Spin-state ordering and magnetic structures in YBaCo2O5.5/5.44
  The antiferromagnetic - ferromagnetic phase transition in YBaCo2O5.50 and YBaCo2O5.44 cobaltites with different types of oxygen-ion ordering in the [YO0.5/0.44] layers has been studied by neutron powder diffraction. Using the magnetic symmetry arguments in combination with group-theoretical analysis, the crystal and magnetic structures above and below the phase transformation temperature, Ti, were determined and successfully refined. In both cases, the proposed models involve a spin-state ordering between diamagnetic (t2g6eg0, S = 0) and paramagnetic (t2g4eg2, S = 2) Co3+ ions with octahedral coordination. Electronic ordering results in a nonzero spontaneous magnetic moment in the high-temperature magnetic phases with isotropic negative exchange interactions. In the case of YBaCo2O5.5, the phase transformation does not change Pmma symmetry of the crystal structure. The wave vectors of magnetic structures above and below Ti are k = 0 and k = c*/2, respectively. In the case of YBaCo2O5.44, a crossover P4/nmm - I4/mmm was involved to solve the low-temperature magnetic structure. The wave vectors in both high-temperature and low-temperature magnetic phases are k = 0. Mechanisms of the phase transformation in both compositions are discussed in the light of obtained magnetic structures. The proposed spin configurations were compared with other models reported in literature. 
D.D. Khalyavin, D.N. Argyriou, U. Amann, A.A. Yaremchenko, V.V.   Kharton