  Voltage-driven spin transfer torque in a magnetic tunnel junction comprising
magnetic insulating electrodes is studied theoretically. In contrast with the
conventional magnetic tunnel junctions comprising transition metal
ferromagnets, the spin transfer torque presents unconventional bias
dependencies, related to the presence of spin-dependent Fowler-Nordheim
tunneling processes. In particular, we find that (i) the out-of-plane torque
generally dominates the in-plane torque, (ii) out-of-plane torque and in-plane
torque are symmetric and asymmetric at low bias voltage, respectively, and
(iii) both of torques show a dramatic enhancement at large bias voltage.
Materials consideration are discussed and we show that due to the low damping
parameter expected in magnetic insulators a spin transfer torque can be
experimentally observed in such systems.
