Superparamagnetism

Superparamagnetism is a form of magnetism, which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, magnetization can randomly flip direction under the influence of temperature. The typical time between two flips is called the Neel relaxation time. In the absence of an external magnetic field, when the time used to measure the magnetization of the nanoparticles is much longer than the Neel relaxation time, their total magnetization is zero. This is due to the fact that the magnetization direction is averaged over time. However, even though their total magnetization is zero, the magnetic susceptibility of these nanoparticles is very large, hence the term superparamagnetism.

History

The phenomenon of superparamagnetism was first described by Louis Néel in 1949, who won the Nobel Prize in Physics in 1970 for his pioneering work in magnetism.

Mechanism

Superparamagnetism occurs in materials composed of tiny magnetic particles, each of which behaves like a tiny magnet. When the size of the particles is reduced to the nanoscale, thermal energy (or Brownian motion) can cause the direction of the magnetic moment of each particle to fluctuate. If the time it takes for the magnetic moment of a particle to flip is shorter than the time scale of the measurement, the particle's magnetic moment averages to zero, and the material appears non-magnetic.

Applications

Superparamagnetic materials have a wide range of applications. They are used in data storage devices, such as hard disk drives, where they allow for increased storage density. They are also used in medicine, for example in magnetic resonance imaging (MRI) as contrast agents, and in drug delivery systems, where they can be directed to specific locations in the body using an external magnetic field.

See also

• Ferromagnetism
• Paramagnetism
• Diamagnetism
• Nanotechnology
• Magnetic nanoparticles