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The low-magnetic-field feature in the magnetic field dependence of the luminescence of negatively charged nitrogen-vacancy (NV − ) centers in diamond has been experimentally observed for the first time. This feature, centered at the zero magnetic field-henceforth the zero field line-and observed with a magnetic field sweep of NV − center luminescence (magnetic spectrum), is quite narrow. The properties of this line are considerably different from those of other narrow NV − magnetic spectrum lines. Its amplitude is weakly dependent on the orientation of the single-crystal sample with respect to the external magnetic field. This line is also observed in a powdered sample. The shape of the line changes drastically with a change in the polarization of excitation light. The amplitude of the line nonlinearly depends on excitation light intensity. For low intensities this dependence is close to the square law. We attribute the occurrence of this line to the dipole-dipole interaction between different NV − centers.

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