Large magnetoresistance of the polycrystalline metamagnetic layered manganite La1.2Nd0.2Sr1.6Mn2O7

Magnetization and magnetotransport have been investigated for polycrystalli ne samples of the bilayer Ruddlesden-Popper compound La1.2Nd0.2Sr1.6Mn2O7 t hat shows the colossal magnetoresistance effect connected with ferromagneti c ordering at T-C=74 K. At T-2=38 K the compound reveals a transition from the ferromagnetic to an antiferromagnetic spin arrangement of alternating f erromagnetic bilayers, which undergoes a first order metamagnetic transitio n towards ferromagnetism in a magnetic field of similar to 3.5 kOe at 20 K. In measurements of resistance vs magnetic field, this transition is accomp anied by a large resistance drop of [R(5 kOe)-R(0)]/R(0)=-60% at 5 K. The t unneling-like magnetoresistance (MR) known for current transport perpendicu lar to the layers in (La, Nd)(1.4)Sr1.6Mn2O7 single crystals contributes to this large MR observed in a polycrystalline sample. While current is expec ted to mainly follow the bilayers within the grains due to the anisotropy o f conductivity, the high value of resistivity of our sample (similar to 8 O mega cm at 5 K) suggests that current paths are partially directed perpendi cular to the layers. (C) 2000 American Institute of Physics. [S0021-8979(00 )03102-9].