Role of polar compensation in interfacial ferromagnetism of ${\mathrm{LaNiO}}_3/{\mathrm{CaMnO}}_3$ superlattices

Polar compensation can play an important role in the determination of interfacial electronic and magnetic properties in oxide heterostructures. Using x-ray absorption spectroscopy, x-ray magnetic circular dichroism, bulk magnetometry, and transport measurements, we find that interfacial charge redistribution via polar compensation is essential for explaining the evolution of interfacial ferromagnetism in ${\mathrm{LaNiO}}_3/{\mathrm{CaMnO}}_3$ superlattices as a function of ${\mathrm{LaNiO}}_3$ layer thickness. In insulating superlattices (four unit cells or less of ${\mathrm{LaNiO}}_3$), magnetism is dominated by Ni–Mn superexchange, while itinerant electron-based Mn–Mn double exchange plays a role in thicker metallic superlattices. X-ray magnetic circular dichroism and resonant x-ray scattering show that Ni–Mn superexchange contributes to the magnetization even in metallic superlattices. This Ni–Mn superexchange interaction can be explained in terms of polar compensation at the ${\mathrm{LaNiO}}_3–{\mathrm{CaMnO}}_3$ interface. These results highlight the different mechanisms responsible for interfacial ferromagnetism and the importance of understanding compensation due to polar mismatch at oxide-based interfaces when engineering magnetic properties.

Figure 1

(a) Schematic of ${[{\left({\mathrm{LaNiO}}_3\right)}_N/{\left({\mathrm{CaMnO}}_3\right)}_M]}_{L=10}$ superlattices on ${\mathrm{LaAlO}}_3$ substrates showing net charge of oxide layers. (b) $2\theta -\theta$ scan of an $N=6, M=4$ superlattice around the (002) LAO peak. Superlattice Bragg peaks and superlattice period thickness fringes are clearly seen, indicating high structural quality.

Figure 2

LNO layer thickness dependence of (${\mathrm{LNO})}_N/{\left(\mathrm{CMO}\right)}_4$ superlattice saturated magnetic moment at 2 T and 10 K. Note that even below the LNO metal–insulator transition a small ferromagnetic contribution remains. The background colors indicate the shift from primarily ${\mathrm{Ni}}^{2+}-{\mathrm{Mn}}^{4+}$ superexchange at low $N$ to ${\mathrm{Mn}}^{3+}-{\mathrm{Mn}}^{4+}$ double exchange at high $N$.

Figure 3

Temperature dependence from 10–200 K of superlattice resistivity for $N=2$–6 superlattices. Included is temperature dependence from 5–200 K of LNO thin film resistivity for comparison. A metal–insulator transition at $N=4$ and gradual approach to bulk LNO value is observed, consistent with previous results [12, 14].

Figure 4

(a) X-ray absorption spectrum of Ni $L$ edge at 30 K for an $N=2, M=4$ superlattice. (b) Corresponding Ni $L$ edge x-ray magnetic circular dichroism. (c) Mn $L$ edge x-ray absorption for the same superlattice. (d) Mn x-ray magnetic circular dichroism. (e) Ni $L$ edge x-ray absorption spectra of $N=2$–8 superlattices with La $M_4$ background subtracted (solid lines). Ni $L$ edge reference spectra of NiO (dashed gray line) (top) and an LNO thin film (dashed gray line) (bottom) shown for comparison. The shift in the $L_2$ edge is highlighted with the dashed black line.

Figure 5

(a) Specular x-ray reflectometry scan showing nonresonant (1000 eV) and Ni $L_2$ resonant (871 eV) reflectivity spectra of an $N=8$ superlattice. (b) Energy scans of (001) (solid black) and (002) (dash-dot blue) superlattice Bragg peaks. Simultaneously measured $N=8$ superlattice Ni $L_2$ XAS (dash red) and NiO Ni $L_2$ XAS (dot gray) are included for comparison.