  We design and demonstrate a compact, robust and simple to assemble and tune
permanent magnet suitable for NMR relaxometry measurements of microfluidic
flows. Soft-magnetic stainless steel plates, incorporated inside the magnet
airgap, are key for obtaining substantially improved and tunable field
homogeneity. The design is scalable for different NMR probe sizes with the
region of suitable field homogeneity, less than 200 ppm, achievable in a
capillary length of about 50% of the total magnet length. The built physical
prototype, having 3.5x3.5x8.0 cm3 in size and 5 mm high airgap, provides a
field strength of 0.5 T and sufficient field homogeneity for NMR relaxometry
measurements in capillaries up to 1.6 mm i.d. and 20 mm long. The magnet was
used for test flow rate measurements in a wide range, from 0.001 ml/min to 20
ml/min.
