Low-temperature Ferromagnetism in Tensile-Strained LaCoO2.5 Thin Film

The origin of ferromagnetism in epitaxial strained LaCoO3-x films has long been controversial.Here,we investigated the magnetic behavior of a series of oxygen vacancy-ordered LaCoO3-x films on different substrates.Obvious ferromag-netism was observed in perovskite LaCoO3/LSAT(LSAT=(LaAlO3)0.3(SrAlTaO6)0.7)and LaCoO3/SrTiO3 films,while LaCoO3/LaAlO3 films showed weak ferromagnetic behavior.Meanwhile,LaCoO2.67 films exhibited antiferromagnetic behavior.An unexpected low-temperature ferromagnetic phenomenon with a Curie temperature of～83 K and a saturation magnetization of～1.2 μB/Co was discovered in 15 nm thick LaCoO2.5/LSAT thin films,which is probably related to the change in the interface CoO6 octahedron rotation pattern.Meanwhile,the observed ferromagnetism gradually disappeared as the thickness of the film increased,indicating a relaxation of tensile strain.Analysis suggests that the rotation and rhombohedral distortion of the CoO6 octahedron weakened the crystal field splitting and promoted the generation of the ordered high-spin state of Co2+.Thus the super-exchange effect between Co2+(high spin state),Co2+(low spin state)and Co2+(high spin state)produced a low-temperature ferromagnetic behavior.However,compressive-strained LaCoO2.5 film on a LaAlO3 substrate showed normal anti-ferromagnetic behavior.These results demonstrate that both oxygen vacancies and tensile strain are correlated with the emergent magnetic properties in epitaxial LaCoO3-x films and provide a new perspective to regulate the magnetic properties of transition oxide thin films.