Materials and Structures Laboratory, Tokyo Institute of Technology^* Photon Factory, KEK, Japan^**
○Satoshi Sasaki^* Norio Shimizu^* Takeshi Ohno^* Takayasu Hanashima^* Koichi Ohkubo^* Maki Okube^* Takeharu Mori^**

Co ions in ferrites, which are notable to be ferrimagnetic at room temperature, give strong magnetic moments through the super exchange interaction between the tetrahedral and octahedral sites. Since the spinel structure has the geometrical frustration of a pyrochlore type, site occupancy between Fe and Co ions and the valence and spin states are key factors to control the physical properties. On the other hand, an intermediate-spin state of Co ions can be stabilized Co³⁺ or Co⁴⁺ with the strong Co 3d- O 2p hybridization in perovskite structures. Nonmagnetic LaCoO₃ transforms to paramagnetic with the spin-crossover phenomenon around T = 100 K. By substituting Sr for La, (La,Sr)CoO₃ becomes ferromagnetic. A combination study of x-ray diffraction, XANES and XMCD at Co K absorption edges has been carried out to determine the valence and spin states of Co ions at PF-BL-3A and BL-10A. Compositional dependence on the valence state of Co ions has been examined for cubic spinel of Fe_xCo_3-xO₄ (x = 0, 0.8, 1.0, 1.1, 1.4, 1.8, 2.0 and 2.1) and rhombohedral perovskite of La_1-xSr_xCoO₃ (x = 0, 0.2, 0.3, 0.4, 0.5 and 0.6). The present study suggests (1) mixing of two spin states of Co ions in Fe cobaltite, (2) the existence of an intermediate-spin state of Co³⁺ in LaCoO₃ and (3) the double-exchange interaction between Co³⁺ and Co⁴⁺ in La-Sr-Co oxide.