Schematic representations of superparamagnetic iron oxide (SPIO). (a) Spinal crystal structure for SPIO.
 (b) Comparison of a SPIO nanoparticle, which possesses a single magnetic domain (ie, all of the individual moments of the atoms are aligned) to a larger iron oxide particle that has multiple magnetic domains. In general, the presence of multiple magnetic domains will result in a reduced net magnetization because the domains will interfere with each other. (c) Transmission electron micrograph of SPIO nanoparticles. (d) Schematic illustrating the behavior of SPIO in the presence and absence of an external magnetic field. In the presence of a magnetic field the magnetic moment of SPIO aligns in the direction the magnetic field; however, in the absence of a magnetic field SPIO become randomly oriented due to Brownian fluctuations. Brownian motions are also sufficient to prevent aggregation of the nanometer-sized SPIO, when prepared appropriately.