  Magnetometers based on nitrogen-vacancy (NV) centers in diamond are promising
room-temperature, solid-state sensors. However, their reported sensitivity to
magnetic fields at low frequencies (<1 kHz) is presently >10 pT s^{1/2},
precluding potential applications in medical imaging, geoscience, and
navigation. Here we show that high-permeability magnetic flux concentrators,
which collect magnetic flux from a larger area and concentrate it into the
diamond sensor, can be used to improve the sensitivity of diamond
magnetometers. By inserting an NV-doped diamond membrane between two ferrite
cones in a bowtie configuration, we realize a ~250-fold increase of the
magnetic field amplitude within the diamond. We demonstrate a sensitivity of
~0.9 pT s^{1/2} to magnetic fields in the frequency range between 10 and 1000
Hz, using a dual-resonance modulation technique to suppress the effect of
thermal shifts of the NV spin levels. This is accomplished using 200 mW of
laser power and 20 mW of microwave power. This work introduces a new dimension
for diamond quantum sensors by using micro-structured magnetic materials to
manipulate magnetic fields.
