Magnetic and electrical properties ofLBaMn2O6−γ(L=Pr,Nd, Sm, Eu, Gd, Tb) manganites

Abstract

Influence of cation order-disorder phenomena on the crystal structure, magnetic, and electrical transport properties of new CMR perovskites for LBaMn 2 O 6 - γ (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 LBaMn 2 O 6 compound obtained in air to be cubic with disordered L 3 + and Ba 2 + cations whereas the oxygen-deficient LBaMn 2 O 5 is tetragonal with ordered L 3 + and Ba 2 + and alternate stacking of rare earth and barium containing layers along c. This crystal structure is similar to the YBaCuFeO 5 related one. Another form of oxygen-stoichiometry LBaMn 2 O 6 compound obtained by reoxidation of oxygen-deficient LBaMn 2 O 5 is also tetragonal and retains the ordering of L 3 + and Ba 2 + cations. It is notable that the reoxidized EuBaMn 2 O 6 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 EuBaMn 2 O 6 compound has magnetic properties similar to spin glass and shows freezing temperature of magnetic moments T f 40 K while ordered EuBaMn 2 O 6 is an inhomogeneous ferromagnet with Curie point T C 260 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 180° indirect superexchange interactions taking into account an ion size effect in A sublattice of perovskite.