  Optically detected magnetic resonance of nitrogen vacancy centers in diamond
offers novel routes to both DC and AC magnetometry in diamond anvil cells under
high pressures ($>3$ GPa). However, a serious challenge to realizing
experiments has been the insertion of microwave radiation in to the sample
space without screening by the gasket material. We utilize designer anvils with
lithographically-deposited metallic microchannels on the diamond culet as a
microwave antenna. We detected the spin resonance of an ensemble of
microdiamonds under pressure, and measure the pressure dependence of the zero
field splitting parameters. These experiments enable the possibility for
all-optical magnetic resonance experiments on sub-$\mu$L sample volumes at high
pressures.
