Abstract
We prepared doped perovskite manganites of La₀.₇₇Ca₀.₂₃MnO₃ and La₀.₆₇Ba₀.₃₃MnO₃ by the solid-state reaction method. The samples were characterized electrically, magnetically, and structurally, and the effect of different preparation routines was studied. The X-ray diffraction patterns showed a cubic unit cell with parameter a varying between 3.83 and 3.90 Å. The resistivity measurements as a function of temperature showed a marked decrease when a magnetic field was switched on. All the samples displayed a transition from metallic to semiconductor behavior at temperatures Tp between 175 and 250 K; this temperature, however, becomes higher with increasing magnetic field. The typical semiconductor behavior at T > Tp can be reasonably well fitted to a non-adiabatic small polaron hopping model.
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