  Magnetic fields are a standard tool in the toolbox of every physicist, and
are required for the characterization of materials, as well as the polarization
of spins in nuclear magnetic resonance or electron paramagnetic resonance
experiments. Quite often a static magnetic field of sufficiently large, but
fixed magnitude is suitable for these tasks. Here we present a permanent magnet
assembly that can achieve magnetic field strengths of up to 1.5T over an air
gap length of 7mm. The assembly is based on a Halbach array of neodymium
(NdFeB) magnets, with the inclusion of the soft magnetic material Supermendur
to boost the magnetic field strength inside the air gap. We present the design,
simulation and characterization of the permanent magnet assembly, measuring an
outstanding magnetic field stability with a drift rate of |D| < 2.8 ppb/h. Our
measurements demonstrate that this assembly can be used for spin qubit
experiments inside a dilution refrigerator, successfully replacing the more
expensive and bulky superconducting solenoids.
