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Interface engineering in complex oxide superlattices is a growing field, enabling manipulation of the exceptional properties of these materials, and also providing access to new phases and emergent physical phenomena. Here we demonstrate how interfacial interactions can induce a complex charge and spin structure in a bulk paramagnetic material. We investigate a superlattice (SLs) consisting of paramagnetic LaNiO3 (LNO) and highly spin-polarized ferromagnetic La2/3Ca1/3MnO3 (LCMO), grown on SrTiO3 (001) substrate. We observed emerging magnetism in LNO through an exchange bias mechanism at the interfaces in X-ray resonant magnetic reflectivity. We find non-symmetric interface induced magnetization profiles in LNO and LCMO which we relate to a periodic complex charge and spin superstructure. High resolution scanning transmission electron microscopy images reveal that the upper and lower interfaces exhibit no significant structural variations. The different long range magnetic order emerging in LNO layers demonstrates the enormous potential of interfacial reconstruction as a tool for tailored electronic properties. [ABSTRACT FROM AUTHOR]

For ABO perovskites, the magnetic and electronic properties couple strongly to the BO octahedral rotations and distortions. Therefore, precise control of the octahedral rotations and distortions via epitaxial strain and interfacial octahedral connectivity has become the key for engineering novel functionalities in ABO heterostructures and superlattices. In this paper, we investigated the local octahedral rotations in a [(1 unit cell (u.c.)//4 u.c.) × 13] LaNiO/LaGaO superlattice grown on a (001) SrTiO substrate. By using aberration-corrected high-resolution transmission electron microscopy, we found that the octahedral rotations of NiO adopted the same [100] and [010] rotational magnitudes as the neighboring GaO till the surface of the superlattice. Our results indicate that in LaNiO-based superlattices, the NiO rotations can be precisely controlled via interfacial octahedral connectivity when the thickness of the LaNiO layer is only 1 unit cell. [ABSTRACT FROM AUTHOR]

For ABO perovskites, octahedral rotations and distortions couple strongly to the functional properties. However, in short period perovskite superlattices, the characterization of the octahedral behavior remains challenging due to the local structural variations of the BO octahedra. By aberration-corrected high-resolution transmission electron microscopy, we investigated the local octahedral rotations in a [(4 unit cell (u.c.)//4 u.c.) × 8] LaNiO/LaGaO superlattice grown on a (001) SrTiO substrate. The octahedral behavior varies along the growth direction even though the superlattice is coherently strained. Near the substrate, octahedral rotations about [100] and [010] axes in the superlattice are suppressed due to the octahedral connectivity-rotational magnitudes and patterns-between the NiO and TiO octahedra. Away from the substrate, the magnitudes of [100] and [010] rotations are enhanced as a response to substrate-induced tensile strain. Near the surface of the superlattice, the [100] and [010] rotational magnitudes of NiO and GaO relax to the bulk values of LaNiO and LaGaO, respectively. Our results indicate that the response of octahedral rotations to epitaxial strain in superlattices is significantly different from that in thin films. [ABSTRACT FROM AUTHOR]

We present an x-ray orbital reflectometry and linear dichroism study of PrNiO3-PrAlO3 superlattices grown on substrates that induce either compressive or tensile strain. For superlattices under tensile strain, we observe a pronounced change of the local nickel electronic structure and a decrease of orbital polarization below the metal-insulator transition temperature, in qualitative agreement with theoretical scenarios for charge disproportionation. In contrast, the x-ray absorption spectra of the superlattice under compressive strain with suppressed metal-insulator transition show no temperature dependence at the magnetic transition, consistent with a spin density wave transition driven by epitaxial strain and spatial confinement of the conduction electrons. The layer-resolved orbital occupations indicate a linear orbital-strain coupling previously found in LaNiO3-based superlattices and suggest that, when present, the charge order encompasses the entire PrNiO3 layer stack. [ABSTRACT FROM AUTHOR]

The interfacial atomic structure of a metallic LaNiO3/LaAlO3 superlattice grown on a LaSrA1O4 substrate was investigated using a combination of atomically resolved electron energy loss spectroscopy (EELS) at the A1 K, A1 L2,3, Sr L2,3, Ni L2,3, La M4,5, and O K edges as well as hybridization mapping of selected features of the O K-edge fine structure. We observe an additional La1-xSrx Al1-yNiy,O3 layer at the substrate-superlattice interface, possibly linked to diffusion of A1 and Sr into the growing film or a surface reconstruction due to Sr segregation. The roughness of the LaNiO3/LaAlO3 interfaces is found to be on average around one pseudocubic unit cell. The O K-edge EELS spectra revealed reduced spectral weight of the prepeak derived from Ni-0 hybridized states in the LaNiO3 layers. We rule out oxygen nonstoichiometry of the LaNiO3 layers and discuss changes in the Ni-O hybridization due to heterostructuring as possible origin. [ABSTRACT FROM AUTHOR]

