Spin-direction-dependent tunneling transitions in layered manganites La_2-2xSr_1+2xMn₂O₇ (x=0.35 and 0.4)

The transport and magnetic properties of layered manganites, La_2-2xSr_1+2xMn₂O₇ (x=0.35 and 0.40), synthesized by solid-state reactions at various temperatures are reported. In contrast to the compounds with other doping levels, these compounds show metal-nonmetal transitions at 70-90 K in addition to the double-exchange-driven transitions at about 120 K. The lower-temperature transition is most pronounced when the synthesis temperature is low and gradually decreases in magnitude as the reaction temperature is increased. Temperature-dependent magnetic data show that there is no magnetic change corresponding to the transition at the lower temperatures, whereas the higher-temperature transition accompanies a ferromagnetic ordering, consistent with the double-exchange mechanism. Low-temperature (10 K) neutron diffraction data of selected samples show that these compounds order ferromagnetically with co-linear Mn spins that have tilting angles of about 70° against the c axis in all samples, intermediate between the x=0.30 and 0.40 compounds in the literature, and that the different synthesis temperatures do not influence the magnetic structures of these compounds to a significant level. A mechanism that involves the spin orientation in the ferromagnetic state and the grain boundary effect is proposed to explain why the low-temperature transitions are observed only from samples with limited doping range and low synthesis temperatures.