The Barkhausen effect relates magnetism to acoustics. In 1919, a German scientist Heinrich Barkhausen found that whenever he moved a magnet close to an iron-cored wire, an audible roaring sound (called Barkhausen noise or Barkhausen emission) was heard through an amplified speaker. The sound reflects a sudden (instead of a smooth or gradual) shifting or realignment of magnetic domains in the iron under an external magnetic field (see Figure 1).

The sudden change in the magnetic field around the iron induces a current pulse in the coil and creates a clicking sound in the speaker. These sudden jumps are interpreted as discrete changes in the size or orientation of FM domains that occur during magnetization or demagnetization. Crystalline imperfections temporarily hold up the movement of the domain walls causing the abrupt movements.

Two types of crystalline imperfections can interrupt the movement of domain walls: inclusions and residual stresses. Inclusions, such as impurities and voids, are the regions in a material with different magnetization. Large inclusions impede the domain wall motion because they provide the areas to lower their magnetostatic energy. Small inclusions lower the overall surface energy, and thus delay the movement of walls. The causes of residual stress are crystalline imperfections, such as dislocations.

When a domain wall moves through a region of varying residual stress, there is an energy increase in the wall. This is because the residual stress is trying to create a restoring force when the domain wall moves. If there is sufficient energy to overcome the restoring force, the domain wall can take irreversible jumps to equivalent energy levels.

Barkhausen effect involves a large number of very complicated interrelated mechanisms, making it difficult to express the effect analytically. In addition, magnetic behavior is inhomogeneous and does not “scale” easily as dimensions change.

Williams and Shockley demonstrated the direct relation between changing magnetization and domain wall motion in single crystals of iron with 3.8% silicon, and showed that the wall position is linearly proportional to the magnetization.

Barkhausen Sensor Working Principle

In typical Barkhausen sensors, the domain arrangement in an FM specimen is controlled by a varying, externally applied magnetic field; the Barkhausen jumps in the specimen are detected as voltage pulses induced into a coil near to the surface of the FM material or wound around it (see Figure 2).

Because Barkhausen jumps or noises are sensitive to the microstructural discontinuity of the FM materials such as impurities, dislocations, grain boundaries, precipitated carbides, and stresses, Barkhausen sensors can detect these defects thus.

Existing methods of examining the FM material defects include fast Fourier transform (FFT), averaging over several cycles of magnetization of the sample, or using maximum entropy spectral estimation.

Applications of Barkhausen Sensors 

Barkhausen effect sensors are broadly used for nondestructive detection of structure defects. Application examples include examining stresses in a material, fatigue testing of steels, evaluating piezoelectric properties of metallic glasses, and pointing defects, dislocations, grain boundaries, and impurities of metallic materials.

Impact Toughness Tester: Korean Heavy Industries & Construction Company developed a Barkhausen noise (BN) sensor to obtain information on the impact toughness of the forged shell of SA 508 class 3 steel for a pressurized water reactor vessel.

The sensor consists of a ferrite core and a 2000-turn pickup coil (see Figure 3a). The varying magnetic field is generated by a 1000-turn yoke magnet excited by 10 V, 0.5 Hz triangle waves from a function generator.

The sensor is placed vertically on the surface of the specimen to measure the variation of a magnetic flux developed. The measured BN signal after passing an amplifier and a band-pass filter is displayed in Figure 3b.

Barkhausen Stress Sensor: Figure 4 is a Barkhausen sensor, developed by Technical University of Szczecin in Poland, to detect material changes introduced by stresses. It contains an 800-turn excitation coil, two 2000-turn pickup coils, and an aluminum case (shielding).

The excitation coil with a 0.25 mm diameter is wound on a C-shaped ferrite core and is driven by a 30 Hz sinusoidal current source. The pickup coils, made of 0.02 mm diameter copper wires, are connected in series and placed one over the other separated by pole pieces of the ferrite core. The bottom coil is directly affected by the specimen property.

The top coil picks up nonmaterial related signals. The inner distance between poles of ferrite core is 10 mm. The sensor’s output is fed into a signal conditioning circuit including a highpass filter (500 Hz cutoff frequency, 50 dB gain), followed by a low-pass filter (30 kHz cutoff frequency and 10 dB gain). An A/D (analog-to-digital) converter with a sampling frequency of 500 kHz is used.

Several measurements are taken at each point of interest on the sample, and the median energy value of the signal is obtained.

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