Charge transfer effect for the La0.7Ca0.3MnO3/NiO heterostructure and novel interfacial ferromagnetism

We report the formation of new ferromagnetic (FM) states in antiferromagnetic (AFM) NiO at the interface with FM La0.7Ca0.3MnO3 (LCMO). The LCMO/NiO heterostructures exhibit an exchange bias field of 209 Oe that vanishes as the temperature rises above 90 K. A new magnetization temperature at 90 K is observed and can be ascribed to Ni3+-O-Mn3+ superexchange interactions. Mn 3 s and Ni 3p core-level spectra, measured by X-ray photoelectron spectroscopy, show a direct evidence of charge transfer effects of the type Mn4+-Ni2+ → Mn3+-Ni3+ at the interface region. The valence band offset (VBO) at the LCMO/NiO interface can be determined to be ΔEVBO ∼ 0.77 eV. The valence band of LCMO is shifted to higher binding energy compared with NiO. Thus, charge transfer occurred because of the shifting of the valence band edge at the heterostructure interfaces. We speculate that the superexchange interactions of the Ni3+ and Mn3+ at the interface give rise to the novel low-temperature FM order and the magnetic regions that pin the FM LCMO layer as the temperature decreases.