  The spin-torque in magnetic tunnel junctions possesses two components that
both depend on the applied voltage. Here, we develop a new method for the
accurate extraction of this bias-dependence from experiments over large voltage
ranges. We study several junctions with different magnetic layer structures of
the top electrode. Our results obtained on junctions with symmetric CoFeB
electrodes agree well with theoretical calculations. The bias-dependences of
asymmetric samples, with top electrodes containing NiFe, however, are twisted
compared to the quadratic form generally assumed. Our measurements reveal the
complexity of spin-torque mechanisms at large bias.
