Magnetic Barkhausen noise (MBN) occurs when a changing magnetic field is applied to a polycrystalline ferromagnetic material, and is due to irreversible motion of domain walls as they overcome local pinning sites. MBN is an effective tool for investigating magnetic behaviour because of its high sensitivity to strain. It is usually sensitive to the outer region of the sample (~30 microns).
Under an applied magnetic field, those magnetic domains with magnetization vectors most closely aligned with the applied field increase in size at the expense of less favourably oriented domains. However, domain walls are not necessarily free to move under the influence of the external field. Wall displacement can be temporarily retarded by local pinning sites. Dislocation tangles present in the local structure, inclusions, and grain boundaries can act as pinning sites. Movement of domain walls across these pinning sites requires energy, and makes the process irreversible. This phenomenon gives rise to the Barkhausen effect, and accounts for magnetic hysteresis in ferromagnetic materials. It occurs at intermediate fields, in the steep part of the magnetization curve, and is evidenced by small discontinuous changes in magnetization.
Magnetic Barkhausen noise is termed after its discoverer and is related to the noise heard in the loudspeaker employed in the original experiment. The word 'magnetic' serves to distinguish it from 'acoustic' Barkhausen noise, also known as magnetoacoustic emission.