Resistivity and thermoelectric power of La1−xYxSr2Mn2−zCrzO7 (x or z=0, 0.01, 0.03, 0.05 and 0.1) layered manganite

Abstract

The electrical transport properties of La1−xYxSr2Mn2O7 and LaSr2Mn2−zCrzO7 (x or z=0, 0.01, 0.03, 0.05 and 0.1) samples were studied by means of resistivity and thermoelectric power (TEP) from 10 to 600 K in zero field. The metal–insulator transition occurs at a temperature (TMI) lower than the Curie temperature (TC). The data above TMI was analysed in terms of two conduction processes, viz. the variable range hopping (VRH) and the conduction by adiabatic hopping of small polarons (AHSP). Our resistivity data agrees with the resistivity behaviour predicted by magnetic localisation model only within a small range of temperature around TC. On the other hand it was well described both below and above TC by the AHSP model. The metal–insulator transition in both Cr and Y-doped samples smears out with increasing content of both the dopants. The decrease in TMI of the Y-doped samples appears to be governed by the cation disorder in the rocksalt layer while in the Cr-doped samples it decreases as a result of a reduction in the double exchange interaction (DEX) brought on by the substitution of Mn3+ by Cr3+. TEP was well described in terms of transport by magneto-elastic polarons. Also the exchange coupling constant (J) obtained from the TEP data was seen to decrease with the content of the Cr and Y ions.