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

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Lattice structure, magnetic and transport properties of polycrystalline samples of Nd 0.2 La 1.8-2 x Sr 1+2 x Mn 2 O 7 in the range 0.5⩾ x ⩾0.3 and La 1.5 Sr 1.5 Mn 2 O 7 have been investigated. A pronounced decrease of the Curie temperature T C and the metal-insulator transition temperature T MI of Nd 0.2 La 1.8-2 x Sr 1+2 x Mn 2 O 7 compared to Nd-free compounds La 2-2 x Sr 1+2 x Mn 2 O 7 with the same doping level has been observed. The shift of T C , which is increased with decreasing Sr content, indicates a weakening of the ferromagnetic ordering for the Nd-substituted samples. Nd 0.2 La 1.2 Sr 1.6 Mn 2 O 7 orders ferromagnetically at T C = 75 K and reveals a transition to an antiferromagnetic (afm) spin arrangement below T N = 38 K, whereas polycrystalline samples of the composition La 1.4 Sr 1.6 Mn 2 O 7 remain ferromagnetically ordered down to 5 K. A transition to afm spin structure in a polycrystalline sample of the composition La 2-2 x Sr 1+2 x Mn 2 O 7 has been observed for x = 0.25. La 1.5 Sr 1.5 Mn 2 O 7 orders ferromagnetically at 66 K in a field of 0.5 T; the transition to an afm ordering occurs below T N = 34 K. In contrast to the Nd-substituted phase the afm transition only occurs for zero-field cooled measurements. At low temperature, T < T N , La 1.5 Sr 1.5 Mn 2 O 7 undergoes a metamagnetic transition; the threshold field was found to depend strongly on the temperature (≈0.8 T at 20 K/≈3.5 T at 5 K). With the T MI of 102 K the compound shows metallic behaviour clearly above T C . It becomes insulating again at T N , which is in line with the properties of Nd 0.2 La 1.2 Sr 1.6 Mn 2 O 7 . Large magnetoresistance ratios (MR) have been measured for Nd 0.2 La 1.8-2 x Sr 1+2 x Mn 2 O 7 with x = 0.4, 0.3 and La 1.5 Sr 1.5 Mn 2 O 7 near T C . Furthermore, the metamagnetic transition causes high values of MR for La 1.5 Sr 1.5 Mn 2 O 7 and Nd 0.2 La 1.2 Sr 1.6 Mn 2 O 7 at low fields.