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

Sr_1−xY_xCoO_2.65 (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√2a _p × 2√2a _p × 4a _p (with a _p corresponding to the unit cell parameter of the primitive cell) while basic diffraction peaks are well indexed in the space group I4/mmm (2a _p × 2a _p × 4a _p ) shows an almost standard magnetization. The basic magnetic structure is G-type antiferromagnetic with average magnetic moments of 2.7 μ _B/Co and 1.7 μ _B/Co in anion-deficient CoO_4+γ and stoichiometric CoO₆ layers, respectively. A ferromagnetic component of about 0.27 μ _B/Co is determined from the magnetization measurements at 8 K. Sr_0.8Y_0.2CoO_2.65 shows an almost standard magnetization versus temperature dependence whereas Sr_0.75Y_0.25CoO_2.65 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 T_N 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 T_N 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 CoO_4+γ layer.