  Well controlled and highly stable magnetic fields are desired for a wide
range of applications in physical research, including quantum metrology,
sensing, information processing, and simulation. Here we introduce a low-cost
hybrid assembly of rare-earth magnets and magnetic field coils to generate a
field strength of $\simeq\,10.9\,$mT with a spatial variation of less than
10$^{-6}$ within a diameter of spherical volume of $150\,$um. We characterise
its tuneability and stability performance using a single Mg$^{+}$ atom confined
in a radio-frequency surface-electrode trap under ultra-high vacuum conditions.
The strength of the field can be tuned with a relative precision of $\leq
2\,\times\,10^{-5}$ and we find a passive temporal stability of our setup of
better than $1.0\,\times\,10^{-4}$ over the course of one hour. Slow drifts on
time scales of a few minutes are actively stabilised by adjusting electric
currents in the magnetic field coils. In this way, we observe coherence times
of electronic superposition states of greater than six seconds using a
first-order field insensitive (clock) transition. In a first application, we
demonstrate sensing of magnetic fields with amplitudes of $\geq0.2\,$uT
oscillating at $\simeq 2\pi\,\times\,60\,$MHz. Our approach can be implemented
in compact and robust applications with strict power and load requirements.
