Magnetic Tunnel Junctions (MTJs) based oscillators harvest enough RF energy to drive an LED.
Researchers at the National University of Singapore (NUS) and Tohoku University have discovered a way to use magnetic tunneling junctions (MTJs) for harvesting 2.4GHz RF energy and turning it into DC current.
Magnetic Tunnel Junctions (MTJs) consist of two layers of magnetic material separated by an insulator layer with a thickness of about 1nm. The insulating layer is so thin that electrons can tunnel through the barrier if a bias voltage is applied between the two metal electrodes. Because of their non-linear electrical characteristics, these junctions can act as high frequency oscillators and rectifiers.
Earlier studies have indicated that the frequency response of MTJs can be enhanced if several were coupled through electrical connections. The researchers at the Tohoku University fabricated 80 x 200nm ‘canted anisotropy’ MTJs. Here four MTJs were connected in series or parallel with some external DC bias. The researchers found that they indeed self-oscillate. The system initially started as unsynchronized oscillators but as the current increased, they became more harmonious and were fully synchronized by a few milliamps.
The ferromagnetic resonance of these MTJs can be adjusted by applying an external magnetic field. Talking about the output power, the parallel combination produced a particularly clean output at 2.4GHz: with 8.4MHz bandwidth at 850nW.
With this system, 30mV was generated without any magnetic bias, which is enough to power a red LED via an LTC3108 DC-DC converter.
The study is published in the Nature Communications paper ‘Electrically connected spin-torque oscillators for 2.4 GHz WiFi band transmission and energy harvesting‘.