Low-temperature ferromagnetism in tensile-strained LaCoO2.5 thin film

Abstract

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 ferromagnetism 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 μ B/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.