Transition metal oxides exhibit rich electrical and magnetic properties. The epitaxial strain plays a vital role to manipulate their magnetic ground state. In addition, Oxygen vacancies (Vo) are one of the most common point defects to play a key role in regulating the physicochemical properties of oxides. (1-4) In this context, LCO exhibits interesting phenomena in terms of the origin of magnetism. In this work, Perovskite LaCoO3 (PV-LCO) films were grown at STO and LAO substrate. Then brownmillerite LaCoO2.5 (BM-LCO) films are formed by annealing in a vacuum at 450 °C for 4h and 500 °C for 4h, respectively. The XRD spectra show that there is no impurity phase during phase transitions, indicating that the films can be fully transformed. (Fig.1a) The magnetic properties were confirmed by SQUID-VSM. The 50-nm-thick PV-LCO film on STO exhibits an obvious ferromagnetism phase. (Fig.1b-c) The tensile strain leads to a decrease of ΔCF due to distortion of CoO6 octahedron, promotes the spin state transition. (5,6) However, the same thickness coherently strain film in compression on LAO exhibits extremely weak ferromagnetism. We anticipate the reason for this is because the eg orbitals are split into two separate energy levels due to existence of the JTD distortion, it is always accompanied by an increase of the bond distance rCo–O and suppression of CoO6 octahedral rotation and distortion. Interestingly, the BM-LCO shows no hysteresis loop. We believe the reason for this novel phenomenon is that the increase in ordered Vo leads to the expansion of the lattice, thus inhibiting the rotation and distortion of the CoO6 octahedron and causing the generation of anti-ferromagnetic behavior. (7,8) In sum, we find that BM-LCO films in epitaxial strain and Vo ordering show no ferromagnetism. This phenomenon provided an effective way to regulate the physical properties of cobalt oxides. It would have a good application value in the fields of ion electrochemical sensors.

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