  We study cross-relaxation features near zero magnetic field with ensembles of
nitrogen-vacancy (NV) centers in diamond and examine their properties in
samples with a range (0.9 ppm - 16.0 ppm) of NV concentrations. The observed
NV-NV cross-relaxation features between differently oriented NV centers in high
(greater than 0.9 ppm)-NV-density samples hold promise for a variety of
magnetometry applications where microwave fields (or any bias field) disturb
the system under study. We theoretically determine the values of the bias
magnetic fields corresponding to cross-relaxations between different axes and
experimentally validate them. The behavior of zero-field cross-relaxation
features as a function of temperature is also investigated.
