Magnetic saturation

The maximum magnetization of a material is called saturation magnetization. The background is an initially proportional to the magnetic field strength increasing magnetic force in the case of magnetization of a ferromagnetic body. However, from a certain point of time, this magnetization increases more and more slowly until it finally reaches an end point, the so-called magnetic saturation. It is the reason that permanent magnets have a limited magnetic field strength - therefore there are no magnets that are arbitrarily strong.

Physical explanation of magnetization and magnetic field saturation

The attempt to increase the magnetization after reaching the magnetic saturation results in the following behavior: The material behaves as if the external magnetic field is increased in a vacuum. The magnetization can be observed in this context, especially in ferromagnetic materials: The magnetic flux density increases very strong as soon as ferromagnetic material is introduced into an external magnetic field. The physical explanation for this are the electron spins. These align themselves in the ferromagnet after the externally applied magnetic field. With increasing magnetization, more and more of these so-called magnetic moments (effect of the electron spin) align themselves parallel to the magnetic field. This process is also called magnetic polarization. Due to the alignment itself, the outer field is strengthened. It comes in this case to a strong increase in the magnetic flux density and the magnetic field in the vicinity of the ferromagnet. Logically, this process only takes place until all existing magnetic moments are aligned.

Once that's done, the magnetic saturation is reached. From now on, the external magnetic field of the ferromagnet can not be further amplified, even if it is further increased from the outside. The flux density of this field behaves from now on as if the magnetic field is amplified in vacuum. So there is no amplification by the ferromagnet instead - after all, no more electron spin can be aligned.

Experiment shows magnetic saturation

This experiment requires a coil with an iron core and a magnetic flux density meter (such as a Hall probe) and an adjustable voltage source. In the experiment one always measures the magnetic flux density directly at the iron core with changing currents. It turns out that the magnetic flux density initially increases very significantly with an increase in the electrical current through the iron coil. If the current or the current is doubled, the magnetic flux density approximately doubles as well. However, at some point, there is a slower increase. Finally, the magnetic saturation of the iron core is reached (for iron at a maximum flux density of 2 Tesla). The magnetic permeability of the ferromagnetic material decreases during the saturation effect until it approaches unity. This means that the magnetic conductivity of the ferromagnetic material is equal to that of the vacuum - thus confirming the previous statements that the magnetic field of the ferromagnet behaves like a magnetic field in a vacuum after saturation.

Remanence and magnetic saturation

As already explained, the maximum saturation is reached when all atomic spins are aligned with the magnetic field. The magnetization of the material can not continue to increase from now on, which is why, after switching off the external magnetic field, this is the state of the maximum possible remaining magnetization. Remaining magnetization is basically called remanence.

Importance in technology

Magnetic saturation leads to many disadvantages in technical applications. An example is transformers. These convert voltage based on a changing magnetic field through two coils that have the same iron core. As long as the current in the primary circuit of the transformer is very low, the transformer operates with high efficiency because the magnetization with the current is in the proportional range. However, if the current becomes too high, the efficiency will decrease as the range of saturation magnetization is reached. The efficiency of the transformer also drops. One can counteract this effect by cutting an air gap in the iron core. The magnetic saturation then occurs later, because the magnetic flux density increases more slowly - after all, the magnetic resistance of the air gap is much higher than that of the iron core. This in turn increases the efficiency. In most transformers, however, such an air gap can be dispensed with. A counter-example would be so-called high-current transformers. These may be known from physics lessons: the teacher usually tries to make a nail or other rod-like metal object glow with a high current.


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