Crystal Structure and Magnetic Properties of an oxygen deficient n = 2 Ruddlesden-Popper phase Sr$_{3}$Co$_{2}$O$_{5.67}$

Interest in charge, orbital, and spin state phenomena in perovskite and related cobalt oxides is a growing area of transition metal oxide physics. Recently, J. Matsuno \textit{et al.}\footnote{ J. Matsuno \textit{et al}., PRL \textbf{93}, 167202 (2004).} have found that epitaxial films of the n = 1 Ruddlesden-Popper (R-P) phase Sr$_{2}$CoO$_{4}$ are metallic ferromagnets with relatively high T$_{C} \quad \sim $ 250 K. This is particularly interesting in light of the formal oxidation state of Co, Co$^{4+}$, offering no clear source of carriers. To extend the materials chemistry and physics of the R-P series of cobaltites, we have synthesized the n = 2 R-P phase Sr$_{3}$Co$_{2}$O$_{7-\delta }$ in bulk form. The crystal structure [from neutron powder diffraction (NPD) data] of our most oxygen-deficient sample, Sr$_{3}$Co$_{2}$O$_{5.67}$ is orthorhombic \textit{Immm} with a = 3.94025(9) {\AA}, b = 3.67479(9) {\AA} and c = 20.6642(5) {\AA}. The magnetization versus temperature data show two antiferromagnetic transitions at approximately 170 K and 220 K. To further elucidate the magnetic properties of this material, we have conducted a temperature-dependent NPD study. The low temperature magnetic structure is surprisingly complex and suggestive of an incommensurate ordering wave vector. Full details and results of the NPD study will be given.