  We investigate the spin dynamics of a Nitrogen-Vacancy (NV) center contained
in an individual diamond nanocrystal in the presence of continuous microwave
excitation. Upon periodic reversal of the microwave phase, we observe a train
of 'Solomon echoes' that effectively extends the system coherence lifetime to
reach several tens of microseconds, depending on the microwave power and phase
inversion rate. Starting from a model where the NV center interacts with a bath
of paramagnetic defects on the nanocrystal surface, we use average Hamiltonian
theory to compute the signal envelope from its amplitude at the echo maxima.
Comparison between the effective Rabi and Solomon propagators shows that the
observed response can be understood as a form of higher-order decoupling from
the spin bath.
