Nitrogen-annealing effects on the electrical resistivity and thermopower of layered La1.4Sr1.6Mn2O7+δ ceramics

We report measurements of thermopower and resistivity as a function of temperature on bilayered La1.4Sr1.6Mn2O7+delta ceramics treated in different atmosphere in H = 0 and 5 T. Chemical titration is used to determine both valence of manganese and content of oxygen. As the sample is annealed in nitrogen, the resistivity of La1.4Sr1.6Mn2O7+delta reduces; the peak resistivity shifts to a higher temperature; the magnetoresistance ratio is enhanced; and the thermopower shifts to a more negative value over the investigated temperature range as compared to the sample annealed in oxygen. These results show that annealing samples in flowing nitrogen has the same effect as the pressure-induced shift of transport behavior. In addition, thermopower data of our ceramic materials resemble those within the ab plane of a single crystal. Transport data will be discussed in terms of the polaron hopping.