Barkhausen effect - a stepwise change in the magnetisation of ferromagnets under a continuous change in external conditions, for example a magnetic field. The effect was first observed in 1919 by the German physicist H. G. Barkhausen: on slowly magnetising a ferromagnetic sample he found, in the circuit of a measuring coil placed over the sample, current pulses caused by the stepwise change in the magnetisation of the sample. The Barkhausen effect shows itself especially clearly in magnetically soft materials on the steep parts of the magnetisation curve and of the hysteresis loop, where the domain structure changes as a result of the displacement of the boundaries of ferromagnetic domains. Inhomogeneities of various kinds present in a ferromagnet (foreign inclusions, dislocations, residual mechanical stresses and so on) hinder the rearrangement of the domain structure. When the boundary of a domain, moving as the magnetic field H increases, meets an obstacle (for example, a foreign inclusion), it stops and remains stationary as the field is increased further. At some increased value of the field the boundary overcomes the obstacle and jumps further, to the next obstacle, without any further increase of the field. Because of such delays the magnetisation curve of a ferromagnet is stepped (Fig.). A stepwise change of magnetisation may be caused not only by a field, but by other external actions (for example, by a smooth change in elastic stresses or in temperature) that alter the domain structure of the sample.
Barkhausen effect - one of the direct proofs of the domain structure of ferromagnets; it makes it possible to determine the volume of an individual domain. For most ferromagnets this volume is 10-6—10-9 cm3. The study of the Barkhausen effect has made it possible to understand the dynamics of the domain structure better and to establish a relation between the number of jumps and the main characteristics of the hysteresis loop (the coercive force and others). By analogy with the Barkhausen effect in ferromagnets, the jumps of repolarisation in ferroelectrics are also called Barkhausen jumps.