Hysteresis: The progression between magnetisation and demagnetisation

Hysteresis describes the phenomenon that a magnetic material shows a time-delayed or non-linear course during magnetisation and demagnetisation. The magnetisation of a material depends not only on the current magnetic field, but also on its magnetic history. This behaviour is represented by the so-called hysteresis curve.

How does hysteresis work?

Hysteresis occurs in materials that have magnetic domains - small areas with uniform magnetisation. During magnetisation, these domains align themselves along the external magnetic field. When the field is removed, some domains remain in the aligned position, resulting in remanence (residual magnetisation). To completely demagnetise the material, an opposing magnetic field is required, known as coercive field strength.

The hysteresis curve

The hysteresis curve is a graphical representation of the relationship between the magnetic field strength ( H ) and the magnetic flux density ( B ). It shows the progression of magnetisation and demagnetisation of a material. The most important points of the curve are

• Remanence (B_r): The remaining magnetisation after the magnetic field has been removed.
• Coercive field strength (H_c): The opposite magnetic field required to reduce the magnetisation to zero.
• Saturation magnetisation: The point at which all magnetic domains are aligned and no further magnetisation is possible.

Hysteresis properties

• Energy losses: The loop of the hysteresis curve shows the energy losses that occur during each magnetisation cycle.
• Material dependence: The shape of the hysteresis curve depends on the material. Soft magnetic materials have a narrow loop, while hard magnetic materials show a wide loop.
• Temperature dependence: The hysteresis of a material changes with temperature, especially near the Curie temperature.

Applications of hysteresis

Understanding hysteresis is crucial for many technical applications:

• Permanent magnets: Materials with a broad hysteresis curve are used in electric motors, loudspeakers and generators.
• Transformers: Soft magnetic materials with a narrow hysteresis curve minimise energy losses.
• Data storage: Magnetic data carriers such as hard drives use hysteresis to store data as different magnetisation states.
• Sensor technology: Magnetic field sensors often rely on the hysteresis effect to make precise measurements.

Interesting facts about hysteresis

Did you know that hysteresis is not only found in magnetism? This non-linear behaviour can also be seen in other areas such as the elasticity of materials, in thermal processes and in biology (e.g. in the reaction of cells to stimuli).