Barkhausenin

The Barkhausen effect, also known as Barkhausen noise, refers to the discontinuous change in magnetization observed in ferromagnetic materials as an external magnetic field is slowly varied. It was discovered by Heinrich Barkhausen in 1919 while studying the alternating-current response of iron. The effect is detected by winding a coil around a ferromagnetic sample and recording the induced voltage as the magnetic field changes. The recorded noise consists of discrete pulses corresponding to sudden jumps of magnetic domain walls.

The underlying mechanism involves magnetic domains and their walls. In ferromagnets, magnetic moments align within regions called domains, separated by domain walls. As the external field changes, these walls move, but their motion is hindered by defects, impurities, and internal stresses. When the driving field reaches a threshold, walls depin and jump to new positions, producing abrupt changes in magnetization and resulting in measurable electrical pulses in the detection coil.

The Barkhausen effect provided direct experimental evidence for the existence of magnetic domains and for domain-wall motion, contributing to the acceptance of domain theory in magnetism. It has since been used to study microstructure and mechanical state in ferromagnetic materials. Barkhausen noise is influenced by factors such as grain size, dislocation density, and residual stress, and can be exploited in non-destructive testing and material characterization to assess heat treatment, work hardening, and quality control. In some texts, the term Barkhausenin ilmiö is used in Finnish, reflecting the same phenomenon.