Negative Charge-Transfer Energy in SrCoO2.5 Thin Films: An Interplay between O-2p Hole Density, Charge-Transfer Energy, Charge Disproportionation, and Ferromagnetic Ordering

SrCoO2.5 (SCO) bulk is found to be a charge-transfer insulator with a positive value of charge-transfer energy (Δ) and 3d6 ground state, which achieves the antiferromagnetic nature through conventional Co3+–O–Co3+ superexchange interaction. However, in the epitaxial SCO thin film, it is observed, with the help of resonant photoemission spectroscopy and X-ray absorption spectroscopy, that substrate-induced strain modifies the ground state of SCO film to 3d7L (L: O-2p hole), causing a negative value of Δ. The presence of the O-2p hole in the negative Δ SCO film arises because of strong O 2p–Co 3d hybridization and induces charge disproportionation (CD) in the system. Consequently, the Co3+–O–Co3+ superexchange interaction is modified, and a hole-mediated unconventional ferromagnetic (or ferrimagnetic) ordering is observed in the SCO film. The magnetic moment is found to depend on the values of Δ and CD, which are controlled by the lattice-induced strain. This is manifested from the strain-dependent valence band and conduction band spectra of the SCO films. Because of the negative Δ and CD, additional spectral features appear in the Co L-edge of the films, which are absent in its bulk counterpart. The energy positions and intensities of such features vary with the film thickness, divulging the role of strain in modifying Δ (and CD). Tuning SCO from the positive Δ regime in the bulk to the negative Δ regime in the thin film provides an opportunity to modulate the electronic structure vis-à-vis magnetic property via strain engineering with huge technological potential applications.