Influence of pressure, temperature, and magnetic field on the resistivity and magnetoresistive effect of lanthanum manganite ceramics and films with the composition La0.7Mn1.3O3 +/-delta

The effects of magnetic fields H=0-8 kOe and high hydrostatic pressures P=0 -2.2 GPa on the resistivity rho, magnetoresistive effect DeltaR/R-0, metal- semiconductor phase transition temperature T-ms, and peak temperature T-p o f the magnetoresistive effect are investigated over a wide range of tempera tures T=77-350 K in a ceramic target and a laser film of the same cationic compound La0.7Mn1.3O3 +/- delta. Increasing H and P leads to a decrease of the resistivity. The observed difference in rho, T-ms, and T-p between the ceramics and film samples is explained by the difference of their oxygen no nstoichiometry. The magnetoresistive effect decreases with increasing H and decreases with increasing P. The temperatures T-ms and T-p of the ceramics and film increase with increasing P, but the effect is stronger in the fil m. The observation of two phase transitions in the ceramics (the main trans ition at T-ms=250 K and an additional transition at T-ms(')=210 K) is expla ined by its mesoscopic inhomogeneity of the cluster type. (C) 2001 American Institute of Physics.