Control of the charge-ordering-insulating phase in epitaxial La_1-xCa_xMnO₃ (x=0.30-0.45) thin films under the anisotropic strain

The control of charge-ordering-insulating (COI) phase in epitaxial La_1−xCaxMnO₃/NdGaO₃ (001) (x=0.30–0.45)thin films with essentially the ferromagnetic metal ground state as observed for the bulk counterparts has been realized via the anisotropicstrain relaxation. This epitaxial system is special in that there is a negligible average lattice mismatch but a large anisotropic strain in between the film and the substrate. By changing the film thickness, postannealing temperature, along with the doping level for strain relaxation, the COI phase in the films can be tuned to either melt completely under 1 T, producing a huge low-field magnetoresistance(MR) in a wide temperature range (e.g., for the 20 nm film with x=0.33 and annealed at 780 °C, the MR can be over 70% at 0.2 T and 97% at 0.5 T in 10–200 K), or survive under a high magnetic field of 6 T. The results demonstrate the crucial role of anisotropicstrain relaxation in inducing the inhomogeneity in manganites films, thus providing a forward understanding of the strain field in manganite physics.