  We calculate the efficiency with which magnetic tunnel junctions can be used
as resonant detectors of incident microwave radiation via the spin-torque diode
effect. The expression we derive is in good agreement with the sensitivities we
measure for MgO-based magnetic tunnel junctions with an extended (unpatterned)
magnetic pinned layer. However, the measured sensitivities are reduced below
our estimate for a second set of devices in which the pinned layer is a
patterned synthetic antiferromagnet (SAF). We suggest that this reduction may
be due to an undesirable coupling between the magnetic free layer and one of
the magnetic layers within the etched SAF. Our calculations suggest that
optimized tunnel junctions should achieve sensitivities for resonant detection
exceeding 10,000 mV/mW.
