Spin state and orbital moments across the metal-insulator-transition of REBaCo₂O_5.5 investigated by XMCD

DyBaCo₂O_5.5 has shown a complex phase diagram, which is based on the interplay of different energy scales, related to magnetism, orbital ordering and for example Co spin-state transitions. For a detailed understanding of these fascinating materials it is therefore necessary to identify the order of the different energy scales. Small changes in the corresponding energy relations strongly influence the electronic structure and ground state properties, like low and high spin configurations, which have been controversially discussed in order to interpret the metal-to-insulator (MIT) transition in REBaCo₂O_5.5 (RE = rare earths). To clarify unambiguously the microscopic nature of the spinstate evolution associated with this MIT, we performed detailed temperature and angular dependent x-ray magnetic circular dichroism measurements in DyBaCo₂0_5.5 single crystals above and below the MIT and at the onset of the ferromagnetic phase. Anisotropic contributions of spin and orbital moments have been observed with an extremely high signal-to-noise ratio. We can identify a higher-spin- to lower-spin-state change across the MIT, which is in contrast to previous macroscopic experimental findings. Only the Co ions in octahedral environment are found to be in a reduced spin configuration in the hightemperature metallic state.