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

Highlights: • Controllable oxygen vacancies, orbital occupancy and magnetic ordering are achieved through the facile application of epitaxial strain. • The oxygen vacancies are enhanced greatly with δ ∼ 0.28 as the strain is in moderate compressive −1.0%. • The orbital occupancy under different strain states is resolved for the first time. • Robust magnetization is obtained in tensile strained SrCoO_3−δ films. • The strong coupling between lattice, charge, spin and orbital degrees of freedom in cobaltites is experimentally demonstrated. 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 SrCoO_3−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 SrCoO_3−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 La_0.3Sr_0.7Al_0.65Ta_0.35O₃ 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⁺ (t_2g⁴e_g¹)/high spin-Co³⁺ (t_2g⁴e_g²) in different proportions depending on strain states. Such controllable spin and orbital configurations lead to more robust magnetization in tensile strained SrCoO_3−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.