Barkhausen effect

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What is the Barkhausen effect?

The Barkhausen effect was discovered at the beginning of the 20th century by the physicist Heinrich Barkhausen and is named after him. The Barkhausen effect describes the discontinuous change in magnetisation in ferromagnetic materials caused by microscopic, abrupt changes in the direction of magnetisation of so-called Weiss domains, in which the magnetic moments of the electron spins present (indicated by arrows) are aligned parallel to each other (see illustration 1). These abrupt changes, known as Barkhausen jumps, can be made audible in an experiment as a crackling sound in a loudspeaker and generate measurable magnetic noise signals, also known as Barkhausen noise (see illustration 2).

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The Barkhausen effect has significant applications in materials science and non-destructive material testing as it provides insight into the microstructure and stress states of the material. Analysing Barkhausen jumps and Barkhausen noise enables the assessment of material fatigue and the detection of microcracks, which is essential for monitoring critical components in industrial settings.

Illustration 1: The Barkhausen effect occurs at the boundaries of so-called Weiss domains. These boundaries are also known as Bloch walls (solid lines on the left and dashed lines on the right).

Illustration 1 shows the Barkhausen effect: The alignment of the electron spins between different Weiß domains changes abruptly when they cross the Bloch wall (see illustragion 1). This effect is called the Barkhausen effect. In ferromagnetic materials, there are generally Weiss domains of a few tenths of a millimetre in size in which the electron spins of the matter are aligned parallel to each other. However, the electron spins in various neighbouring Weiss domains are not aligned in parallel. This is why, in a demagnetised, ferromagnetic material, no magnetic field can be measured. The electron spins of one Weiss domain oppose the electron spins of another Weiss domain and thus offset each other's magnetic effect.

The influence of an external magnetic field, in particular, causes the orientations of the electron spins within a Weiss domain to change. If such a collective rearrangement of the spin orientation occurs, as in the transition from the left to the right-hand side in illustration 1, it is referred to as a Barkhausen jump. This can be induced externally by the magnet.

What is a Barkhausen jump?

Barkhausen jumps are abrupt changes in the magnetisation of a magnet, whereby the direction of the magnetisation of a microscopic area, the so-called Weiss domain, changes abruptly. A Barkhausen jump is the simultaneous change in the orientation of all electron spins in a Weiss domain.

However, magnetisation can be used to make a ferromagnetic material appear outwardly magnetic. This is because the electron spins in the material are all largely aligned in parallel during the magnetisation. Different Weiss domains merge with each other to form a large common domain with electron spins aligned in parallel.

To achieve this, the electron spins have to change their alignment. But, due to the strong exchange interaction between the individual electron spins, this does not happen for each spin individually; instead, in a Weiss domain, the alignment of all electron spins in this domain changes instantaneously under the influence of a magnetic field. All electron spins change their orientation together in the form of a collective "jump". This is known as a Barkhausen jump.

Barkhausen jumps are, therefore, associated with a sudden change in magnetisation in a ferromagnetic material.

Experiment for the detection of Barkhausen jumps

Although the Weiss domains that change their orientation are very small (often only a few µm in size), the abrupt collective behaviour of the tiny electron spins can be demonstrated in an experiment (see illustration 2).

Experiment set-up to make Barkhausen jumps audible

Illustration 2: The experiment shows a set-up for making Barkhausen jumps audible. Inside a coil is a ferromagnetic material. The Weiss domains of the ferromagnetic material are not aligned parallel to each other, and the material is non-magnetic. During the magnetisation process, an external permanent magnet is brought into close proximity, triggering an abrupt change in the direction of the Weiss domains. This causes the magnetisation of the material in the coil to change abruptly and a minute current (which is proportional to the size of the Weiss domain that has changed direction) becomes measurable. The short current pulse can be led through an amplifier to a loudspeaker, for example, which then begins to "crackle" softly with each jump. The signal can be further amplified with a microphone.

This experiment makes use of the fact that, with careful magnetisation, individual Weiss domains perform Barkhausen jumps one after the other. A ferromagnetic probe is carefully magnetised using a permanent magnet (see illustration 2). This causes the spins of the Weiss domains to "flip", resulting in a brief magnetic pulse. If the material is wound in a coil, this magnetic pulse briefly induces a current in the coil. This current pulse can be amplified and then made visible via a needle deflection or made audible via a loudspeaker.

Barkhausen noise

Barkhausen noise is a phenomenon that characterises the micromagnetic changes in ferromagnetic materials through discrete jumps in magnetisation. This noise is a direct result of the realignment of domain walls and provides valuable insights into material properties such as microstructure and stress states. It is used in non-destructive testing to identify material fatigue and microcracks, which is particularly important in safety-critical industrial applications.

Author:
Dr Franz-Josef Schmitt

Dr Franz-Josef Schmitt is a physicist and academic director of the advanced practicum in physics at Martin Luther University Halle-Wittenberg. He worked at the Technical University from 2011-2019, heading various teaching projects and the chemistry project laboratory. His research focus is time-resolved fluorescence spectroscopy in biologically active macromolecules. He is also the Managing Director of Sensoik Technologies GmbH.

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