Optical-Spectroscopic Detection of Spin-Exchange Interaction Pyrochlore Molybdates

Optical spectroscopy was used to determine nearest-neighbor spin correlations in pyrochlore molybdates [Formula presented] ([Formula presented], Sm, and Nd), which exhibit ferromagnetic metal to spin-glass insulator transition as the [Formula presented] ion size decreases. Using an analysis based on the orbitally degenerate Hubbard model, we could estimate important physical parameters, such as the effective on-site Coulomb energy [Formula presented] and the Hund rule exchange coupling [Formula presented]. We demonstrated experimentally and theoretically that the effective superexchange interaction between the Mo ions depends on [Formula presented]/[Formula presented], which determines the phase boundary of the magnetic ground states.