Controllable oxygen vacancies, orbital occupancy and magnetic ordering in SrCoO3−δ films

Epitaxial strain imposed in complex oxide ultrathin films is recognized as a powerful tool for controlling the ground state of correlated electron system. Here, we achieved simultaneous control of oxygen vacancies, orbital occupancy and magnetic ordering through the facile application of epitaxial strain, both tensile and compressive, in SrCoO3−δ oxygen “sponge” material rather than the ordinary manganites. The oxygen vacancies are enhanced greatly as the strain changes from smaller tensile 1.0% to larger tensile 2.0%, then to moderate compressive −1.0% in SrCoO3−δ films, associated with 3 − δ varying from ∼2.90 to ∼2.835, then to ∼2.72. Highest saturated magnetization is found in the thin films in small tension on La0.3Sr0.7Al0.65Ta0.35O3 substrate and lower values are found in larger tension on SrTiO3 and lowest values in moderate compression on LaAlO3. Meanwhile, electrons prefer to occupy the in-plan oriented orbitals for the tensile strain, in contrast to the preferential out-of-plane orbital occupancy for the compressive state associated with coupled intermediate spin-Co4+ (t2g4eg1)/high spin-Co3+ (t2g4eg2) in different proportions depending on strain states. Such controllable spin and orbital configurations lead to more robust magnetization in tensile strained SrCoO3−δ films than in its compressive counterpart. Our findings provide a nostrum for tailoring and controlling new magnetic, electronic and ionically active properties with strain engineering and further enrich orbital physics in cobaltites.