Fermi Level shifting, Charge Transfer and Induced Magnetic Coupling at La_0.7Ca_0.3MnO₃/LaNiO₃ Interface

The research area that focuses on the interfaces between dissimilar complex oxide materials is attracting considerable attention due to the coupling of the spin, charge and orbital freedom of 3d electrons. These interfaces exhibit much richer physical connotation than conventional semiconductor heterostructures because of the novel electronic reconstruction and magnetic states. Recently, remarkable improvement in techniques for growing and characterizing oxide thin films has opened an avenue for the study of the new interfacial electronic states at the interface between ABO3 perovskite oxides. In these systems, novel physical properties such as 2-dimension (2D) superconductivities, artificial topological insulators and unexpected magnetic interaction have been found and envisaged as the promising ideal system for the realization of nanoscale oxide devices.The factors like orbital reconstruction of the 3d electrons and oxygen vacancies at the interface could drastically tune the interfacial properties. The charge transfer as another important factor is also discussed for the origin of the novel physical properties.However, whether the charge transfer would lead to magnetic coupling in the perovskite manganites is still a controversial issue. To understand and explain the role of these factors for the unusual interfacial properties, the detailed spectroscopic studies on these interfaces will be needed. In this study, a large magnetic coupling has been observed at the La0.7Ca0.3MnO3/LaNiO3 (LCMO/LNO) interface. The x-ray photoelectron spectroscopy (XPS) study results show that Fermi level continuously shifted across the LCMO/LNO interface in the interface region. In addition, the charge transfer between Mn and Ni ions of the type Mn3+-Ni3+ → Mn4+-Ni2+ with the oxygen vacancies are observed in the interface region. The intrinsic interfacial charge transfer can give rise to itinerant electrons, which results in a “shoulder feature” observed at the low binding energy in the Mn 2p core level spectra. Meanwhile, the orbital reconstruction can be mapped according to the Fermi level position and the charge transfer mode. It can be considered that the ferromagnetic interaction between Ni2+ and Mn4+ gives rise to magnetic regions that pin the ferromagnetic LCMO and cause magnetic coupling at the LCMO/LNO interface.

Fig. 4 (a) Mn 2p-spectra of the Bulk LCMO that record about 4 nm to the LCMO/LNO interface. (b) Mn 2p-spectra at the LCMO/LNO interface. (c) A two-dimensional contour map of the Mn 2p3/2 and Mn 2p1/2 spectra, recorded all the data across the LCMO/LNO interface in the interface region. The charge transfer may happen in the interface region where the eg electrons of Mn3+ ion near EF may hop to nearby new states, decreasing the Mn3+/Mn4+ ratio.

Fig. 4.Schematic illustrations of the XPS procedures and the binding energies of different core-level electrons.