Charge-orbital ordering near the multicritical point in A-site ordered perovskites SmBaMn2O6 and NdBaMn2O6 -: art. no. 064418

Electronic and magnetic structures of A-site ordered perovskite-type manganites, LnBaMn(2)O(6) (Ln=Sm and Nd), located close to the multicritical point of the competing phases, have been investigated by measurements of resonant x-ray scattering, optical spectroscopy, and neutron diffraction. In the SmBaMn2O6 crystal, two resonant peaks, (5/4 5/4 0) and (3/2 1/2 1/2) in the tetragonal a(p)xa(p)x2a(p) setting (a(p)approximate to3.9 Angstrom), are observed at room temperature and are attributed to the AABB-type stacking of ab planes with a checkerboard-type charge and stripe-type orbital orders. The change of the stacking manner from AABB- to ABAB-type around 190K is evidenced by the disappearance of the resonant peak (3/2 1/2 1/2) and a steep increase in Raman scattering intensity of a Jahn-Teller phonon mode below 190 K. The powder neutron diffraction study has shown that NdBaMn2O6 has an A-type antiferromagnetic state below 275 K. The abrupt expansion of the ab plane and shrinkage along the c axis upon the antiferromagnetic transition at 275 K suggest that uniform d(x)(2)-y(2) orbital ordering occurs.