Enhancing the orthorhombicity and antiferromagnetic-insulating state in epitaxial La0.67Ca0.33MnO3/NdGaO3(001) films by inserting a SmFeO3 buffer layer

摘要

Structural and magnetotransport properties of epitaxial LaCaMnO(30 nm)/NdGaO(001) [LCMO/NGO(001)] films are tuned by inserting an insulating SmFeO (SFO) buffer layer at various thicknesses (t). All the layers and the NGO substrates have the same Pbnm symmetry with the octahedra tilting about the b-axis, but different orthorhombicity (d). We found that as t increases, the fully strained (≤15 nm) or partially relaxed (30–60 nm) SFO layers can produce different d in the upper LCMO films. Correspondingly, the induced antiferromagnetic-insulating (AFI) state in LCMO is greatly enhanced with T shifted from ∼250 K for t ≤ 15 nm to ∼263 K for t = 30–60 nm. We also show that the strain relaxation for t ≥ 30 nm is remarkably anisotropic, with a stable lattice constant a as that of the NGO substrates but increasing b of both SFO and LCMO layers. This indicates the octahedral coupling across the interfaces, leaving the strain along the a-axis accommodated by the octahedral tilts, while along the b-axis most probably by the octahedral deformations. The AFI state in the LCMO layer could be ascribed to the enhanced orthorhombicity with cooperatively increased Jahn-Teller-like distortions and tilting of the MnO octahedra. The results strongly suggest that the interfacial octahedral coupling plays a crucial role in epitaxial growth and in tuning functionalities of the perovskite oxide films.