Preparation and characterization of structurally stable polycrystalline La[sub]0.7Ca[sub]0.3Mn[sub]03 and La[sub]0.7Ca[sup]0.3(Mn[sup](1-x)Fe[sub]x)[sub]03 samples
A new procedure in the preparation of polycrystalline, structurally sound, doped manganites was developed. This method is called "flashing." U sing the new preparation technique, several batches of samples of Lao.7Cao.3Mn03 and Lao.7Cao.3 (Mn(1_x)Fex)03 (with x = 0.05 and 0.07) were produced under different pelletizing pressures and annealing conditions. In the case of the Ca doped manganites, electrical resistivity as a function of temperature measurements, with and without an applied magnetic field of 350 mT, show a negative magnetoresistive effect, and a practically field-independent metal-insulator transition that depends upon heat treatment and annealing conditions. Similar measurements in Ca and Fe doped manganites reveal the same behavior with relatively large resistivity values and lower transition temperatures that depend on concentration. Maximal values for electrical resistivity in Fe doped samples are at least one order of magnitude higher compared to the maximal values for Ca doped specimens. X-ray diffraction, electrical resistivity, and electron-microscope analyses indicate a correlation between grain size, annealing environment, conductivity, and magneto-resistance. Magnetic susceptibility measurements indicate that the metal-insulator transition occurs approximately at the Curie temperature in all the cases studied.