Abstract
Spin-state transitions are the hallmark of rare-earth cobaltates. In order to understand them, it is essential to identify all relevant parameters which shift the energy balance between spin states and determine their trends. We find that , the - crystal-field splitting, increases by 250 meV when increasing pressure to 8 GPa and by about 150 meV when cooling from 1000 K to 5 K. It changes, however, by less than 100 meV when La is substituted with another rare earth. Moreover, the Hund's rule coupling is about the same in systems with very different spin-state transition temperature, like LaCoO and EuCoO. Consequently, in addition to and , the Coulomb-exchange anisotropy and the superexchange energy gain play a crucial role and are comparable with spin-state-dependent relaxation effects due to covalency. We show that in the LnCoO series, with or another rare earth (), superexchange progressively stabilizes a low-spin ground state as the Ln ionic radius decreases. We give a simple model to describe spin-state transitions and show that, at low temperature, the formation of isolated high-spin/low-spin pairs is favored, while in the high-temperature phase, the most likely homogeneous state is high spin rather than intermediate spin. An orbital-selective Mott state could be a fingerprint of such a state.
2 More- Received 2 August 2012
DOI:https://doi.org/10.1103/PhysRevB.86.184413
©2012 American Physical Society