Novel orbital order in the A-site ordered manganite, TbBaMn2O6

The crystal structure and electromagnetic properties of the A-site ordered perovskite manganite, TbBaMn2O6, have been studied. TbBaMn2O6 has successive phase transitions; the structural transition at T-t = 450K, the charge/orbital order transition at T-CO = 430K and the antiferromagnetic transitions on Mn ions at T-N = 210 K and on Tb ions at T-N = 12 K. At T-t, both a- and b-axis expand by 0.7%, and the c-axis shrinks by 1.3% in the low-temperature phase. The magnetic interaction changes from ferromagnetic (the high-temperature triclinic phase) to anti ferromagnetic (the low-temperature monoclinic phase) across T-t. The electrical resistivity clearly shows anisotropic behaviors at T-t; an increase of resistivity along the c-axis and no change within the ab-plane. The obtained results strongly suggest the d(x2-y2)-type orbital order at T-t. (c) 2005 Elsevier Ltd. All rights reserved.