Structure and transport properties of coherently strained La_2/3Ca_1/3MnO₃/SrTiO₃superlattices

Abstract

Abstract We have prepared high quality, coherently strained La 2/3 Ca 1/3 MnO 3 /SrTiO 3 superlattices with different modulation periods by laser molecular beam epitaxy on (001) SrTiO 3 and NdGaO 3 substrates. A detailed structural characterization was performed by high‐angle X‐ray diffraction (HAXRD) and low‐angle X‐ray reflectivity (LAXRR). All superlattices are very flat, show excellent structural coherence and very small mosaic spread (0.02°). The in‐plane coherency strain was varied by changing the thickness ratio of the constituent layers allowing for a systematic variation of the resulting tetragonal distortion of LCMO. The c ‐axis lattice parameter of LCMO could be continuously changed from 3.87 Å to 3.79 Å. The interface roughness was analyzed by offset low‐angle X‐ray reflectivity and low‐angle rocking curve measurements. It was found to be of the order of one unit cell with a significant part of the roughness being vertically correlated. The strain induced tetragonal distortion of LCMO was found to cause strong reduction of the paramagnetic to ferromagnetic transition temperature from about 260 to 120 K and an increase of resistivity. The transport properties in the paramagnetic regime could be well described by a small polaron hopping model. The lattice distortions were found to result in a significant increase of the polaron trapping energy. Our results show that coherently strain superlattices are an interesting model system for the systematic study of the effect of lattice distortions on the magnetic and electronic properties of the doped manganites. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)