CHARGE, ORBITAL, AND MAGNETIC-ORDERING IN LA0.5CA0.5MNO3

The unusual magnetic properties of La0.5Ca0.5MnO3 were found to be ass ociated with structural and magnetic ordering phenomena, resulting fro m the close interplay between charge, orbital, and magnetic ordering. Analysis of synchrotron x-ray and neutron powder diffraction data indi cates that the anomalous and hysteretic behavior of the lattice parame ters occurring between T-C similar to 225 K and T-N similar to 155 K i s due to the development of a Jahn-Teller (J-T) distortion of the MnO6 octahedra, the d(z)(2) orbitals being oriented perpendicular to the o rthorhombic b axis. We observed an unusual broadening of the x-ray Bra gg reflections throughout this temperature region, suggesting that thi s process occurs in stages. Below TN, the development of well-defined satellite peaks in the x-ray patterns, associated with a transverse mo dulation with q = [1/2- epsilon,0,0], indicates that quasicommensurate (epsilon similar to 0) orbital ordering occurs within the a-e plane a s well. The basic structural features of the charge-ordered low-temper ature phase were determined from these satellite peaks. The low-temper ature magnetic structure is characterized by systematic broadening of the magnetic peaks associated with the ''Mn+3'' magnetic sublattice. T his phenomenon can be explained by the presence of magnetic domain bou ndaries, which break the coherence of the spin ordering on the Mn+3 si tes while preserving the coherence of the spin ordering on the Mn+4 su blattice as well as the identity of the two sublattices. The striking resemblance between these structures and the structural ''charge order ing'' and ''discommensuration'' domain boundaries, which were recently observed by electron diffraction and real-space imaging, strongly sug gests that these two types of structures are the same and implies that , in this system, commensurate long-range charge ordering coexists wit h quasicommensurate orbital ordering.