Structure, Magnetism, and Spin Dependent Magnetoresistance in Doped Two-Dimensional Sr/sub 2/CoO/sub 4/ Compounds

In this work, we report our studies on the structures, transport, and magnetic properties in the rare earth element doped two dimensional layer structured perovskite compounds Sr_2-xRE_xCoO₄ (RE= rare earth element). Structure refinement has revealed that these compounds crystallize in the K₂NiF₄-type structure with space group I4/mmm. For Pr doped samples, the lattice parameter c decreases with Pr doping levels. The Curie temperatures are found to decrease gradually from 200 K for x=0.5 to less than 100 K for x>1 for the various dopants Pr, Eu, Nd, and Sm. The resistivities were found to increase with doping levels. A large coercive field of about 1 T was found for Pr and Sm with x=0.75. A negative field hysteresis magnetoresistance in close correlation to the coercive field has been observed for Sr_1.5Sm_0.5CoO₄, which can be explained by grain boundary spin dependent tunneling effects.