Spin-torque diode with metamaterial-based absorbing coating for the efficient waste heat energy harvesting from microwave radiation

Currently, there is a rapid development of spin caloritronics, an emerging field of physics of the interaction of spin and thermoelectric phenomena in magnetic materials, which is applicable for the development of highly efficient miniature thermal energy harvesters. In this work we propose the concept of a spin-torque diode based on a magnetic tunnel junction combined with metamaterial-based absorbing thin-film coating consisting of a metal-dielectric-metal multilayer structure. The numerical simulation of the coating properties demonstrates near unity absorption of microwave energy in a wide frequency range (from hundreds of MHz to several GHz), which leads to a high temperature gradient through the tunnel spacer of the junction. The inclusion of thermal barriers made of materials with low thermal conductivity into the spin-diode structure provides an increase in the temperature gradient up to hundreds of mK, which, due to the tunnel magneto-Seebeck effect, makes it possible to efficiently utilize heat losses during microwave heating and generate a thermo-voltage at the level of several hundreds of μV. Taking into account the high sensitivity of spin-torque diodes (more than 200 mV/μW), as well as the prospects for their scaling down to 1X nm, it opens up the possibility of creating miniature thermoelectric elements, which enable to supply low-power IoT devices.