  Metal-insulator transitions involve a mix of charge, spin, and structural
degrees of freedom, and when strongly-correlated, can underlay the emergence of
exotic quantum states. Mott insulators induced by the opening of a Coulomb gap
are an important and well-recognized class of transitions, but insulators
purely driven by spin correlations are much less common, as the reduced energy
scale often invites competition from other degrees of freedom. Here we
demonstrate a clean example of a spin-correlation-driven metal-insulator
transition in the all-in-all-out pyrochlore antiferromagnet Cd2Os2O7, where the
lattice symmetry is fully preserved by the antiferromagnetism. After the
antisymmetric linear magnetoresistance from conductive, ferromagnetic domain
walls is carefully removed experimentally, the Hall coefficient of the bulk
reveals four Fermi surfaces, two of electron type and two of hole type,
sequentially departing the Fermi level with decreasing temperature below the
N\'eel temperature, T_N. Contrary to the common belief of concurrent magnetic
and metal-insulator transitions in Cd2Os2O7, the charge gap of a continuous
metal-insulator transition opens only at T~10K, well below T_N=227K. The
insulating mechanism resolved by the Hall coefficient parallels the Slater
picture, but without a folded Brillouin zone, and contrasts sharply with the
behavior of Mott insulators and spin density waves, where the electronic gap
opens above and at T_N, respectively.
