Effect of oxygen nonstoichiometry on electrotransport and low-field magnetotransport property of polycrystalline La0.5Sr0.5Mn3-δ thin films

Polycrystalline La0.5Sr0.5MnO3-delta thin films deposited on quartz wafers at 680 degrees C and various oxygen pressures P by pulsed laser deposition are prepared. The effects of oxygen nonstoichiometry on the microstructural, electrotransport and low-field magnetotransport: property of the thin films are investigated in details. A structural distortion from the stoichiometric lattice is identified for the samples deposited at P<0.1 mbar. It is verified that the thin-film conductivity over the Curie point follows variable-range hopping. The carrier density at the Fermi surface falls and the metal-insulating transition shifts toward low temperature with decreasing P, with a jump at P=0.1 mbar. Enhanced low-field magnetoresistance at low temperature is achieved for P>0.1 mbar. Oxygen overdeficiency at P less than or equal to 0.1 mbar essentially prohibits the spin reordering. The temperature dependence of the electro- and magnetotransport properties is explained by the two-channel model where the insulating channels and metallic ones coexist in parallel.