Inversion of magnetization

In Physical of the condensed matter, the inversion of magnetization is the phenomenon by which a material ferrimagnetic Ferromagnétique or can see the direction of its magnetization changed under the action of an external stimulus.

Mechanisms of inversion of magnetization at the time of a hysteresis loop

If a Magnetic field is applied to a ferromagnetic or ferrimagnetic material,

When the field applied to material equalizes the coercitive force, the component of the vector of magnetization of ferromagnetic measured along the direction of the field applied is null. Two principal processes guide this phenomenon: propagation of walls of fields and coherent rotation.

Coherent rotation

On the 'assumptions of the coherent rotation, suggested in first in the model of Stoner and Wohlfarth (1948), the material is compared to one single magnetic moment or macrospin . This magnetic moment is regarded as free rotation. When the sample is elaborate in the form of thin Layer, magnetization is often regarded as confined in the plan of the layer, because of the anisotropy of form. The application of a field external with the system tends to align the magnetization of the system with the field applied, because that minimizes magnetic energy Zeeman. Under the assumption that magnetization changes “in block”, this one thus turns in its plan. The passage to the coercitive force corresponds to an orientation of magnetization perpendicular to the direction of the field applied. This model allows an analytical calculation of the form of the hysteresis loops. it is appropriate for the very pure monocrystals and the magnetic particles of small size. These materials are especially elaborate at ends of basic research.

Inversion of walls

In a certain number of systems, the inversion is not done by coherent rotation. The Modèle of Ising provides a theoretical illustration of a system which cannot adopt this first mode of inversion. The modes of inversion by propagation of walls are more frequent in materials having a strong magnetic anisotropy in the direction of measurement.

Within materials changing by nucleation of fields and propagation of walls, the variation of magnetization measured along the hysteresis loop corresponds not to a projection of magnetization (what was the case for coherent rotation), but to the existence within different material magnetic fields where magnetizations are directed according to two opposite directions. The fields are separated by walls from field, zones where magnetization continuously varies from one direction to another.

At the time of the course of the hysteresis loop close to the coercitive force, the inversion of walls by propagation of fields consists in for the system minimizing its energy by moving the wall so that the field énergétiquement favoured grows. This movement of wall requires a work of the magnetic field and takes part in the energy expenditure which characterizes the phenomenon of hysteresis.

When magnetization changes by propagation of walls, the variation of magnetization along the hysteresis loop can be interpreted as the algebraic sum of magnetizations of the various magnetic fields which were formed. The passage to the coercitive force corresponds to the moment when the volume occupied by the various orientations of the moment of field are compensated exactly.

This movement of wall can also be characterized by a resistance of wall.

A great number of industrial materials developed for the applications of the tape recording show a behavior of inversion of magnetization by inversion of magnetization. That comes from the presence of crystalline defects in magnetic metal considered: impurities, grain boundary, etc the role of the defects in the value of the coercitive force is in addition complex. The defects can at the same time support the creation of fields and trap them, preventing their displacement during a variation of the external field.

The mechanism of inversion of magnetization by propagation of walls makes more difficult the theoretical study of the force coercitive of a given material. Indeed, the concentration of the impurities, their nature and their localization influence the value of the coercitive force largely. However these values can vary from a method of manufacture with another, or from a laboratory with another, from where the difficulty of theoretical developments.

The dynamics of the walls of fields in ferromagnetic materials divides many common points with the phenomenon of Plasticité in physics of materials and Métallurgie. Indeed, the walls of fields are planar defects as well as the grain boundaries which control the phenomena of palsticity.