  Subjecting a magnetic tunnel junction (MTJ) to a spin current and/or electric
voltage induces magnetic precession, which can reciprocally pump current
through the circuit. This results in an ac impedance, which is sensitive to the
magnetic field applied to the MTJ. Measuring this impedance can be used to
characterize the coupling between the magnetic free layer and the electric
current as well as a read-out of the magnetic configuration of the MTJ.
