Magnetoresistance of coherently strained La_2/3Ba_1/3MnO₃/SrTiO₃ superlattices

A series of coherently strained multilayer thin films of SrTiO₃/La_2/3Ba_1/3MnO₃ have been fabricated on (100) SrTiO₃ and (100) NdGaO₃ substrates by laser molecular-beam epitaxy. By the in-plane coherency strain the out-of-plane lattice constant could be continuously adjusted from 3.916 to 3.975 Å by varying the relative thickness of the SrTiO₃ and La_2/3Ba_1/3MnO₃ layers. The strain results in a biaxial distortion ε_bi of La_2/3Ba_1/3MnO₃ that strongly affects the electrical transport properties and the magnetoresistance. Our measurements show that there is a clear correlation between ε_bi and the temperature T_pcorresponding to the maximum in the resistivity versus temperature curves as well as the measured magnetoresistance. By varying the strain we could derive the quantity Delta;=(T_p/T_p^bulk)(d²T _p/dε_bi²) giving the sensitivity of the materials properties to biaxial strain. Our data provide evidence for the relevance of the Jahn-Teller electron-phonon coupling in the doped manganites. Our analysis further shows that biaxial strain is an important variable that has to be taken into account in the design of thin-film structures used for magnetoelectronic devices based on doped manganites.