There are different kinds of magnetism, on which Ferromagnetism is the strongest type. Ferromagnetic materials are the ones, exhibit a spontaneous net magnetization at the atomic level, and even when the absence of an external magnetic field.
When the material is placed in an external magnetic field, the ferromagnetic materials strongly magnetizes in the direction of the field. These materials are strongly attracted to a magnet. Also, they will retain their magnetization for some time, although after the external magnetizing field is removed. This property is known as hysteresis.
What is Ferromagnetism?
The term Ferromagnetism has taken from the word 'ferrous' which means iron, the first metal that is known to exhibit the attractive properties to the magnetic fields. Ferromagnetism is a unique magnetic behaviour which is exhibited by certain materials like iron, alloys, cobalt, and more. It is a phenomenon where the Ferromagnetic materials attain a permanent magnetism, or they acquire some attractive powers. It is also demonstrated as a process where a few of the electrically uncharged materials strongly attract each other. Ferromagnetism is a property that not only considers the chemical make-up of material but also considers the crystalline and microstructureBesidesa.
Causes of Ferromagnetism
In an unmagnetized state of ferromagnetic material, the atomic dipoles in small regions known as domains are aligned in the same direction. These domains exhibit a net magnetic moment, even in the case of external magnetizing field absence.
However, the neighbouring domains' magnetic moments are directed towards the opposite direction. They cancel out, and thus the net magnetic moment of the material results zero. On applying the external magnetic field, all these domains align themselves in the direction of the applied field. In such a way, the material is strongly magnetized in a direction that is parallel to the magnetizing field.
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Ferromagnetic materials are some groups of substances, tend to manifest or display strong magnetism in the direction of the field because of the application of a magnetic field. Mainly, the cause of magnetism in these materials is due to the alignment patterns present in their constituent atoms. These atoms tend to behave as an elementary electromagnet.
Examples of Ferromagnetic Materials
Many ferromagnetic materials are metals. Some common examples of ferromagnetic materials are Cobalt, Iron, Nickel, and more. Besides, rare earth magnets and metallic alloys are also classified as ferromagnetic materials.
Magnetite is a ferromagnetic material, produced by the oxidation of iron into an oxide. It holds a Curie temperature of 580°C and was recognized earlier as a magnetic substance. Magnetite has the greatest magnetism among all the natural minerals that exist on the earth.
Properties of Ferromagnetic Materials
Ferromagnetic substances atoms have permanent dipole moment in domains.
The magnetic dipole moment is so large and present in the magnetizing field direction.
Atomic dipoles in ferromagnetic substances are situated in the same direction as the external magnetic field.
The intensity of magnetization (M) is very large, positive, and varies linearly with the magnetizing field (H). So, saturation depends on the nature of the material.
The magnetic susceptibility is very large, positive. Magnetic susceptibility Xm = M / H, where H is the applied magnetic field's strength, M is the intensity of magnetization.
Also, the relative permeability is very large and varies linearly with the magnetizing field; the field inside the material is much stronger compared to the magnetizing field. They contain a tendency to pull in a large number of lines of force by the material. Relative permeability of material is, r = 1 + m.
The magnetic flux density of the material is very large, positive. Magnetic field lines become very dense inside the ferromagnetic materials. Magnetic flux density B = 0 (H + M), where H is the strength of the applied magnetic field, M is the intensity of magnetization, 0 is the magnetic permittivity of free space.
Ferromagnetic substances are attracted by the field, strongly. So they have a tendency in a nonuniform field, to stick at the poles in the strongest field.
If a ferromagnetic powder is kept in a watch glass placed on two poles pieces that are sufficient apart, then the powder accumulates at sides and shows the depression in the middle due to the field is strongest at poles.
When the same substance is liquefied, it loses ferromagnetic properties because of the higher temperature.
On the removal of the external magnetic field, a ferromagnetic material doesn't get fully demagnetized. To bring the material back to the zero magnetization, a magnetic field in the opposite direction needs to be applied. The property of ferromagnetic materials retaining the magnetization after the external field is removed, known as hysteresis.
The magnetization of the material measured in magnetic flux density (B) plotted against the external applied magnetic field intensity (H) will trace out a loop. This is known as the hysteresis loop.
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Retentivity is the magnetic flux density, which remains when the magnetizing force is decreased to zero.
Coercivity is the strength reverse magnetizing field that needs to be applied to completely demagnetize the material.