  We have measured the bias voltage and field dependence of eigenmode
frequencies in a magnetic tunnel junction with MgO barrier. We show that both
free layer (FL) and reference layer (RL) modes are excited, and that a
cross-over between these modes is observed by varying external field and bias
voltage. The bias voltage dependence of the FL and RL modes are shown to be
dramatically different. The bias dependence of the FL modes is linear in bias
voltage, whereas that of the RL mode is strongly quadratic. Using modeling and
micromagnetic simulations, we show that the linear bias dependence of FL
frequencies is primarily due to a linear dependence of the perpendicular spin
torque on bias voltage, whereas the quadratic dependence of the RL on bias
voltage is dominated by the reduction of exchange bias due to Joule heating,
and is not attributable to a quadratic dependence of the perpendicular spin
torque on bias voltage.
