Restoring the magnetism of ultrathin $\mathrm{LaMn}{\mathrm{O}}_3$ films by surface symmetry engineering

The frustration of magnetization and conductivity properties of ultrathin manganite is detrimental to their device performance, preventing their scaling down process. Here we demonstrate that the magnetism of ultrathin $\mathrm{LaMn}{\mathrm{O}}_3$ films can be restored by a $\mathrm{SrTi}{\mathrm{O}}_3$ capping layer, which engineers the surface from a symmetry breaking induced out-of-plane orbital occupancy to the recovered in-plane orbital occupancy. The stabilized in-plane orbital occupancy would strengthen the intralayer double exchange and thus recovers the robust magnetism. This method is proved to be effective for films as thin as 2 unit cells, greatly shrinking the critical thickness of 6 unit cells for ferromagnetic $\mathrm{LaMn}{\mathrm{O}}_3$ as demonstrated previously [Wang et al., Science 349, 716 (2015)]. The achievement made in this work opens up new perspectives to an active control of surface states and thereby tailors the surface functional properties of transition metal oxides.

Figure 1

(a)–(e) $M-H$ curves at 10 K after zero-field cooling from room temperature for a variety of bare $\mathrm{LaMn}{\mathrm{O}}_3$ films and those heterostructures with the same thickness LMO capped by 8 u.c. STO on STO substrate. Only the range from −5 to 5 kOe is shown to clarify the picture, although they are measured from −30 to 30 kOe. (f) A summary of the enhancement ratio by STO capping.

Figure 2

XAS spectra measured with linearly polarized light for $\mathrm{LaMn}{\mathrm{O}}_3$ films with different thickness: (a) 4 u.c. (b) 6 u.c., (c) 8 u.c., and (d) 12 u.c.. The normalized difference spectra $\left[I_{\sigma }\left(E\right)-I_{\pi }\left(E\right)\right]$ are multiplied by a factor of 5 to make them clearer and are shown directly below the corresponding spectra. (e) A summary of the absolute value of $A_{\mathrm{XLD}}$ dependent on film thickness and film thickness dependent magnetic moments.

Figure 3

(a) A sketch of LMO single layer, with the surface symmetry breaking inducing a preferential occupation of out-of-plane oriented orbit. Yellow layers account for $\mathrm{Mn}{\mathrm{O}}_2$ planes while light green layers account for LaO planes. (b) XLD spectrum of LMO single layer. (c) A sketch of capped LMO heterostructure with STO on top, with the recovered surface symmetry and a preferential occupation of in-plane oriented orbits as stabilized by tensile strain. Yellow layers account for $\mathrm{Mn}{\mathrm{O}}_2$ planes and light green layers account for LaO planes as described above, while the cyan layer presents the capping layer. (d) The XLD signal of capped LMO layer.

Figure 4

$M-H$ curves for a 6 u.c. LMO single layer and those capped by 4, 8, and 12 u.c. STO. (b) $M-H$ curves for a 6 u.c. single LMO layer and those capped by 8 u.c. STO and LAO. (c) $M-T$ curves for a 6 u.c. single LMO layer and those capped by 8 u.c. STO and LAO. (d) XLD spectral of a LMO layer capped by 8 u.c. LAO. The measurements were performed from −30 to 30 kOe, but only the expanded range from −5 kOe to 5 kOe is shown for clarity.

Figure 5

(a) Mn $L$-edge spectra of a bare LMO layer and those capped by 8 u.c. STO and LAO. (b) Ti $L$-edge spectra of a bare LMO layer and those capped by STO with different thicknesses.

Figure 6

(a) XAS spectra measured with linearly polarized light for $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ (LSMO) films (a) without overlayer and (b) with 8 u.c. STO overlayer. The normalized difference spectra $\left[I_{\sigma }\left(E\right)-I_{\pi }\left(E\right)\right]$ are multiplied by a factor of 3 to make them clearer and are shown directly below the corresponding spectra. (c) $M-H$ curves of $\mathrm{L}{\mathrm{a}}_{2/3}\mathrm{S}{\mathrm{r}}_{1/3}\mathrm{Mn}{\mathrm{O}}_3$ films with or without STO overlayer.