  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.
