Ferromagnet-antiferromagnet transition in layered perovskites of Sr3YCo4O10.5 type

Sr1−xYxCoO2.65 (x = 0.2) with layered perovskite structure was studied by neutron diffraction, synchrotron x-ray and magnetometry methods. It is shown that in the 90-375 K temperature range the crystal structure can be described by the monoclinic space group A2/m with the superstructure 4 2ap × 2 2ap × 4ap (with ap corresponding to the unit cell parameter of the primitive cell) while basic diffraction peaks are well indexed in the space group I4/mmm (2ap × 2ap × 4ap) shows an almost standard magnetization. The basic magnetic structure is G-type antiferromagnetic with average magnetic moments of 2.7 B/Co and 1.7 B/Co in anion-deficient CoO4+γ and stoichiometric CoO6 layers, respectively. A ferromagnetic component of about 0.27 B/Co is determined from the magnetization measurements at 8 K. Sr0.8Y0.2CoO2.65 shows an almost standard magnetization versus temperature dependence whereas Sr0.75Y0.25CoO2.65 exhibits an antiferromagnet-ferromagnet transition accompanied by a structural transformation. There is practically no spontaneous magnetization in x = 0.3. The type of the magnetic structure and the high value of TN suggest that the Co3+ ions are in both structural layers predominantly in the low-spin (LS)/high-spin (HS) state mixture. It is proposed that the ferromagnetic component is due to the orbital ordering occurring at TN in the CoO5 pyramids and the concomitant appearance of ferromagnetic coupling between the Co3+(HS) ions located in these CoO5 pyramids in the anion-deficient CoO4+γ layer.