  We report the tunability of the exchange bias effect by the first-order
metal-insulator transition (known as the Verwey transition) of Fe3O4 in CoO (5
nm)/Fe3O4 (40 nm)/MgO (001) thin film. In the vicinity of the Verwey
transition, the exchange bias field is substantially enhanced because of a
sharp increase in magnetocrystalline anisotropy constant from high-temperature
cubic to lowtemperature monoclinic structure. Moreover, with respect to the
Fe3O4 (40 nm)/MgO (001) thin film, the coercivity field of the CoO (5 nm)/Fe3O4
(40 nm)/MgO (001) bilayer is greatly increased for all the temperature range,
which would be due to the coupling between Co spins and Fe spins across the
interface.
