Orbital Ordering of Intermediate-Spin State of Co3+ in Sr3YCo4O10.5

The orbital ordering and spin state in the room-temperature ferromagnet Sr 3 YCo 4 O 10.5 were investigated by a resonant X-ray scattering technique. A resonating signal with a $\sigma\rightarrow\pi'$ scattering component was observed at ($h00$), where $h=2n+1$ ($n$: integer), near the $1s\rightarrow e_{g}$ transition energy of Co. This provides direct evidence of not only $e_{g}$ orbital ordering but also the existence of the intermediate-spin (IS) state. Moreover, we suggest that the ferrimagnetism is induced by the $e_{g}$ orbital ordering of the IS state of Co 3+ . The peculiar $e_{g}$ orbital, spin-state, and magnetic orderings are also discussed.