Disclosed is process for preparing magnetic nanoparticles (MNPs) that results in very sensitive MNPs that can be used in a variety of diagnostic and analytical methods. The MNPs exhibit superparamagnetism and find special use in giant magnetoresistance sensors (GMRS). The MNPs are created by a process that permits one to tune the size of nanoparticles to a range of from 10 to 20 nanometers with a very small particle size distribution of +/−2 nanometers or less. The MNPs can be tagged with a variety of markers and thus find use in many analytical assays, cell sorting techniques, imaging methods, drug delivery methods and cancer treatments. The inventive MNPs can be detected in magnetic file strengths of 2000 Oe or less.
