A magnetic-field sensor device comprises at least two electrodes; an insulating layer separating the at least two electrodes; at least one layer of chemically-synthesized magnetic nanoparticles disposed at or above a level with the insulating layer, and disposed between the at least two electrodes; and an organic spacer surrounding each of the chemically-synthesized magnetic nanoparticles. A deviation between diameters of different ones of the nanoparticles is less than 15%. Moreover, the chemically-synthesized magnetic nanoparticles range in size between 2 nm and 20 nm in diameter.