Magnetic hysteresis is the trend that materials ferromagnetic present to conserve magnetization acquired by them by applying a magnetic field external. The term hysteresis is from Greek origin and means "delay".
Some materials may have different levels of hysteresis, that is, they are able to maintain part of the orientation of the magnetic domains in their interior even after the external magnetic field, commonly generated from a electric current that circulates through a solenoid.
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How does magnetic hysteresis work?
Magnetic hysteresis is done controlling the intensity and direction of a magnetic field that passes through a ferromagnetic material. This external magnetic field, usually denoted by the symbol H, causes the magnetic domains, which are microscopic regions inside the material, to align the magnetic dipoles of the atoms with the external magnetic field. Alignment of these small magnetic domains produces a resulting non-zero magnetic field induced within the material.
Magnetic Hysteresis Cycle
Note in the following figure the relationship between the external magnetic field (horizontal), denoted by the letter H, and the internal magnetic field (vertical direction), denoted by the letter B, which is induced inside a ferromagnetic material.
From the origin of the graph, the intensity of the external magnetic field H is gradually increased. Thus, the material has more and more aligned magnetic domains, thus reaching maximum magnetization in the point A — the point at which the saturationgivescurve of magnetization.
After the saturation of the internal magnetic field, the external magnetic field gradually decreases, however the magnetization curve runs through a different path, since a part of the magnetic domains remains in the same direction even when the external field H is null, as seen in point B. The magnetic field that remains in the material after the magnetic field ceases is called the remnant field.
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Between the points B and C, the direction of the electric current that runs through the solenoid is reversed, hence the direction of the external magnetic field is reversed as well. As the H field increases in the opposite direction to the direction of initial magnetization, the material becomes increasingly demagnetized.
THE demagnetizationcompleteof the material only occurs at point C – at this point, it is possible to measure what the intensity of the external magnetic field must be for the material to lose its magnetization, and this field is called fieldcoercive.
From the point D, if we continue to increase the intensity of the external field, the material will magnetize again, but will have its poles reversed in relation to point A. By decreasing again the external field, the material will have its internal magnetic field reduced to fieldremnant at the point E. However, this remaining field will have the opposite sense to that measured at point B.
At the point F the material is again demagnetized, but if we continue to increase the strength of the H field, the magnetic domains will line up once more, so that the material will return to the saturation state at point A.
It is important to note that, during the hysteresis cycle, a part of the energy that is transferred by the external magnetic field is used to orient the magnetic domains, and the other part of that energy is dissipated in the form of an increase in Thermal energy, since the rotation of the magnetic dipoles occurs in the midst of friction between the molecules. This dissipated energy, in turn, is proportionalthe area formed by the curves of the hysteresis cycle – the larger this area, the greater the amount of heat that is lost to the external environment.
Lookalso: Transformers - devices that lower or raise electrical voltage
Technological applications of magnetic hysteresis
Magnetic hysteresis is used for the write data totapes, cardsmagneticor on hard drives, like those used for data storage on most modern computers.
The bigger the coercivity of a material, the greater is your resistance to demagnetization, that is, the greater must be the intensity of the external magnetic field to nullify the magnetization of the material. Highly coercive materials are interesting for applicationselectronics, since in these applications it is necessary that the stored information is not easily destroyed when exposed to an external magnetic field.
As stated, materials whose hysteresis cycles have large areas dissipate large amounts of heat, so can be used to heat up quickly, as iron or steel pans do when used in induction cookers, by example.
For production of permanent magnets, for example, materials capable of maintaining their magnetization are used, that is, they have high remanent magnetization. At production of magnets artificial, in turn, it is desired that the material is easily magnetized, but that it does not maintain this magnetization after the external magnetic field has ceased.
According to the desired technological application, different materials, with different hysteresis cycles, can be used. Some of them have tighter loops, while others may have more pronounced cycles in the vertical direction, for example.
By Rafael Hellerbrock
Physics teacher
