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An atom’s electron has its own orbital magnetic dipole moment and spins magnetic dipole moment that combines vectorially. In a material sample, the results of each atom combine with that of the other atoms. If all these magnetic dipole moments produce a magnetic field, we can say that the material has magnetism. There are general three types of magnetism ;diamagnetism, paramagnetism, and ferromagnetism.
The permanent magnetic moments of some materials have a strong tendency to align themselves even when there is no external field present. Such materials are known as ferromagnetic materials.
Ferromagnetism:
Ferromagnetism is a characteristic of some elements where the resultant magnetic movements of electrons are aligned, which produces strong dipole moments in the region. The magnetic moment from such regions is then aligned by an external field, resulting in a strong magnetic field for a material sample. Some part of the field is present even after we remove the external field.
Ferromagnetic materials:
The atoms of ferromagnetic material have permanent dipole moments in their domains. Elements like iron, cobalt, nickel, iron exhibit ferromagnetism because of a quantum physics effect called exchange coupling. The adjacent atoms interact with each other through their electron spin.
This results in the alignment of the magnetic dipole moment of the atoms despite the randomisation tendency of atomic collision due to thermal agitation. The permanent magnetism of ferromagnetic elements is due to this persistent alignment. These elements are known as ferromagnetic materials and magnetic materials.
When heated above a specific temperature called the Curie temperature, the ferromagnetic materials lose their magnetic properties and become paramagnetic. At high temperatures, exchange coupling stops being effective. The dipole still aligns with an external field but much more weakly.
Causes of ferromagnetism:
Magnetism occurs due to the interaction between the neighbouring dipole of atoms and the alignment of the permanent dipoles that results from unpaired electrons present in the valence shell.
Theory of ferromagnetism:
Weiss gave the domain theory of ferromagnetism in 1907. This theory explains the hysteresis and the properties of ferromagnetic material.
Magnetic material is divided into many small regions called domains.
In each domain, the magnetic moment is in the same direction.
The net magnetisation is zero, but the magnetic moment is different for different domains.
When the external magnetic field is absent, the magnetic moment is in different directions.
When the magnetic field is applied, two processes take place.
On application of the small amount of magnetic fields, the dipoles present in the domains align themselves parallel to the direction of the applied magnetic field. The movement of the domain wall increases the domain area.
When we increase the applied magnetic field, the rotation of domains causes the domains to rotate parallel to the direction of the field.
Magnetic hysteresis:
When an external magnetic field is applied to a ferromagnetic substance, such as iron, nickel, or cobalt, the atomic dipoles align themselves with it, causing hysteresis. When we remove the field, the alignment is partially retained, and the material becomes magnetised, and when a material is magnetised, it remains magnetised for an indefinite amount of time. It is also known as a hysteresis loop.
B-H curve:
The B-H curve represents the magnetising force(H) versus a ferromagnetic material’s magnetic flux density(B). This curve is characteristic of the observed type of material and can vary in shape and size.
The BH curve is used to explain the nonlinear behaviour of magnetisation that a ferromagnetic material obtains when a magnetic field is applied to it.
Applications of ferromagnetism:
Ferromagnetism applies to various devices like transformers, electromagnets, magnetic tape recordings, hard drives, telephones, loudspeakers, generators, etc.
Antiferromagnetism:
It is a type of magnetism that occurs in solids like magnesium oxide, in which the adjacent ions behave like tiny magnets. They spontaneously aligned themselves at a relatively low temperature into opposite arrangements throughout the material to exhibit net external magnetism.
In antiferromagnetic materials, the magnetism from magnetic ions of one direction is cancelled by the set of magnetic ions aligned in the opposite direction.
Conclusion:
Magnetism is a property by which certain materials obtain a permanent magnet’s property or are attracted to a magnet. An external magnetic field aligns the magnetic dipole movements in a ferromagnetic compound. They remain partially aligned when the external fields are removed hence acquiring the characteristics of a permanent magnet.
When the temperature of the material rises above the curie temperature, the alignment of the magnetic dipole moment is disturbed. Where there is a non-uniform magnetic field, the ferromagnetic substance is generally attracted to the region of the high magnetic field.
