Circular magnetism: Magnetic phenomena in rotating systems

The term circular magnetism describes magnetic effects caused by rotating movements of electric charges or magnetic moments. This term is often used in connection with circularly polarised light and the associated magneto-optical effects, but also plays a role in electrodynamics and materials research.

How does circular magnetism occur?

Circular magnetism occurs when the magnetic moments of a system align themselves in a circular or spiral motion. This can be caused by external magnetic fields or by the interaction with circularly polarised light. Examples include:

• The rotation of charged particles, such as electrons, in a magnetic field (e.g. cyclotron motion).
• Magneto-optical effects in which the polarisation direction of light is changed by magnetic fields.
• Phenomena in spirally arranged magnetic domains or nanostructures.

Circular magnetism in magneto-optics

In magneto-optics, circular magnetism is used to investigate the properties of materials. Two central effects are:

• Faraday effect: The rotation of the plane of polarisation of light propagating through a material in a magnetic field.
• Kerr effect: The change in polarisation of reflected light when it strikes a magnetised material.

These effects are directly related to circular magnetism and provide insights into the magnetic properties of materials.

Applications of circular magnetism

Circular magnetism is used in various technological and scientific fields:

• Magneto-optical data storage: By manipulating light with circular magnetism, data can be stored and read on magneto-optical media such as MO discs.
• Sensor technology: Magneto-optical sensors use circular magnetic effects to precisely measure magnetic fields.
• Material science: The investigation of circular magnetism helps to characterise the magnetic properties of new materials.
• Communication technology: Circular magnetic materials are used in optical isolators and frequency filters.

Mathematical description of circular magnetism

Circular magnetism can be described by the magnetic circular current density generated in a material by the movement of magnetic moments. This is often taken into account in Maxwell's equation for the magnetic field strength:

∇ × H = J + ∂D/∂t

Where:

• H: Magnetic field strength
• J: Current density
• ∂D/∂t: Change in electrical displacement density

Interesting facts about circular magnetism

Did you know that circularly polarised light is used in biology to analyse the structure of molecules? Circular magnetism can be used to study enzymes and proteins that react to magnetic fields and circular polarisation. This effect also enables the development of magneto-optics in quantum communication.