Report on the DFG project "Optimization of different strategies for designing an energy harvester based on spin-torque diodes" (BE 2464/21-1)

Abstract

Steadily growing demand for cheap and green energy has caused a rapid development of so called ‘energy harvesting’ devices for producing dc-power from the ambient microwave radiation from various sources like TV and mobile-phone networks, Wi-Fi routers etc. The energy density of this radiation ranges from 1 to 1000 nW/cm^2, so that corresponding technology could be successfully used by low-power applications (digital thermometer, smoke detectors, some sensors in medicine etc.). The main goal of this project was the optimization of various designs for energy harvesters based on spin-torque-diodes (STDs), i.e. devices where dc-voltage is generated when an ac-current flows through a magnetic tunnel junction (MTJ). Using computer simulations, we have studied and optimized three main types of MTJ-based nanodevices: (i) ‘standard’ MTJ nanopillars of the resonant type employing quasi-homogeneous in-plane magnetization oscillations; (ii) MTJs in the out-of-plane precession regime for broadband rectification and (iii) multilayer stacks with the in-plane shape designed for oscillation of domains walls. As the results of this project we have determined optimal geometric and magnetic parameters for all three kinds of spin-torque-based energy harvesters listed above, and predicted corresponding maximal rectification efficiencies in ambient conditions.