  The coupling between defects in diamond and a superconducting microwave
resonator is studied in the nonlinear regime. Both negatively charged
nitrogen-vacancy and P1 defects are explored. The measured cavity mode response
exhibits strong nonlinearity near a spin resonance. Data is compared with
theoretical predictions and a good agreement is obtained in a wide range of
externally controlled parameters. The nonlinear effect under study in the
current paper is expected to play a role in any cavity-based magnetic resonance
imaging technique and to impose a fundamental limit upon its sensitivity.