Using a complementary combination of x-ray diffraction and atomically resolved imaging we investigated the lattice structure of epitaxial LaNiO3/LaAlO3 superlattices grown on a compressive-strain inducing LaSrAlO4 (001) substrate. A refinement of the structure obtained from the x-ray data revealed the monoclinic I 2/c 1 1 space group. The (Ni/Al)O6 octahedral rotation angle perpendicular to the superlattice plane is enhanced, and the one parallel to the plane is reduced with respect to the corresponding bulk values. High-angle annular dark field imaging was used to determine the lattice parameters within the superlattice unit cell. High-resolution electron microscopy images of the oxygen atoms are consistent with the x-ray results. [ABSTRACT FROM AUTHOR]

A combined analysis of x-ray absorption and resonant reflectivity data was used to obtain the orbital polarization profiles of superlattices composed of four-unit-cell-thick layers of metallic LaNiO3 and layers of insulating RXO3 (R=La, Gd, Dy and X=Al, Ga, Sc), grown on substrates that impose either compressive or tensile strain. This superlattice geometry allowed us to partly separate the influence of epitaxial strain from interfacial effects controlled by the chemical composition of the insulating blocking layers. Our quantitative analysis reveals orbital polarizations up to 25%. We further show that strain is the most effective control parameter, whereas the influence of the chemical composition of the blocking layers is comparatively small. [ABSTRACT FROM AUTHOR]

We have studied the microstructure of LaNiO3-LaAlO3 superlattices using transmission electron microscopy in combination with electron energy loss spectroscopy. In superlattices grown on non-polar SrTiO3 substrates, nanometer-sized NiO precipitates form directly at the interface between the substrate and the initial LaNiO3 layer, while control measurements on polar substrates show no NiO. Because of the drastically different electronic properties of NiO and LaNiO3, such precipitates can strongly affect measurements on atomically thin LaNiO3-based films and multilayers on SrTiO3. In general, polarity-driven secondary phase formation should be carefully evaluated as a possible consequence of the polarity mismatch at metal-oxide interfaces. [ABSTRACT FROM AUTHOR]

The electronic and magnetic properties of epitaxial LaNiO3/LaAlO3 superlattices can be tuned by layer thickness and substrate-induced strain. Here, we report on direct measurements of the k-space-resolved electronic structure of buried nickelate layers in superlattices under compressive strain by soft-x-ray photoemission. After disentangling strong extrinsic contributions to the angle-dependent signal caused by photoelectron diffraction, we are able to extract Fermi surface information from our data. We find that with decreasing LaNiO3 thickness down to two unit cells (2 uc) quasiparticle coherence becomes strongly reduced, in accord with the dimension-induced metal-to-insulator transition seen in transport measurements. Nonetheless, on top of a strongly incoherent background a residual Fermi surface can be identified in the 2 uc superlattice whose nesting properties are consistent with the spin-density-wave (SDW) instability recently reported. The overall behavior of the Ni 3d spectra and the absence of a complete gap opening indicate that the SDW phase is dominated by strong order parameter fluctuations. [ABSTRACT FROM AUTHOR]

We use polarized Raman scattering to probe lattice vibrations and charge ordering in 12 nm thick, epitaxially strained PrNiO3 films, and in superlattices of PrNiO3 with the band insulator PrAlO3. A carefully adjusted confocal geometry is used to eliminate the substrate contribution to the Raman spectra. In films and superlattices under tensile strain which undergo a metal-insulator transition upon cooling, the Raman spectra reveal phonon modes characteristic of charge ordering. These anomalous phonons do not appear in compressively strained films, which remain metallic at all temperatures. For superlattices under compressive strain, the Raman spectra show no evidence of anomalous phonons indicative of charge ordering, while complementary resonant x-ray scattering experiments reveal antiferromagnetic order associated with a modest increase in resistivity upon cooling. This confirms theoretical predictions of a spin density wave phase driven by spatial confinement of the conduction electrons. [ABSTRACT FROM AUTHOR]

We have used resonant x-ray diffraction to develop a detailed description of antiferromagnetic ordering in epitaxial superlattices based on two-unit-cell thick layers of the strongly correlated metal LaNiO3. We also report reference experiments on thin films of PrNiO3 and NdNiO3. The resulting data indicate a spiral state whose polarization plane can be controlled by adjusting the Ni d-orbital occupation via two independent mechanisms: epitaxial strain and spatial confinement of the valence electrons. The data are discussed in light of recent theoretical predictions. [ABSTRACT FROM AUTHOR]

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa2Cu307 and ferromagnetic-metallic Lao.67Cao.33Mn03 or ferromagnetic-insulating LaMn03+&b.delta;. We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the ferromagnetic-metallic Lao.67Cao.33Mn03 and almost absent for ferromagnetic-insulating LaMn03+&b.delta;. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature. [ABSTRACT FROM AUTHOR]