Magnetic Structures and Spin States of NdBaCo2O5:5

Neutron diffraction studies have been carried out on a single crystal of oxygen-deficient perovskite NdBaCo₂O_5:5 in the temperature (T) range between 7K and 320 K. We have found a magnetic transition at T_c ~ 170 K, and a rapid change of the intensities of several reflections at T_c ~ 30 K. The magnetic structures have been analyzed at 7K (< T_c) and 40K (T_c < T < T_c). Non-collinear magnetic structures, which are based on the G-type antiferromagnetic structure, are proposed at these temperatures. In the T region between T* and T_c, Co³⁺ ions of CoO₆ octahedra are in the intermediate spin state, and those of CoO₅ pyramids are in the high spin state. In the region of T < T_c, the magnitude of the Co₃ spins in the octahedra decreases gradually as T decreases, suggesting that the spin state approaches the low spin state. It is pointed out that the spin state of Co ions depends on the local structure around them.