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 3d7 L (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.

The study of magneto-optical (MO) properties is reported in epitaxial cerium (Ce) substituted yttrium iron garnet thin films deposited on Gd3Ga5O12 (GGG) substrates using pulsed laser deposition technique at (i) different O2 partial pressure (OPP) during film deposition and (ii) with different film thickness at a given OPP. The MO activity is estimated by measuring the Kerr rotation using the longitudinal magneto-optical Kerr effect with blue (405 nm), green (532 nm), and red (632 nm) incident laser light. The Kerr rotation is observed to increase with an increase in OPP and vary differently with thickness for a given OPP as a function of incident light wavelength. The results are explained in terms of the possible charge transfer mechanism for the films prepared with OPP variation and the optical interference effects in dielectric films with thickness variation.

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