  Spintronic rf detectors were demonstrated, recently, for energy harvesting
and wireless communication at low input power. Here we report on the
optimization of the rectified output dc voltage using magnetic tunnel junctions
(MTJ) with strong perpendicular anisotropy (PMA) of both the polarizing and the
free layer. The magnetization of the polarizing layer is fixed out of plane,
while the free layer thickness is adjusted so that its magnetization
orientation changes from in plane to out of plane. The rectification dc output
voltage lies in the mV range for moderate rf powers, with a signal to noise
ratio of 10 to 100 for Prf = -25dBm. It shows a strong dependence on the
dimensions of the MTJ: it increases by a factor of 5 to 6 when reducing the
diameter from 150nm to 20nm. This enhancement can be doubled when reducing the
FL thickness from 1.8nm to 1.6nm. This dimensional enhancement is attributed to
the change of the effective anisotropy of the excited free layer, and the MTJ
resistance. The results are of interest for the design of spintronic based rf
detectors with optimized sensitivity.
