What exactly is the hysteresis? - The effect of hysteresis simply explained
As a property of ferromagnetic materials, hysteresis (of hysteresis: the lagging of an effect behind the variable force causing it) is a graphic illustration of magnetization and remanence. Thus, the magnetization of a material increases under application of an external magnetic field and decreases after its removal again. In the case of ferromagnetic materials, however, this does not happen so quickly: The demagnetization in the course of hysteresis takes place here more slowly as soon as the magnetic field is switched off. When the external magnetic field is completely switched off, so-called remanence remains with ferromagnetic materials - this is nothing more than a residual magnetization. It is directed counter to the once applied external magnetic field.
The hysteresis is therefore a physical phenomenon that describes the non-proportionality between the course of the external magnetic field and the course of the magnetization in the ferromagnetic material. It follows from this fact that the magnetization of a magnet with twice the magnetic field does not automatically cause it to double.
How to visualize the hysteresis graphically?
As already mentioned, one has to visualize the hysteresis graphically by means of a coordinate system:
- The x-axis stands for the magnetic field H
- The y-axis stands for the magnetic flux density B
Initially, the magnetization of the ferromagnetic material increases with the external magnetic field - as long as the external magnetic field strengthens, so does it. But as soon as it weakens, the magnetization goes back down - but not as fast as the magnetic field, which is down-regulated.
This effect of hysteresis is responsible for the fact that at the end - ie when the external magnetic field has been completely switched off - residual magnetization remains in the material: the remanence. The magnetic flux density always depends on the magnetization field strength. At the end, there are two curves that are symmetric at the point of origin. They meet in the first and third quadrants of the coordinate system and form two limit values, which we will explain in more detail later.
What does the hysteresis curve in the field of magnetism describe?
The so-called hysteresis curve depends on the respective material. It exists only with ferromagnetic materials:
- If the distance between the two curves is large, it indicates a hard magnetic material.
- A smaller distance is called a soft magnetic material.
Of course, if a material is not magnetized and exposed to an external magnetic field, the magnetic flux density curve will naturally start at the origin. It goes up to one of the mentioned corner points and is referred to by professionals as a new curve. This has a special property: it approximates a straight line. This means that the magnetic flux density and the magnetization of the material to the external magnetic field are approximately linear. From this context one can derive the following formula (with μ for the magnetic permeability) to calculate the hysteresis:
Inside the ferromagnetic material, the magnetic field is the sum of the magnetization M and the external magnetic field H. If an already magnetized ferromagnetic body is exposed to an opposite magnetic field, its magnetization weakens initially, but it still exists. Only at a certain field strength of the external magnetic field, the ferromagnetic material is demagnetized, so it is no longer magnetic at this certain limit - also called coercive field strength. As the external magnetic field continues to increase in strength, the magnetization of the material eventually reverses - in effect, the entire magnetization process, already explained above, begins anew. This time just in the other direction.
Another interesting quantity that can be taken from the hysteresis curve: The energy that is in the magnet per unit volume. It is taken from the area enclosed by the hysteresis curve. This energy is converted into heat during the magnetization process. For magnetically soft materials, the energy is smaller than for magnetically hard materials. Therefore, magnetically hard materials are less sensitive to external influences. These include, for example:
- Magnetic fields
- Mechanical shocks
Hard magnetic materials are therefore perfectly suited for use as a permanent magnet. Transformers, on the other hand, are made of soft magnetic materials. In a transformer, the magnetic field is constantly reversed. Finally, this changing magnetization of the material should consume as little energy as possible.
Physical background of the hysteresis
One can imagine a ferromagnetic material once at the atomic level. Every atom has electrons and they have an electron spin. These spins, in turn, account for the elementary magnetic moments of each ferromagnetic substance (eg, iron). Basically, they are equivalent to the well-known elementary magnet, which were always drawn in the physics class as small arrows. By means of an external magnetic field they align themselves according to this and stabilize themselves by the so-called exchange interaction.
Finally, the meaning of the two corner points (limits) of the hysteresis curve is understood. At these two points, the elementary magnets are all aligned in parallel. Therefore, they are also called saturation flux density. In order to destroy the alignment of the elementary magnets, the exchange interaction must be overcome. This means that in order to demagnetize the magnetized material again, it requires a certain amount of energy - for example in the form of a hard impact, high heat or a corresponding magnetic field.