Exchange bias in a singleLaMnO3film induced by vertical electronic phase separation

We verify that the exchange bias effect unexpectedly emerges in a single ${\mathrm{LaMnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ film, one of the most studied correlated oxides. We combine x-ray absorption spectroscopy results, which serves as a fingerprint of the electronic structure, with microstructure characterizations and magnetization data to explore the origin of the exchange bias behavior. Taken together, these measurements provide compelling evidence that the formation of a Mn${}^{2+}$ component associated with the double exchange between Mn${}^{2+}$-O-Mn${}^{3+}$ produces robust ferromagnetism in the upper part of the film, which is exchanged coupled with the antiferromagnetic bottom part that is dominated by Mn${}^{3+}$. Thus, the Mn ions are found to be distributed unevenly in the depth profile, with vertical electronic phase separation, in contrast to the often accepted view of the lateral phase separation in manganites. The observation of exchange bias in a chemically single film paves the way for interface engineering induced by vertical electronic phase separation towards technological applications.