Magnetic Structures and Spin States of NdBaCo₂O_5.5

Abstract

Neutron diffraction studies have been carried out on a single crystal of oxygen-deficient perovskite NdBaCo₂O_5.5 in the temperature (T) range between 7 K 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^*∼30 K. The magnetic structures have been analyzed at 7 K (<T^*) and 40 K (T^*<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^*, 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.