Interplay of structural, magnetic and transport properties in the layered Co-based perovskite LnBaCo_2O_5 (Ln = Tb, Dy, Ho)

摘要

The oxygen deficient cobaltites LnBaCo_2O_5 (Ln = Tb, Dy, Ho) exhibit two successive crystallographic transitions at T_N ～ 340 K and at T_(CO) ～ 210 K. Whereas the first transition (P4/mmm to Pmmm) is related to the long-range antiferromagnetic ordering of the Co ions (spin ordering), the second transition (Pmmm to Pmmb) corresponds to the long-range ordering of the Co~(2+) and Co~(3+) species (charge ordering) occurring in 1:1 ratio in the structure. The charge ordered (CO) state was directly evidenced by the observation of additional superstructure peaks using neutron and electron diffraction techniques. The CO state was also confirmed indirectly from refinement of high resolution neutron diffraction data as well as from resistivity and DSC measurements. From the refined saturated magnetic moment values only, ～3.7μ_B and ～2.7μ_B, the electronic configuration of the Co ions in LnBaCo_2O_5 remains conjectural. Two pictures, with Co~(3+) ions either in intermediate spin state (t_(2g)~5e_g~1) or in high spin state (t_(2g)~4e_g~2), describe equally well our experimental data. In both cases, the observed magnetic structure can be explained using the qualitative Goodenough-Kanamori rules for superexchange. Finally, in contrast to the parent Ln = Y compound [Vogt et al., Phys. Rev. Lett. 84, 2969 (2000)], we do not report any spin transition in LnBaCo_2O_5 (Ln = Tb, Dy, Ho).