  Shallow nitrogen-vacancy (NV) centers in diamond are promising for
nano-magnetometry for they can be placed proximate to targets. To study the
intrinsic magnetic properties, zero-field magnetometry is desirable. However,
for shallow NV centers under zero field, the strain near diamond surfaces would
cause level anti-crossing between the spin states, leading to clock transitions
whose frequencies are insensitive to magnetic signals. Furthermore, the charge
noises from the surfaces would induce extra spin decoherence and hence reduce
the magnetic sensitivity. Here we demonstrate that the relatively strong
hyperfine coupling (130 MHz) from a first-shell 13C nuclear spin can provide an
effective bias field to an NV center spin so that the clock-transition
condition is broken and the charge noises are suppressed. The hyperfine bias
enhances the dc magnetic sensitivity by a factor of 22 in our setup. With the
charge noises suppressed by the strong hyperfine field, the ac magnetometry
under zero field also reaches the limit set by decoherence due to the nuclear
spin bath. In addition, the 130 MHz splitting of the NV center spin transitions
allows relaxometry of magnetic noises simultaneously at two well-separated
frequencies (~2.870 +/- 0.065 GHz), providing (low-resolution) spectral
information of high-frequency noises under zero field. The hyperfine-bias
enhanced zero-field magnetometry can be combined with dynamical decoupling to
enhance single-molecule magnetic resonance spectroscopy and to improve the
frequency resolution in nanoscale magnetic resonance imaging.
