Magnetic and transport properties of Nd0.2La1.8-2xSr1+2xMn2O7 (x=0.5, 0.4,0.3) and La1.5Sr1.5Mn2O7

Lattice structure, magnetic and transport properties of polycrystalline sam ples of Nd0.2La1.8-2xSr1+2Mn2O7 in the range 0.5 greater than or equal to x greater than or equal to 0.3 and La1.5Sr1.5Mn2O7 have been investigated. A pronounced decrease of the Curie temperature T-C and the metal-insulator t ransition temperature T-MI of Nd0.2La1.8-2xSr1+2Mn2O7 compared to Nd-free c ompounds La2-2xSr1+2xMn2O7 with the same doping level has been observed. Th e shift of T-C, which is increased with decreasing Sr content, indicates a weakening of the ferromagnetic ordering for the Nd-substituted samples. Nd0.2La1.2Sr1.6Mn2O7 orders ferromagnetically at T-C = 75 K and reveals a t ransition to an antiferromagnetic (afm) spin arrangement below T-N = 38 K, whereas polycrystalline samples of the composition La1.4Sr1.6Mn2O7 remain f erromagnetically ordered down to 5 K. A transition to afm spin structure in a polycrystalline sample of the composition La2-2xSr1+2xMn2O7 has been obs erved for x = 0.25, La1.5Sr1.5Mn2O7 orders ferromagnetically at 66 K in a f ield of 0.5 T; the transition to an afm ordering occurs below T-N = 34 K. I n contrast to the Nd-substituted phase the afm transition only occurs for z ero-field cooled measurements. At low temperature, T < T-N, La1.5Sr1.5Mn2O7 undergoes a metamagnetic transition; the threshold field was found to depe nd strongly on the temperature (<approximate to> 0.8 T at 20 K/approximate to 3.5 T at 5 K). With the T-M, Of 102 K the compound shows metallic behavi our clearly above T-C. It becomes insulating again at T-N, which is in line with the properties of Nd0.2La1.2Sr1.6Mn2O7. Large magnetoresistance ratios (MR) have been measured for Nd0.2La1.8-2xSr1 +2xMn2O7 with x = 0.4, 0.3 and La1.5Sr1.5Mn2O7 near T-C. Furthermore, the m etamagnetic transition causes high values of MR for La1.5Sr1.5Mn2O7 and Nd0 .2La1.2Sr1.6Mn2O7 at low fields.