Magnetic Properties of Nanoparticles, Superparamagnetism
|For a single isolated particle, coercitivity or remanence as function of the particle size, are sketched in the figure.
Large magnetic particles are subdivided by Bloch walls into magnetic domains. Therefore, remanence and coercitivity are
independent of the particle size. Decreasing particle size leads to a size range, where the particles consist of only a
single magnetic domain. Therefore, coercitivity and remanence are increasing drastically. This range of particle sizes is
used for magnetic data storage.
|Further decrease of the particle size leads to a decrease of remanence and coercitivity to zero. However, this step depends
on the measurement time. The shorter the time constant of the measuring method tm the
more is this step shifted to smaller particle sizes. Magnetic properties of a single isolated particle are strongly
influenced by the particle size. This phenomenon, called "superparamagnetism" is observed, when the thermal energy of
the particle kT is larger than the energy of magnetic anisotropy KV.