  Ensembles of nitrogen vacancy centres (NVCs) in diamond can be employed for
sensitive magnetometry. In this work we present a fiber-coupled NVC
magnetometer with an unshielded sensitivity of (30 $\pm$ 10)
pT/$\sqrt{\textrm{Hz}}$ in a (10 - 500)-Hz frequency range. This sensitivity is
enabled by a relatively high green-to-red photon conversion efficiency, the use
of a [100] bias field alignment, microwave and lock-in amplifier (LIA)
parameter optimisation, as well as a balanced hyperfine excitation scheme.
Furthermore, a silicon carbide (SiC) heat spreader is used for microwave
delivery, alongside low-strain $^{12}\textrm{C}$ diamonds, one of which is
placed in a second magnetically insensitive fluorescence collecting sensor head
for common-mode noise cancellation. The magnetometer is capable of detecting
signals from sources such as a vacuum pump up to 2 m away, with some
orientation dependence but no complete dead zones, demonstrating its potential
for use in remote sensing applications.
