Probing GHz Gravitational Waves with Graviton-magnon Resonance
  A novel method for extending frequency frontier in gravitational wave
observations is proposed. It is shown that gravitational waves can excite a
magnon. Thus, gravitational waves can be probed by a graviton-magnon detector
which measures resonance fluorescence of magnons. Searching for gravitational
waves with a wave length $\lambda$ by using a ferromagnetic sample with a
dimension $l$, the sensitivity of the graviton-magnon detector reaches spectral
densities, around $5.4 \times 10^{-22} \times (\frac{l}{\lambda /2\pi})^{-2} \
[{\rm Hz}^{-1/2}]$ at 14 GHz and $8.6 \times 10^{-21} \times (\frac{l}{\lambda
/2\pi})^{-2} \ [{\rm Hz}^{-1/2}]$ at 8.2 GHz, respectively.
