Influence of strain and microstructure on magnetotransport in La0.7Ca0.3MnO3 thin films

A ${\mathrm{La}}_{\mathrm{0.7}}{\mathrm{Ca}}_{\mathrm{0.3}}{\mathrm{MnO}}_{\mathrm{3}}$ thin film made by pulsed laser deposition (PLD) and another film of the same composition made by metal organic chemical vapor deposition (MOCVD), both on single crystal ${\mathrm{LaAlO}}_{\mathrm{3}},$ were subject to a series of six, short, controlled anneals. The oxygen content was purposely not changed in the films from the first anneal to subsequent anneals. After each anneal, the film microstructures were characterized to determine average grain size, lattice constants, nonuniform strain, and crystalline mosaic spread, and these parameters were correlated with the magnetotransport properties. For both sets of films, the influence of annealing was to both increase the temperature at which the maximum in the magnetoresistance occurs ${\mathrm{(T}}_m)$ and the maximum magnetoresistance (MR) value. The improvement in film properties occurred in conjunction with stress relaxation and improved crystallinity, as a result of grain growth. The MOCVD films showed poorer grain coupling and poorer epitaxy compared to the PLD films. These features did not significantly influence the absolute values of the resistivity, but did produce spin canting in the MOCVD film, as seen in magnetization and resistivity versus field data. The canting resulted in a lower $T_m$ and depressed MR value for the MOCVD film which increased only marginally with annealing. The work highlights the importance of controlling microstructure for optimizing properties of colossal magnetoresistance films.