Magnetisation curve: The behaviour of materials in a magnetic field

The magnetisation curve, also known as the hysteresis curve, describes the magnetic behaviour of a material when it is exposed to an external magnetic field. It shows the relationship between the magnetic field strength (H) and the resulting magnetic flux density (B) or the magnetisation (M) of the material. The curve is a key tool for analysing the magnetic properties of materials.

Structure of a magnetisation curve

A typical magnetisation curve consists of various characteristic sections that provide important information about the material:

• Linear gradient: At low field strengths, the magnetisation increases linearly with the field strength. This range is determined by the magnetic permeability of the material.
• Saturation: When all magnetic moments in the material are aligned, the magnetisation reaches a maximum limit, which is referred to as saturation magnetisation.
• Hysteresis: When the external field is reduced, a residual magnetisation remains in the material. This is represented by the width of the curve.
• Coercive field strength: The field strength required to completely cancel out the magnetisation is a measure of the hardness of the material.

Properties of the magnetisation curve

• Hysteresis: The curve shows that the magnetisation process is not completely reversible. This is represented by the hysteresis loop.
• Material-dependent: The shape and size of the magnetisation curve vary greatly between soft magnetic and hard magnetic materials.
• Energy losses: The area within the hysteresis loop represents the energy losses during magnetisation and demagnetisation.

Differences between soft and hard magnetic materials

The magnetisation curve differs depending on the material:

• Soft magnetic materials: A narrow hysteresis loop shows low energy losses and easy magnetisability. They are used in transformers and in electromagnets 
• Hard magnetic materials: A wide hysteresis loop indicates high coercive field strengths and permanent magnetisation. They are used for permanent magnets.

Applications of the magnetisation curve

The analysis of magnetisation curves is essential in many technical and scientific fields:

• Material development: The curve helps to determine the properties of new magnetic alloys and materials.
• Electrical engineering: Transformers, motors and generators are based on materials with optimised magnetisation properties.
• Memory technology: Magnetic data carriers use materials with specific hysteresis curves to store data efficiently.
• Medical technology: Magnetic materials in MRI systems are selected based on their magnetisation curves.

Interesting facts about magnetisation curves

Did you know that the shape of the magnetisation curve is not only crucial for technical applications, but also for the analysis of natural phenomena? In geophysics, magnetisation curves are used to determine the age and origin of rock formations. Volcanic rocks, for example, retain their magnetisation according to the Earth's magnetic field at the time of their formation. This property enables scientists to reconstruct changes in the Earth's magnetic field over millions of years.

The magnetisation curve also plays a crucial role in nanotechnology. Tiny magnetic particles, known as nanomagnets, are optimised based on their hysteresis loops. These particles are used in areas such as cancer therapy, where they can be specifically controlled in tumour tissue using magnetic fields.

It is also interesting to note that modern electrical devices such as smartphones and computers rely on materials with customised magnetisation curves. These materials enable the precise control of magnetic fields in tiny components such as memory chips, sensors or loudspeakers. Without these advances in materials research, today's miniaturisation of electronic devices would be unthinkable.