Charge transfer and interfacial magnetism in (LaNiO${}_3$)${}_n$/(LaMnO${}_3$)${}_2$ superlattices

(LaNiO${}_3$)${}_n$/(LaMnO${}_3$)${}_2$ superlattices were grown using ozone-assisted molecular beam epitaxy, where LaNiO${}_3$ is a paramagnetic metal and LaMnO${}_3$ is an antiferromagnetic insulator. The superlattices exhibit excellent crystallinity and interfacial roughness of less than one unit cell. X-ray spectroscopy and dichroism measurements indicate that electrons are transferred from the LaMnO${}_3$ to the LaNiO${}_3$, inducing magnetism in LaNiO${}_3$. Magnetotransport measurements reveal a transition from metallic to insulating behavior as the LaNiO${}_3$ layer thickness is reduced from five to two unit cells and suggest an inhomogeneous magnetic structure within LaNiO${}_3$.

Figure 1

X-ray reflectivity (a) and high-resolution x-ray diffraction (b) scans for a series of [(LaNiO${}_3$)${}_n$/(LaMnO${}_3$)${}_2$]${}_m$ superlattices on SrTiO${}_3$. The $c$-axis lattice parameters are found to be 0.3847, 0.3824, 0.3823, and 0.3816 nm for $n=2,3,4,5$, respectively.

Figure 2

(a) Temperature dependence of the magnetization for [(LaNiO${}_3$)${}_n$/(LaMnO${}_3$)${}_2$]${}_m$ superlattices with $n=2$–5 and 80-unit-cell-thick films of LMO and LNO. Solid and open symbols show field-cooled and zero-field-cooled measurements, respectively. The field is applied in the plane of the film along the [100] direction. (b) Variation with number of LNO layers of the remanent magnetization (open symbols) and saturation magnetization (filled symbols). (c) Coercive field variation with number of LNO layers.

Figure 3

(a) Mn L${}_3$ and (b) Ni L${}_3$ XAS and XMCD spectra for [(LaNiO${}_3$)${}_2$/(LaMnO${}_3$)${}_2$]${}_{20}$ and [(LaNiO${}_3$)${}_4$/(LaMnO${}_3$)${}_2$]${}_{13}$ superlattices. Mn${}^{3+}$ and Mn${}^{4+}$ references adapted from Ref. 25. (c) and (d) Cation-resolved magnetization as a function of field for the superlattices with $n=2$ and 4, along with average magnetization (weighted by concentration of Mn and Ni) and magnetization obtained by SQUID magnetometry.

Figure 4

Magnetotransport behavior of [(LaNiO${}_3$)${}_n$/(LaMnO${}_3$)${}_2$]${}_m$ superlattices, $2\le n\le 5$. (a) Temperature dependence of longitudinal resistivity. The arrows indicate positions of the resistivity minima at $T=90$ K $(n=4)$ and $T=30$ K $(n=5)$. (b) Resistivity of a pure LaMnO${}_3$ film with a fit to thermally activated transport. (c) Fit to 2D variable-range hopping for $n=2$ for 60 K $\le T\le$ 300 K. (d) Magnetoresistance ratio (MRR) of $n=3$, 4, and 5 superlattices at $T=5$ K with the field applied normal to the film.

Figure 5

(a) Ordinary Hall coefficient as a function of temperature for $n=4$ and 5 superlattices. (b) Temperature dependence of the anomalous Hall resistivity for the $n=4$ and 5 superlattices. (c) Linear variation of the anomalous Hall resistivity with in-plane magnetization.