  We report on a combined soft x-ray absorption and magnetic circular dichroism
(XMCD) study at the Co-$L_{3,2}$ on the hybrid 3$d$/5$d$ solid state oxide
Sr$_2$Co$_{0.5}$Ir$_{0.5}$O$_4$ with the K$_2$NiF$_4$ structure. Our data
indicate unambiguously a pure high spin state $(S=2)$ for the Co$^{3+}$
(3$d^6$) ions with a significant unquenched orbital moment $L_z/2S_z=0.25$
despite the sizeable elongation of the CoO$_6$ octahedra. Using quantitative
model calculations based on parameters consistent with our spectra, we have
investigated the stability of this high spin state with respect to the
competing low spin and intermediate spin states.
