Novel structures and electromagnetic properties of the A-site-ordered/disordered manganites RBaMn2O6/R0.5Ba0.5MnO3 (R = Y and rare earth elements)

The A-site-ordered/disordered manganese perovskites, RBaMn2O6/R-0.5 Ba0.5MnO3 (R = Y and rare earth elements), are reviewed. RBaMn2O6 displays remarkable features: (1) the charge/orbital order (CO) transition at relatively high temperatures far above 300 K, (2) a new stacking variation of the CE-type CO with a fourfold periodicity along the c-axis, (3) the presence of structural transition possibly accompanied by dx(2)-y(2) orbital order and (4) electronic phase segregation. These novel structural and electromagnetic properties are discussed in terms of the structural characteristic that the MnO2 square sublattice is sandwiched by two types of rock-salt layer, RO and BaO, with different lattice sizes. Such structure introduces a strong frustration to the MnO2 sublattice and gives a new perturbation to the competition of multiple degrees of freedom among charge, orbital, spin and lattice. In R0.5Ba0.5MnO3 with a primitive cubic perovskite cell, on the other hand, the magnetic glassy states are dominant as the ground state. A peculiar behaviour, steplike ultrasharp magnetization and resistivity changes, has been observed in Pr0.5Ba0.5MnO3.