Superparamagnetism

Superparamagnetism

Graph of the Langevin function, which describes the magnetization of superparamagnetic materials.

Superparamagnetism is a form of magnetism which appears in small ferromagnetic or ferrimagnetic nanoparticles. In sufficiently small nanoparticles, the entire particle can act like a single magnetic domain, and thermal fluctuations can randomly flip the direction of magnetization. This phenomenon is distinct from paramagnetism and ferromagnetism.

Characteristics

Superparamagnetism occurs in nanoparticles that are small enough such that the energy required to change the direction of magnetization is comparable to the thermal energy of the environment. This results in the magnetization of the particles being able to randomly flip direction under the influence of temperature.

In the absence of an external magnetic field, the average magnetization of a superparamagnetic material is zero. However, when an external magnetic field is applied, the magnetization can align with the field, similar to paramagnetic materials, but with a much larger magnetic susceptibility.

Langevin Function

The behavior of superparamagnetic materials can be described by the Langevin function, which is used to model the magnetization of a collection of non-interacting magnetic moments in thermal equilibrium. The function is given by:

\[ L(x) = \coth(x) - \frac{1}{x} \]

where \(x = \frac{\mu B}{k_B T}\), \(\mu\) is the magnetic moment, \(B\) is the magnetic field, \(k_B\) is the Boltzmann constant, and \(T\) is the temperature.

Applications

Superparamagnetic materials have a variety of applications, particularly in the field of biomedicine and data storage. In biomedicine, superparamagnetic nanoparticles are used in magnetic resonance imaging (MRI) as contrast agents. In data storage, they are used in magnetic recording media to increase the density of information storage.

Comparison with Other Magnetic Phenomena

Superparamagnetism is distinct from other forms of magnetism such as:

• Ferromagnetism: where materials exhibit permanent magnetization.
• Antiferromagnetism: where adjacent spins align in opposite directions, canceling out the overall magnetization.
• Ferrimagnetism: similar to antiferromagnetism, but with unequal opposing magnetic moments, resulting in a net magnetization.

Related pages

• Magnetism
• Nanoparticle
• Magnetic susceptibility
• Magnetic domain