  We theoretically investigate the effective exchange interaction,
$J_\mathrm{eff}$, mediated by conductive electrons within a nonmagnetic metal
spacer, in the presence of a bias voltage, sandwiched by two ferromagnetic
insulators. On the basis of the tight-binding model, we show the voltage and
spacer thickness dependences of $J_\mathrm{eff}$, and its contorollability is
demonstrated. We also propose a new magnonic device with the functions of both
field effect transistor and non-volatile memory.
