Disclosed are biocompatible magnetic nanoparticle compositions for various therapeutic or biological applications, and methods related thereto. Specifically, the present invention pertains to magnetic nanoparticle compositions prepared via high-pressure homogenization processes that include a turbulent flow zone. The methods of production may involve a two-step or a one-step process. The disclosed magnetic nanoparticle compositions may be useful in the treatment of the body, body part, tissue, cell, or body fluid of a subject for a variety of indications. The disclosed magnetic nanoparticle compositions may also be useful in the fixation, separation, transportation, marking or coding of targets, or energy transformation processes.