Low-temperature ferromagnetism in tensile-strained LaCoO_2.5 thin film

【摘要】 The origin of ferromagnetism in epitaxial strained La Co O_3-x films has long been controversial. Here, we investigated the magnetic behavior of a series of oxygen vacancy-ordered La Co O_3-x films on different substrates. Obvious ferromagnetism was observed in perovskite La Co O₃/LSAT（LSAT =（La Al O3）0.3（Sr Al Ta O6）0.7） and La Co O3/Sr Ti O3films, while La Co O₃/La AlO₃ films showed weak ferromagnetic behavior. Meanwhile, La Co O2.67 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 La Co O_2.5/LSAT thin films, which is probably related to the change in the interface Co O6octahedron 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 Co O6octahedron 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 La Co O_2.5 film on a La Al O₃ 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 La Co O_3-x films and provide a new perspective to regulate the magnetic properties of transition oxide thin films.更多还原

【Abstract】 The origin of ferromagnetism in epitaxial strained La Co O_3-x films has long been controversial. Here, we investigated the magnetic behavior of a series of oxygen vacancy-ordered La Co O_3-x films on different substrates. Obvious ferromagnetism was observed in perovskite La Co O₃/LSAT（LSAT =（La Al O3）0.3（Sr Al Ta O6）0.7） and La Co O3/Sr Ti O3films, while La Co O₃/La AlO₃ films showed weak ferromagnetic behavior. Meanwhile, La Co O2.67 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 La Co O_2.5/LSAT thin films, which is probably related to the change in the interface Co O6octahedron 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 Co O6octahedron 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 La Co O_2.5 film on a La Al O₃ 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 La Co O_3-x films and provide a new perspective to regulate the magnetic properties of transition oxide thin films.更多还原