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  Khalyavin, D.D.; Argyriou, D.N.; Amann, U.; Yaremchenko, A.A.; Kharton, V.V.: Spin-state ordering and magnetic structures in the cobaltites YBaCo₂O_5+δ (δ=0.50 and 0.44). Physical Review B 75 (2007), p. 134407/1-15

10.1103/PhysRevB.75.134407

Abstract:
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 in combination with group-theoretical analysis. As a result, 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 (t_2g⁶e_g⁰, S=0) and paramagnetic (t_2g⁴e_g², 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 the Pmma (2ap×2ap×2ap) 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 (3sqrt(2)ap×3sqrt(2)ap×2ap)-->I4/mmm (3sqrt(2)ap×3sqrt(2)ap×4ap) 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.