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Hysteris in ferromagnetic materials - BH curve

Hysteresis is present in ferromagnetic material, as shown in the figure below. The 'magnetic field strength' H is shown along the x-axis and the degree of magnetization B is shown along the y-axis. If there is no 'magnetic field', there is no magnetization (at the beginning) and we begin at the origin. As soon as a magnetic field is applied, the ferromagnetic material will become magnetic. This continues until all the 'Weiss regions' in the material have the same orientation. The material is now at its maximum magnetization and increasing the magnetic field has no further influence on the degree of magnetization. If the magnetic field is reduced, the domain walls of the Weiss regions will mostly maintain their position. Only when the field becomes more negative will the total magnetization also change sign. This continues until all the spins are oriented in the other direction and the magnetization is reversed (product is 'demagnetized'). There are also three other factors that play a role in this nonlinearity: a curve characteristic, a dead zone and saturation.

Hysteresis curve (BH curve)

BH curve

 

Physical origins of ferromagnetism

Ferromagnetism occurs in materials containing atoms with partially filled shells (i.e. unpaired spins). These atoms interact with each other, resulting in the alignment of their atomic magnetic moments. This gives rise to spontaneous, permanent magnetic fields around an object made of ferromagnetic material.

Ferromagnetism

Ferromagnetisme

Anti-Ferromagnetisme

Anti-Ferromagnetism

 

Although a material usually contains interactions that tend to align the spins as well as those that tend to set them in opposite directions, the former forces predominate in a ferromagnet (otherwise the result is 'antiferromagnetism')

In principle, all the spins in a ferromagnet can be aligned in the same direction – in which case the object reaches its magnetic saturation and it possesses a strong, spontaneous magnetic field. However, the alignment of the spins may also take place in smaller domains, in what are referred to as Weiss regions. If the magnetization of the domains is random, the total field of the object is zero, even though there is magnetic orientation. Through exposure to a strong magnetic field all the domains can be drawn in the same direction (become magnetized).

As the temperature rises, the molecular excitement gradually disrupts the spin alignment. At a certain temperature, the 'Curie temperature', the alignment collapses because the thermal energy has exceeded the energy of the magnetic interaction. Above TC the material behaves paramagnetically, the reciprocal susceptibility as a function of the absolute temperature then forms the characteristic straight line of a paramagnet. However, the line passes through T = TC instead of through T = 0 K because the interaction between the spins remains, even though the thermal energy inhibits the ordering.

Applications

There are a wide range of applications for the hysteresis that occurs in ferromagnets. Many of these applications utilize this phenomenon to serve as ‘memory’, such as magnetic tapes, hard disks and credit cards. For these applications the use of hard magnets (high coercivity), such as iron, is desirable, so the memory cannot be easily erased.

Soft magnets (low coercivity) are used as cores for electromagnets.

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Anisotropní

Když je magnetický materiál natlačen do magnetického pole, nazývá se preferenčně orientovaným a anisotropním. Anisotropní materiál lze zmagnetizovat pouze v preferenční orientaci.