Combined strain and high pressure oxygen treatment effects and phase separation in La$_{067}$Ca$_{0.33}$MnO$_3$ thin films

An evidence for phase separation in high-pressure oxygen treated La$_{0.67}$Ca$_{0.33}$MnO$_3/$(100)MgO thin films is presented. This phase separated state is not observed for the $x=0.33$ classical doping level neither for bulk material, nor for as prepared thin films. It originates from the combined influence of two factors: large strains due to the film-substrate lattice mismatch and high pressure (100 bar) annealing in oxygen. These factors weaken double exchange, drastically enhance low-field magnetoresistance, and lead to hysteresis phenomena. The experimental results are interpreted in terms of phase separation of the film volume into the ferromagnetic conductive matrix with inclusions of spin-glass-like clusters.