Enhancing the orthorhombicity and antiferromagnetic-insulating state in epitaxial La_0.67Ca_0.33MnO₃/NdGaO₃(001) films by inserting a SmFeO₃ buffer layer

Structural and magnetotransport properties of epitaxial La_0.67Ca_0.33MnO₃(30 nm)/NdGaO₃(001) [LCMO/NGO(001)] films are tuned by inserting an insulating SmFeO₃ (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_AFI 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 MnO₆ 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.