Small polaron hopping conduction in samples of ceramic La1.4Sr1.6Mn2O7.06

The ceramic sample of La1.4Sr1.6Mn2O7.06 exhibits the metal-insulator trans ition and a negative magnetoresistance in the vicinity of the Curie tempera ture (T-C similar or equal to 100 K). The de magnetic susceptibility betwee n 100 K and 280 K is nearly constant and decreases gradually with increasin g temperature above 280 K. The measurements of de resistivity and the therm oelectric power indicate that small polaron hopping conduction takes place at T > 280 K. The spin ordering due to the two-dimensional d(x2-y2) state o ccurring at T > 280 K is directly related to the hopping conduction above 2 80 K, although the spin ordering due to the one-dimensional d(3z2-r2) state takes place at T > T-C. The two-dimensional d(x2-y2) state extending withi n the MnO2 sheets starts to narrow and leads to the carrier localisation at 280 K. The effective number of holes in this sample estimated from the the rmoelectric power is considerably smaller than the nominal value. This indi cates that the small polaron hopping conduction takes place predominantly w ithin the in-plane MnO2 sheets. A discussion is given of the experimental r esults of the ceramic sample of La2/3Ca1/3MnO2.98.