Influence of cation and oxygen vacancy ordering on magnetic properties of RE$_{1-x}$Sr$_{x}$CoO$_{3-\delta }$ (RE=Ho, La)

The attractive physical properties of RE$_{1-x}$Sr$_{x}$CoO$_{y}$ (RE - rare earth and Y) are strongly dependent upon overall oxygen content y and the ordering of oxygen and RE/Sr, as well as charge and spin ordering on the Co$^{3+}$/Co$^{4+}$- sites. Structural, magnetic and thermoelectric properties of Ho$_{1/3}$Sr$_{2/3}$CoO$_{y}$ have been studied within the oxygen content range 2.67$<$y$<$2.80. The studied samples demonstrate antiferromagnetic order (G-type) and semiconductive behavior. No ferromagnetic ordering has been observed and compared with A. Maignan et al. on a closely related Sr$_{2/3}$Y$_{1/3}$CoO$_{y}$ with similar y. We have observed that a the same high degree of oxygen ordering and antiferromagnetic structure exists for both cation disordered (La) and ordered (Y,Ho) materials. We conclude that the high degree of oxygen vacancy ordering determines the observed properties via strong localization of Co$^{3+ }$electrons. Work at NIU was supported by the NSF-DMR-0302617 and at ANL by the U.S. DOE under contract No. DE-AC02-06CH11357.