Magnetic ordering in Dy1-xCaxBaCo2O5.5 for x = 0.0 and 0.1

Abstract

The crystal and magnetic structures of Dy1-xCaxBaCo2O5.5 for x = 0.0 and 0.1 have been studied by neutron powder diffraction and the crystal structures of both compounds were found to be best described in space group Pmmm with a ap × 2ap × 2ap unit cells where ap is the lattice parameter of the cubic perovskite unit cell. The a- and b-axes were found to decrease and increase abruptly between 315 and 350 K as the temperature increases and the unit cell volumes exhibit signs of excess thermal expansion in the temperature range from 260 to 315 K. Dy0.9Ca0.1BaCo2O5.5 orders antiferromagnetically for T ≤ 305 K into a G-type magnetic structure with a 2ap × 2ap × 2ap magnetic unit cell. DyBaCo2O5.5 exhibits two magnetically ordered phases and a G-type magnetic structure was observed at the investigated temperatures 260 and 290 K. A 2ap × 2ap × 4ap magnetic unit cell was needed for indexing of the magnetic reflections observed for T ≤ 230 K. The low temperature magnetic structure of DyBaCo2O5.5 is different from the observed magnetic structures of TbBaCo2O5.5 and HoBaCo2O5.5 despite the proximity of Tb, Dy and Ho in the periodic table. It is a relatively complex antiferromagnetic structure with both pyramidally and octahedrally coordinated Co ions in the intermediate spin state. It contains both ferro- and antiferromagnetic interactions and the magnetic moments are canted in the a, b-plane. The canting angles between the magnetic moments and the b-axis are 6.6 and 50.0° at 20 K for the pyramidally and octahedrally coordinated Co ions, respectively. The high and low temperature magnetic phases were found to coexist at 230 K.