Size-dependent enhancement of passive microwave rectification in magnetic tunnel junctions with perpendicular magnetic anisotropy

Spintronic rf detectors are efficient nanoscale counterparts to conventional semiconductor-based components 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 (MTJs) with strong perpendicular anisotropy of both the polarizing and free layers. 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. Rectification dc output voltages in the mV range are obtained for moderate rf source powers with a signal-to-noise ratio of 26-39 dB for P-rf = -25 dBm and a sensitivity epsilon of 300 mV/mW. The rectified signal shows a strong dependence on MTJ dimensions: it increases by a factor of 5-6 when reducing the diameter from 180 to 50 nm. Furthermore, this enhancement can be doubled when reducing the free layer thickness from 1.8 to 1.6 nm. This size-related enhancement is attributed to several jointly acting effects: the amplitude of the spin transfer torque that depends inversely on the diameter, the effective anisotropy that depends on the thickness of the excited layer, and the tunneling magneto-resistance ratio that for the devices studied here depends on diameter. The obtained results indicate that the geometry of the MTJ can be used to design spintronic based rf detectors with optimized sensitivity.