Structures and Electromagnetic Properties of New Metal-Ordered Manganites; RBaMn₂O₆ (R = Y and Rare Earth Elements)

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New metal-ordered manganites RBaMn₂O₆ have been synthesized and investigated in the structures and electromagnetic properties. RBaMn₂O₆ can be classified into three groups from the structural and electromagnetic properties. The first group (R = La, Pr and Nd) has a metallic ferromagnetic transition, followed by an A-type antiferromagnetic transition in PrBaMn₂O₆. The second group (R = Sm, Eu and Gd) exhibits a charge-order transition, followed by an antiferromagnetic long range ordering. The third group (R = Tb, Dy and Ho) shows successive three phase transitions, the structural, charge/orbital-order and magnetic transitions, as observed in YBaMn₂O₆. Comparing to the metal-disordered manganites (R³⁺₀.₅A²⁺₀.₅)MnO₃, two remarkable features can be recognized in RBaMn₂O₆; (1) relatively high charge-order transition temperature and (2) the presence of structural transition above the charge-order temperature in the third group. We propose a possible orbital ordering at the structural transition, that is a possible freezing of the orbital, charge and spin degrees of freedom at the independent temperatures in the third group. These features are closely related to the peculiar structure that the MnO₂ square-lattice is sandwiched by the rock-salt layers of two kinds, RO and BaO with extremely different lattice-sizes.