Magnetoresistance in double-layered perovskite-type manganese oxides

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Single-crystal growth of double-perovskite NdBaMn2O6, which locates near the multicritical point among ferromagnetic metal, antiferromagnetic charge-ordered insulator, and antiferromagnetic two-dimensional metal, was tried. Transport measurements of twin single crystals show the significant reduction of Neel temperature of about 280 K by application of a magnetic field. Giant negative magnetoresistance along the c axis was observed ; resistivity at 240 K decreases by 1/30.Competing among the abovementioned three phases in the A-site ordered RBaMn2O6 system was investigated by X-ray diffuse scattering. Raman scattering spectroscopy was also performed to observe the effect of charge/orbital ordering in this system, where charge/orbital ordered MnO2 sheets attack along the c axis in a different manner from the conventional A-site-disordered perovskite manganese oxides (R,A)MnO3.Detail study of neutron diffraction in NdBaMn2O6 shows that ferromagnetically ordered MnO2 sheets antiferromagnetically stack along the c-axis. Lattice parameters a and b are suddenly elongated below Neel temperature, while the c length shrinks. This anomaly in lattice parameters clearly demonstrates that the ferro-type ordering of x2-y2 orbitals is formed simultaneously at the ordering of the Mn spin sector.