electron reconstruction and the emergence of ferromagnetism in polar-mismatched ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ (001) heterostructures. Using a combination of element-specific x-ray absorption spectroscopy and dichroism, and first-principles calculations, interfacial electron accumulation, and ferromagnetism have been observed within the polar, antiferromagnetic insulator ${\mathrm{LaMnO}}_3$. Our results show that the critical thickness for the onset of electron accumulation is as thin as 2 unit cells (UC), significantly thinner than the observed critical thickness for ferromagnetism of 5 UC. The absence of ferromagnetism below 5 UC is likely induced by electron overaccumulation. In turn, by controlling the doping of the ${\mathrm{LaMnO}}_3$, we are able to neutralize the excessive electrons from the polar mismatch in ultrathin ${\mathrm{LaMnO}}_3$ films and thus enable ferromagnetism in films as thin as 3 UC, extending the limits of our ability to synthesize and tailor emergent phenomena at interfaces and demonstrating manipulation of the electronic and magnetic structures of materials at the shortest length scales.

Figure 1

(a) Mn XMCD spectra for various ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ heterostructures. The inset shows a schematic of the experimental configurations for the x-ray spectroscopy studies. Thickness dependence of the XAS spectra of the ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ heterostructures at the (b) Ti $L_{2,3}$, and (c) Mn $L_{2,3}$ edges with reference spectra for ${\mathrm{SrTiO}}_3$, bulk ${\mathrm{SrMnO}}_3$, ${\mathrm{LaMnO}}_3$, and MnO are shown for comparison. (d) XAS of Mn $L_{2,3}$ edges for various ${\mathrm{LaMnO}}_3/{\mathrm{NdGaO}}_3$ heterostructures where no reconstruction has occurred.

Figure 2

(a) Schematic of the symmetric, vacuum-terminated ${({\mathrm{LaMnO}}_3)}_m/{({\mathrm{SrTiO}}_3)}_n/{({\mathrm{LaMnO}}_3)}_m$ slab with two identical $n$-type interfaces (here $m=4$ and $n=4.5$). (b) The in-plane average (oscillating blue line) and macroscopic average (red line) electrostatic potential across the ${({\mathrm{LaMnO}}_3)}_4/{({\mathrm{SrTiO}}_3)}_{4.5}/{({\mathrm{LaMnO}}_3)}_4$ simulation slab. (c) Comparison of the macroscopic average potential between the ${({\mathrm{LaMnO}}_3)}_4/{({\mathrm{SrTiO}}_3)}_{4.5}/{({\mathrm{LaMnO}}_3)}_4$ and ${({\mathrm{LaMnO}}_3)}_2/{({\mathrm{SrTiO}}_3)}_{4.5}/{({\mathrm{LaMnO}}_3)}_2$ heterostructures; the average intrinsic electric field is indicated by the black lines with a value of $0.177\mathrm{V}/\AA$. (d) Schematic band diagram of ${\mathrm{LaMnO}}_3/{\mathrm{SrTiO}}_3$ interface.

Figure 3

(a) Mn $L_{2,3}$ XAS spectra of the ${\mathrm{La}}_{0.95}{\mathrm{MnO}}_3/{\mathrm{SrTiO}}_3$ heterostructures. (b) Mn XMCD spectra for the 3 UC ${\mathrm{La}}_{0.95}{\mathrm{MnO}}_3/{\mathrm{SrTiO}}_3$ heterostructure. The XMCD features reverse in sign when the magnetic field is reversed, confirming the reliability of the observation.