  The recently discovered spin defects in hexagonal boron nitride (hBN), a
layered van der Waals material, have great potential in quantum sensing.
However, the photoluminescence and the contrast of the optically detected
magnetic resonance (ODMR) of hBN spin defects are relatively low so far, which
limits their sensitivity. Here we report a record-high ODMR contrast of 46$\%$
at room temperature, and simultaneous enhancement of the photoluminescence of
hBN spin defects by up to 17-fold by the surface plasmon of a gold-film
microwave waveguide. Our results are obtained with shallow boron vacancy spin
defects in hBN nanosheets created by low-energy He$^+$ ion implantation, and a
gold-film microwave waveguide fabricated by photolithography. We also explore
the effects of microwave and laser powers on the ODMR, and improve the
sensitivity of hBN spin defects for magnetic field detection. Our results
support the promising potential of hBN spin defects for nanoscale quantum
sensing.
