Resistivity and thermoelectric power of La1-xYXSr2Mn2-zCrZO7 (x or z=0, 0.01, 0.03, 0.05 and 0.1) layered manganite

The electrical transport properties of La1-xYxSr2Mn2O7 and LaSr2Mn2-zCr2O7 (x or z = 0, 0.01, 0.03, 0.05 and 0.1) samples were studied by means of res istivity and thermoelectric power (TEP) from 10 to 600 K in zero field. The metal-insulator transition occurs at a temperature lower than the Curie te mperature (T-C). The data above T-MI was analysed in terms of two conductio n processes, viz. the variable range hopping (VRH) and the conduction by ad iabatic hopping of small polarons (AHSP). Our resistivity data agrees with the resistivity behaviour predicted by magnetic localisation model only wit hin a small range of temperature around T-C. On the other hand it was well described both below and above T-C by the AHSP model. The metal-insulator t ransition in both Cr and Y-doped samples smears out with increasing content of both the dopants. The decrease in T-MI of the Y-doped samples appears t o 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 exchang e 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 decr ease with the content of the Cr and Y ions. (C) 2001 Elsevier Science BY. A ll rights reserved.