The thickness evolution of orthorhombic lattice distortions in heteroepitaxial La0.67Ca0.33MnO3/NdGaO3(110)Or observed by x-ray reciprocal space mapping

La0.67Ca0.33MnO3 (LCMO) films of 6-60 nm thickness were grown epitaxially on orthorhombic NdGaO3(1 1 0)(Or) (NGO) substrates by the pulsed laser deposition method. Like NGO, the films when relaxed should also have an orthorhombic structure that can be described by a pseudocubic perovskite unit, with the angle beta between a and c axes deviating from 90 degrees. Using high-resolution off-specular x-ray reciprocal space mapping, we clearly observed the angle deviations in pseudocubic LCMO(0 0 1)/NGO(0 0 1) and investigated the thickness evolution of lattice distortions in the films. At above 30 nm the films are angularly relaxed, while the thinner ones suffer from the shear strain. We argue that for this system although the lattice mismatch in between is negligible the shear strain is crucial for transport properties of the ultrathin LCMO films.