Processes and compositions are described for preparing new, colloidally stable, coated nanomagnetic particles useful for both in-vitro and in-vivo biomedical applications, including cell targeting and capturing cells, microorganisms, and cellular organelles or entities such as exosomes. These nanomagnetic particles can also be used as imaging contrast agents due to their small size and high magnetic moment. The nanomagnetic particles include a series of sequentially added, stabilizing surface coatings rendered onto nano-sized magnetic crystal clusters (e.g., magnetite particles) to impart colloidal stability in complex biological samples with minimal leaching of the coating materials, high binding capacity, and low non-specific binding. Another benefit of this invention is the ability to utilize both external and internal magnetic field-generating separation devices to effect separation of the magnetic nanoparticles.