Hysteresis is magnetic, ferroelectric, dynamic, elastic. It is also found in biology, soil science, economics. Moreover, the essence of this definition is almost the same. But the article will talk specifically about magnetic, you will learn in more detail about this phenomenon, on what it depends and when it appears. This phenomenon is studied in universities with a technical focus, is not included in the school curriculum, so not everyone knows about it.
Magnetic hysteresis
This is an irreversible and ambiguous dependence of the magnetization index of a substance (moreover, these are, as a rule, magnetically ordered ferromagnets) on an external magnetic field. In this case, the field is constantly changing - decreasing or increasing. A common reason for the existence of hysteresis is the presence at a minimum of the thermodynamic potential of an unstable and stable state, and there are also irreversible transitions between them. Hysteresis is also a manifestation of a magnetic orientation phase transition of the first kind. With them, transitions from one phase to another occur due to metastable states. A characteristic is a graph called a hysteresis loop. Sometimes it is also called the “magnetization curve”.
Hysteresis loop
On the graph of the dependence of M from H, you can see:
- From the zero state, in which M = 0 and H = 0, M. increases with increasing H.
- When the field increases, the magnetization becomes almost constant and is equal to the value of saturation.
- When H decreases, the opposite change occurs, but when H = 0, the magnetization M will not be zero. This change can be seen in the demagnetization curve. And when H = 0, M takes a value equal to the residual magnetization.
- With an increase in H in the interval –HT ... + NT, a change in the magnetization occurs along the third curve.
- All three curves describing the processes are connected and form a kind of loop. It also describes the phenomenon of hysteresis - the processes of magnetization and demagnetization.
Magnetization energy
The loop is considered asymmetric in the case when the maxima of the H1 field, which are applied in the opposite and forward directions, are not the same. The loop described above, which is characteristic of a slow process of magnetization reversal, was described above. With them, the quasi-equilibrium relationships between the values of H and M are maintained. It should be noted that when magnetizing or demagnetizing, M lags behind N. And this leads to the fact that all the energy that is acquired by the ferromagnetic material during magnetization is not given completely during the demagnetization cycle. And this difference goes all in heating a ferromagnet. And the magnetic hysteresis loop turns out to be asymmetric in this case.
Loop shape
The shape of the loop depends on many parameters - magnetization, field strength, the presence of losses, etc. The chemical composition of a ferromagnet, its structural state, temperature, the nature and distribution of defects, and the presence of processing (thermal, thermomagnetic, mechanical) also have a considerable influence. Consequently, the hysteresis of ferromagnets can be changed by machining the materials. From this, all the characteristics of the material change.
Hysteresis losses
During the dynamic magnetization reversal of a ferromagnet with an alternating magnetic field, losses are observed. Moreover, they make up only a small fraction of the total magnetic losses. If the loops have the same height (the same maximum magnetization value M), the loop of the dynamic form is wider than the static one. This happens due to the fact that new losses are added to all losses. These are dynamic losses, they are usually associated with eddy current, magnetic viscosity. In sum, quite significant hysteresis losses are obtained.
Single domain ferromagnets
In the event that the particles have a different size, the rotation process proceeds. This is due to the fact that the formation of new domains is disadvantageous from an energy point of view. But the process of particle rotation is hindered by anisotropy (magnetic). It can have a different origin - to form in the crystal itself, to arise as a result of elastic stress, etc.). But it is precisely with the help of this anisotropy that the magnetization is retained by the internal field. It is also called the effective field of magnetic anisotropy. And magnetic hysteresis arises due to the fact that the magnetization changes in two directions - direct and reverse. During magnetization reversal of single-domain ferromagnets, several jumps occur. The magnetization vector M rotates in the direction of the field N. Moreover, the rotation can be uniform or inhomogeneous.
Multi-domain ferromagnets
In them, the magnetization curve is constructed in a similar way, but the processes proceed differently. During magnetization reversal, domain boundaries shift. Consequently, one of the reasons for the occurrence of hysteresis may be a delay in the displacement of boundaries, as well as irreversible jumps. Sometimes (if the ferromagnets have a rather large field), the magnetic hysteresis is determined by the growth retardation and the formation of magnetization reversal nuclei. It is from these nuclei that the domain structure of ferromagnetic substances is formed.
Hysteresis theory
It should be borne in mind that the phenomenon of magnetic hysteresis also occurs when the field H is rotated, and not only when it changes in sign and magnitude. This is called the hysteresis of magnetic rotation and corresponds to a change in the direction of magnetization M with a change in the direction of the field N. The occurrence of a hysteresis of magnetic rotation is also observed when the sample under study is rotated relative to a fixed field N.
The magnetization curve also characterizes the magnetic structure of the domain. The structure changes during the processes of magnetization and magnetization reversal. Changes depend on how much the domain boundaries are shifted, on the effects of an external magnetic field. Absolutely everything that can delay all the processes described above translates ferromagnets into an unstable state and is the reason that magnetic hysteresis occurs.
It should be noted that hysteresis depends on many parameters. The magnetization changes under the influence of external factors - temperature, elastic stress, therefore, hysteresis occurs. In this case, hysteresis appears not only of magnetization, but also of all those properties on which it depends. As can be seen from this, the hysteresis phenomenon can be observed not only during magnetization of the material, but also in other physical processes associated directly or indirectly with it.