superparamagnetism (rus. суперпарамагнетизм) — Quasiparamagnetic behaviour of substances and materials that are composed of nanoscale ferro- or ferrimagnetic particles, weakly interacting with each other.

Description

At temperatures below the Curie or Neel point, ferro-and ferrimagnets transition from the paramagnetic to the ferro- or ferrimagnetic state. If the particles of such materials are isolated, and their linear dimensions do not exceed 1-10 nm (different values for different materials), then due to thermal fluctuations, the prevailing magnetisation direction will also randomly change at these temperatures, just as the magnetic moment direction of atoms or ions change in a paramagnet under the effect of thermal motion. As a result, a small particle system behaves in magnetic fields and at changing temperatures as if it were a paramagnetic gas of N atoms (where N is the number of single-domain particles, each with a magnetic moment M). It meets the Curie law at low magnetic fields and the Langevin equation for magnetism within the magnetic saturation area.

The magnetism of superparamagnets can be many times higher than that of conventional paramagnets. For the magnetisation vectors of particles to chaotically change their spatial orientation, the thermal motion energy kT (where k is the Boltzmann constant, and T is the temperature) must be higher than the particle’s magnetic anisotropy energy KV (where K is the anisotropy constant, and V is the volume of the particle). As the temperature goes down, the thermal motion energy reduces, and below the blocking temperature Tb (whose value depends on the type of material and the size of the particles) the change in the prevailing magnetisation direction ceases to be chaotic, and the particles begin to behave as if they were conventional coarse-crystalline ferro- and ferrimagnetic materials.

The typical representatives of superparamagnetic systems are small Co particles, emitted during the decay of Cu-Co solid solution (2% Co), some nodules of Fe in brass (0.1% Fe), Cu in Mn, Ni in Au, as well as some antiferromagnetic oxides.

Authors

  • Goodilin Evgeny A.
  • Shlyakhtin Oleg A.

Sources

  1. A.K. Zvezdin. Superparamagnetism today: Tiny magnets on their way to the world of quanta (in Russian)// Priroda. 2001. #9. P. 9–18.
  2. Superparamagnetism// Wikipedia, the free encyclopedia. — http://en.wikipedia.org/wiki/Superparamagnetism (reference date 01.08.2010).

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