Structural “$\delta$ Doping” to Control Local Magnetization in Isovalent Oxide Heterostructures

Modulation and $\delta$-doping strategies, in which atomically thin layers of charged dopants are precisely deposited within a heterostructure, have played enabling roles in the discovery of new physical behavior in electronic materials. Here, we demonstrate a purely structural “$\delta$-doping” strategy in complex oxide heterostructures, in which atomically thin manganite layers are inserted into an isovalent manganite host, thereby modifying the local rotations of corner-connected ${\mathrm{MnO}}_6$ octahedra. Combining scanning transmission electron microscopy, polarized neutron reflectometry, and density functional theory, we reveal how local magnetic exchange interactions are enhanced within the spatially confined regions of suppressed octahedral rotations. The combined experimental and theoretical results illustrate the potential to utilize noncharge-based approaches to “doping” in order to enhance or suppress functional properties within spatially confined regions of oxide heterostructures.

Figure 1

(a) X-ray reflectivity data (red circles) and fit (black line) using GenX software [39] of the $n=2$ superlattice. Inset: Scattering length density profile obtained from the fit. (b) A low-magnification STEM HAADF image of the superlattice. (c) A high-magnification STEM HAADF image of the superlattice, viewed from the pseudocubic [110] direction.

Figure 2

(a) Representative STEM ABF images of the LCMO and LSMO layers in the superlattice, viewed from two pseudocubic [110] directions that are 90° away ($[110{]}_{\mathrm{pc}}A$ and $[110{]}_{\mathrm{pc}}B$). The proposed polyhedral models are shown overlapping the image. (b) The layer-resolved projected inclination angles of the ${\mathrm{MnO}}_6$ octahedra, viewed from the $[110{]}_{\mathrm{pc}}A$ direction, revealing an increased $\mathrm{Mn}─\mathrm{O}─\mathrm{Mn}$ bond angle within the 2-uc LSMO layer. The error bars in (b) indicate the standard deviation obtained from the measurement of inclination angles averaged within each plane.

Figure 3

(a) DFT-optimized structure of a ${(\mathrm{LSMO})}_2/{(\mathrm{LCMO})}_6$ superlattice confirms the reduction of the octahedral rotations in the LSMO region. Here, ${\theta }_{\mathrm{in}}$ (${\theta }_{\mathrm{out}}$) indicates the in-plane (out-of-plane) $\mathrm{Mn}─\mathrm{O}─\mathrm{Mn}$ bond angles. (b) In-plane $J_{\mathrm{in}}$ (via ${\mathrm{Mn}}_1{\text{−}\mathrm{O}}_p\text{−}{\mathrm{Mn}}_2$) and out-of-plane $J_{\mathrm{out}}$ (via ${\mathrm{Mn}}_1{\text{−}\mathrm{O}}_{\mathrm{ap}}\text{−}{\mathrm{Mn}}_2$) nearest-neighbor isotropic exchange interactions shown for the LCMO and LSMO region.

Figure 4

(a) Polarized neutron reflectivity data and fits multiplied by $Q^4$ from the $n=2$ sample obtained at 50 (top) and 250 K (bottom) with a 0.5 T field. Green and magenta symbols indicate the reflectivity measured with neutrons polarized parallel ($R^{++}$) and antiparallel ($R^{--}$) to the sample magnetization, respectively. The blue box highlights the superlattice Bragg peak. (b) The corresponding fits (in red) to the spin asymmetry, defined as the difference in $R^{++}$ and $R^{--}$ divided by the sum. Error bars in (a) and (b) indicate $\pm 1$ standard deviation. (c) Magnetic depth profiles obtained at both temperatures. Orange enclosed regions indicate the 2-uc LSMO layers. The obtained magnetic depth profile demonstrates that the magnetization is enhanced within the LSMO layers which exhibit reduced octahedral rotations but the same Mn valence as the LCMO layers. (d) The average magnetization within the LSMO and LCMO layers as a function of the temperature is indicative of a higher Curie temperature within the 2-uc LSMO layers compared to the 20-uc LCMO layers.