Barkhausen effect: The erratic behaviour of magnetic domains

The Barkhausen effect describes a physical phenomenon in which magnetic domains in a ferromagnetic material abruptly change their orientation during magnetisation. Named after the German physicist Heinrich Barkhausen, who discovered the effect in 1919, it is a key concept in magnetism research and is used to analyse materials.

What are magnetic domains?

Magnetic domains are small areas in a ferromagnetic material in which the direction of magnetisation is uniformly aligned. These areas are formed due to the interaction between atoms, which ensures that their magnetic moments are aligned in parallel.

In an unmagnetised state, these domains are randomly aligned so that their magnetic fields cancel each other out. During magnetisation, the domains align themselves along the external magnetic field.

What happens during the Barkhausen effect?

The Barkhausen effect occurs when a ferromagnetic material is slowly exposed to an external magnetic field. The domains do not change their orientation continuously, but rather abruptly. These abrupt changes generate measurable signals that can be registered with a sensor or microphone as cracking noises.

Cause of the jumps:

• Obstacles in the material, such as impurities, defects or stresses, prevent the domain boundaries from moving uniformly. As soon as the magnetic field is strong enough, the domains overcome these obstacles and realign themselves abruptly.

Measuring the Barkhausen effect

The effect is measured using a simple experiment:

1. Set-up:
   • A coil is wound around the ferromagnetic material.
   • A slowly changing magnetic field is applied.
2. Detection:
   • The abrupt changes in magnetisation generate induction currents in the coil, which are registered as electrical signals.
3. Analysis:
   • These signals are recorded in order to analyse the structure and properties of the material.

Applications of the Barkhausen effect

1. Material testing:

   • The Barkhausen effect is used to detect defects, stresses or microstructures in ferromagnetic materials.
   • Typical applications:
     • Testing steel for cracks or manufacturing defects.
     • Quality control in the automotive and aerospace industries.

2. Research and development:

   • Scientists use the effect to analyse magnetic materials and improve their properties.

3. Non-destructive testing (NDT):

   • As the Barkhausen effect does not damage the material, it is ideal for testing sensitive components.

5. Magnetic sensor technology:

   • The effect is used in sensor development to characterise magnetic fields

Factors that influence the Barkhausen effect

1. Material composition:

   • Pure and homogeneous materials show less erratic changes as they provide fewer obstacles for the movement of domains.

2. Tensions in the material:

   • Mechanical tensions increase the number of obstacles and intensify the effect.

3. Magnetisation rate:

   • The faster the magnetic field is applied, the more pronounced the effect.

4. Temperature:

   • At high temperatures (close to the Curie temperature), the effect decreases as the magnetisation becomes weaker.

Scientific significance of the Barkhausen effect

1. Domain movement:

   • The Barkhausen effect provides direct insight into the movement of domain walls and the interaction between magnetic moments.

2. Material analysis:

   • The effect is used to evaluate the microstructure and mechanical properties of materials.

3. Theoretical models:

   • The effect plays a key role in the development of models to describe magnetic hysteresis.

Application example: Barkhausen noise

The Barkhausen noise is the sum of the signals generated by the abrupt changes. It is used in practice to:

• Measure material stresses.
• Analyse the influence of manufacturing processes on the material.
• Monitor ageing processes and material fatigue.

Did you know?

• The Barkhausen effect was one of the first experimental proofs of the existence of magnetic domains.
• Modern non-destructive testing methods use the effect to extend the service life of components in power plants, aeroplanes and vehicles.

Conclusion

The Barkhausen effect offers a fascinating insight into the world of magnetic domains and their movement. From research to industrial quality assurance, this effect is an indispensable tool for analysing magnetic materials. Its ability to detect material stresses and defects makes it a key technology in many areas of engineering and science.