In the pursuit of high-temperature superconductivity like that in cuprates, artificial heterostructures or interfaces have attracted tremendous interest. It has been a long-sought goal to find similar unconventional superconductivity in nickelates. However, as far as we know, this has not yet been experimentally realized. To approach this objective, we synthesized a prototypical superlattice that consists of ultrathin LaNiO and LaSrMnO layers. Both zero resistance and the Meissner effect are observed using resistive and magnetic measurements of the superlattice. These are experimental indicators for superconductivity in new superconductors. X-ray linear dichroism causes the NiO planes to develop electron-occupied x-y orbital order similar to that of cuprate-based superconductors. Our findings demonstrate that artificial interface engineering is suitable for investigating novel physical phenomena, such as superconductivity.

Paramagnetic LaNiO (LNO)-based heterostructures have been attracting the attention of researches, especially since the interesting exchange bias (EB) effect has been observed in (111)-oriented LaMnO (LMO)/LNO superlattices (SLs). However, this effect is not expected to occur in the (001) direction SLs. In this paper, we report the observation of an unexpected EB effect in (001)-oriented (LMO)/(LNO) SLs. The orbits of interfacial Mn/Ni ions preferentially occupy the strain-stabilized x - y in ultrathin LNO layers [t ≤ 4 unit cells (u.c.)]. Conversely, as the LNO layer becomes thicker (t ≥ 6 u.c.), the EB effect is absent, and the orbits are reconstructed to form the 3z - r preferential occupancy. The absence of the EB in thicker LNO-based SLs is attributed to the interfacial charge transfer suppressed by orbital reconstruction as a consequence of the increasing LNO thickness. In the thinner LNO-based SLs, the larger charge transfer results in stronger localized magnetic moments for the cause of the EB effect. These results provide a useful interpretation of the relationship between macroscopic magnetic properties and the microscopic electronic structure in oxide-based heterostructures.

The achievement of high temperature ferromagnetism in perovskite manganites has proved both fundamentally and technologically important for spintronics devices. However, high operating temperatures have not been achieved due to the depression of the Curie temperature and the rapid spin filtering efficiency loss, which are the main obstacles for practical applications. Here, we report unexpected room temperature insulating ferromagnetism in ultrathin (110) oriented La0.7Sr0.3MnO3 (LSMO) films. The relationships between room temperature ferromagnetism, charge transfer, and orbital occupancy are investigated, with X-ray absorption spectroscopy (XAS) and X-ray linear dichroism (XLD) measurements. Our results suggest that the room temperature insulating ferromagnetism is originated from super-exchange interaction between Mn2+ and Mn3+. The formation of Mn2+ ions is related to the charge transfer induced by oxygen vacancies. Moreover, a preferential orbital occupancy of eg(3z2-r2) in Mn3+ ions is crucial to the in-plane super-exchange coupling in ultrathin (110) LSMO films, resulting in insulating ferromagnetic behavior. This work may lead to the development of barrier materials in spin filter tunnel junctions and understanding of ferromagnetic coupling in insulating perovskite films.

With the goal of observing and explaining the unexpected exchange bias effect in paramagnetic LaNiO3-based superlattices, a wide range of theoretical and experimental research has been published. Within the scope of this work, we have grown high-quality epitaxial LaMnO3(n)-LaNiO3(n) (LMO/LNO) superlattices (SLs) along (001)-, (110)-, and (111)-oriented SrTiO3 substrates. The exchange bias effect is observed in all cases, regardless of growth orientation of the LMO/LNO SLs. As a result of a combination of a number of synchrotron based x-ray spectroscopy measurements, this effect is attributed to the interfacial charge transfer from Mn to Ni ions that induces localized magnetic moments to pin the ferromagnetic LMO layer. The interaction per area between interfacial Mn and Ni ions is nearly consistent and has no effect on charge transfer for different orientations. The discrepant charge transfer and orbital occupancy can be responsible for the different magnetic properties in LMO/LNO superlattices. Our experimental results present a promising advancement in understanding the origin of magnetic properties along different directions in these materials.