  Spin-orbit torque nano-oscillators based on bilayers of ferromagnetic (FM)
and nonmagnetic (NM) metals are ultra-compact current-controlled microwave
signal sources. They serve as a convenient testbed for studies of spin-orbit
torque physics and are attractive for practical applications such as microwave
assisted magnetic recording, neuromorphic computing, and chip-to-chip wireless
communications. However, a major drawback of these devices is low output
microwave power arising from the relatively small anisotropic magnetoresistance
(AMR) of the FM layer. Here we experimentally show that the output power of a
spin-orbit torque nano-oscillator can be enhanced by nearly three orders of
magnitude without compromising its structural simplicity. Addition of a FM
reference layer to the oscillator allows us to employ current-in-plane giant
magnetoresistance (CIP GMR) to boost the output power of the device. This
enhancement of the output power is a result of both large magnitude of GMR
compared to that of AMR and different angular dependences of GMR and AMR. Our
results pave the way for practical applications of spin-orbit torque
nano-oscillators.
