Abstract

The ceramic sample of La₁ · ₄Sr₁ · ₆Mn₂O₇ · ₀₆ exhibits the metal-insulator transition and a negative magnetoresistance in the vicinity of the Curie temperature (T ⋍ 100 K). The dc magnetic susceptibility between 100 K and 280 K is nearly constant and decreases gradually with increasing temperature above 280 K. The measurements of dc resistivity and the thermoelectric power indicate that small polaron hopping conduction takes place at T > 280 K. The spin ordering due to the two-dimensional d x 2 ¡ y 2 state occurring at T > 280 K is directly related to the hopping conduction above 280 K, although the spin ordering due to the one-dimensional d 3z 2 ¡ r 2 state takes place at T > TC. The two-dimensional d x 2 ¡ y 2 state extending within 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 thermoelectric power is considerably smaller than the nominal value. This indicates that the small polaron hopping conduction takes place predominantly within the in-plane MnO₂ sheets. A discussion is given of the experimental results of the ceramic sample of La_2/3Ca_1/3MnO_2·98.