Effect of oxygen nonstoichiometry on electrotransport and low-field magnetotransport property of polycrystallineLa0.5Sr0.5MnO3−δthin films

Polycrystalline ${\mathrm{La}}_{0.5}{\mathrm{Sr}}_{0.5}{\mathrm{MnO}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin films deposited on quartz wafers at 680 \ifmmode^\circ\else\textdegree\fi{}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 $Pl0.1\mathrm{mbar}.$ 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 $P=0.1\mathrm{mbar}.$ Enhanced low-field magnetoresistance at low temperature is achieved for $Pg0.1\mathrm{mbar}.$ Oxygen overdeficiency at $Pl~0.1\mathrm{mbar}$ 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.