Wideband and On-Chip Excitation for Dynamical Spin Injection into Graphene

Graphene is an ideal material for spin transport, as very long spin-relaxation times and lengths can be achieved even at room temperature. However, electrical spin injection is challenging due to the conductivity-mismatch problem. Spin pumping driven by ferromagnetic resonance is a neat way to circumvent this problem, as it produces a pure spin current in the absence of a charge current. Here, we show spin pumping into single-layer graphene in micron-scale devices. A broadband on-chip radio-frequency (rf) current line is used to bring micron-scale permalloy (Ni80Fe20) pads to ferromagnetic resonance with a magnetic-field-tunable resonance condition. At resonance, a spin current is emitted into graphene, which is detected by the inverse spin Hall voltage in a close-by platinum electrode. Clear spin-current signals are detected down to a power of a few milliwatts over a frequency range of 2-8 GHz. This compact device scheme paves the way for more complex device structures and allows the investigation of novel materials.