Magnetoresistance of La$_{0.67}$Ca$_{0.33}$MnO$_3$ films the coherent growth of which is disturbed by mechanical stress relaxation

Because of a large ($m$ = 1.8%) lattice mismatch between La$_{0.67}$Ca$_{0.33}$MnO$_3$ and LaAlO$_3$, manganite films grown on a lanthanum aluminate substrate experience biaxial mechanical compression stresses. Strong adhesion to the substrate causes a substantial tetragonal distortion ($\gamma\approx$ 1.04) of the unit cell in a 20-nm-thick layer of the manganite film coherently grown on (001)LaAlO$_3$, while in the remaining part ($\approx$ 75%) of the manganite film, stresses partially relax. The stress relaxation decreases $\gamma$ and increases the effective volume of the unit cell of the La$_{0.67}$Ca$_{0.33}$MnO$_3$ film. The relaxed part of the La$_{0.67}$Ca$_{0.33}$MnO$_3$ film consists of crystallites 50–200 nm across azimuthally misoriented by approximately 0.3$^\circ$. The temperature dependences of the resistivity and negative magnetoresistance of the manganite films exhibit maxima at 240 and 215 K, respectively. At temperatures below 50 K, the dependence of the resistivity on the magnetic induction taken with the induction varying from 0 to 14 T and vice versa becomes hysteresis.