Insight into Superparamagnetism in Magnetic Nanoparticles

A set of magnetic nanoparticles at different temperatures may have different magnetic states, including anisotropic superparamagnet, anisotropic superpromagnet, locked state, and super-spin glass and ferromagnetic glass phases. The superparamagnetism phenomenon is observed when the size of particles approaches to the single domain area in such a way that they behave as ferromagnetic materials below Curie temperature. Superparamagnetic (SPM) can be described based on the equations that are employed for paramagnetic particles, e.g. the Langevin function, with only a difference that the magnetic moment of individual particles is much larger in the case of SPM. Thus, the hysteresis loop, M–H or B–H, of SPM is more similar to that of a paramagnetic particle, but much steeper with higher magnetization. This is why they are named superparamagnetic.