Spin-direction-dependent tunneling transitions in layered manganites La2-2xSr1+2xMn2O7 (x=0.35 and 0.4) - art. no. 054437

The transport and magnetic properties of layered manganites, La2 - 2xSr1 2xMn2O7 (x = 0.35 and 0.40), synthesized by solid-state reactions at variou s temperatures are reported. In contrast to the compounds with other doping levels, these compounds show metal-nonmetal transitions at 70-90 K in addi tion to the double-exchange-driven transitions at about 120 K. The lower-te mperature transition is most pronounced when the synthesis temperature is l ow and gradually decreases in magnitude as the reaction temperature is incr eased. Temperature-dependent magnetic data show that there is no magnetic c hange corresponding to the transition at the lower temperatures, whereas th e higher-temperature transition accompanies a ferromagnetic ordering, consi stent with the double-exchange mechanism. Low-temperature (10 K) neutron di ffraction data of selected samples show that these compounds order ferromag netically with co-linear Mn spins that have tilting angles of about 70 degr ees against the c axis in all samples, intermediate between the x = 0.30 an d 0.40 compounds in the literature, and that the different synthesis temper atures do not influence the magnetic structures of these compounds to a sig nificant level. A mechanism that involves the spin orientation in the ferro magnetic state and the grain boundary effect is proposed to explain why the low-temperature transitions are observed only from samples with limited do ping range and low synthesis temperatures.