  Diamond-based quantum magnetometers are more sensitive to oscillating (AC)
magnetic fields than static (DC) fields because the crystal impurity-induced
ensemble dephasing time $T_2^*$, the relevant sensing time for a DC field, is
much shorter than the spin coherence time $T_2$, which determines the
sensitivity to AC fields. Here we demonstrate measurement of DC magnetic fields
using a physically rotating ensemble of nitrogen-vacancy centres at a precision
ultimately limited by $T_2$ rather than $T_2^*$. The rotation period of the
diamond is comparable to $T_2$ and the angle between the NV axis and the target
magnetic field changes as a function of time, thus upconverting the static
magnetic field to an oscillating field in the physically rotating frame. Using
spin-echo interferometry of the rotating NV centres, we are able to perform
measurements for over a hundred times longer compared to a conventional Ramsey
experiment. With modifications our scheme could realise DC sensitivities
equivalent to demonstrated NV center AC magnetic field sensitivities of order
$0.1$\,nT\,Hz$^{-1/2}$.
