Magnetic and electrical properties of LBaMn2O6-γ (L=Pr, Nd, Sm, Eu, Gd, Tb) manganites -: art. no. 184424

Influence of cation order-disorder phenomena on the crystal structure, magnetic, and electrical transport properties of new CMR perovskites for LBaMn2O6-gamma (L=Pr, Nd, Sm, Eu, Gd, Tb) series has been investigated. For each rare-earth ion three compounds have been synthesized by the topotactic reduction-oxidation method. Structural investigations have shown the oxygen-stoichiometry LBaMn2O6 compound obtained in air to be cubic with disordered L3+ and Ba2+ cations whereas the oxygen-deficient LBaMn2O5 is tetragonal with ordered L3+ and Ba2+ and alternate stacking of rare earth and barium containing layers along c. This crystal structure is similar to the YBaCuFeO5 related one. Another form of oxygen-stoichiometry LBaMn2O6 compound obtained by reoxidation of oxygen-deficient LBaMn2O5 is also tetragonal and retains the ordering of L3+ and Ba2+ cations. It is notable that the reoxidized EuBaMn2O6 compound has an orthorhombic unit cell. It is observed that this type of cation ordering leads to considerable increase of transition temperature to paramagnetic state. For example, disordered EuBaMn2O6 compound has magnetic properties similar to spin glass and shows freezing temperature of magnetic moments T(f)approximate to40 K while ordered EuBaMn2O6 is an inhomogeneous ferromagnet with Curie point T(C)approximate to260 K. Electrical resistivity behavior correlates with magnetization. Below the T-C the Pr, Nd, Sm based compounds undergo a transition to metallic state and demonstrate a peak of magnetoresistance. It is supposed that the remarkable changing of the magnetic and electrical properties of the reoxidized compounds is a consequence of the L/Ba ordering and can be explained on the base of the Goodenough-Kanamori rules for 180degrees indirect superexchange interactions taking into account an ion size effect in A sublattice of perovskite.