Magnetic nanoparticles are of great interest for researchers from a wide range of disciplines, including magnetic fluids,catalysis,  biotechnology/biomedicine, magnetic resonance imaging, data storage, and environmental remediation. While a number of suitable methods have been developed for the synthesis of magnetic nanoparticles of various different compositions, successful application of such magnetic nanoparticles in the areas listed above is highly dependent on the stability of the particles under a range of different conditions. In most of the envisaged applications, the particles perform best when the size of the nanoparticles is below a critical value, which is dependent on the material but is typically around 10–20 nm. Then each nanoparticle becomes a single magnetic domain and shows superparamagnetic behavior when the temperature is above the so‐called blocking temperature. Such individual nanoparticles have a large constant magnetic moment and behave like a giant paramagnetic atom with a fast response to applied magnetic fields with negligible remanence (residual magnetism) and coercivity (the field required to bring the magnetization to zero). These features make superparamagnetic nanoparticles very attractive for a broad range of biomedical applications because the risk of forming agglomerates is negligible at room temperature.

REF: Lu, A.‐H., Salabas, E. and Schüth, F. (2007), Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application. Angewandte Chemie International Edition, 46: 1222-1244. doi:10.1002/anie.200602866