Effect of Strain and Dimensionality on the Properties of Manganites

Strain and dimensional confinement can be used to tune magnetic and ferroelectric properties or enhance device performance. In epitaxial films, the different lattice spacing of an underlying substrate can be used to impose a biaxial strain. We have studied the effect of the substrate-induced biaxial strain on the electrical conductivity and magnetic properties of manganite compounds. Epitaxial (001) La0.7Sr0.3MnO3 thin films have been grown by reactive molecular-beam epitaxy (MBE) on single crystalline substrates, varying the substrate-induced biaxial strain from -2.3% to +3.2%. The strain caused the Curie temperature to decrease by up to 100 K in very good agreement with the predictions of Millis et al. [A. J. Millis, T. Darling, and A. Migliori, J. Appl. Phys. 83, 1588 (1998)]. The effect of dimensional confinement on manganites was also investigated in thin films by synthesizing a superlattice of two formula-unit-thick layers of CaMnO3 separated by CaO double layers, i.e., the n=2 Ruddlesden-Popper phase Ca3Mn2O7. Magnetization measurements on 30 nm thick (001)-oriented Ca3Mn2O7 thin films grown on (110) YAlO3 substrates by MBE reveal a Neel transition temperature of T-N=115 K, similar to bulk Ca3Mn2O7, but 10 K lower than thick CaMnO3 films grown on this same substrate.