What’s A Ferromagnetic Substance And How Is It Related To Domain Theory?
A ferromagnetic substance is a substance that is attracted to a magnet. These substances can also be magnetized. This attraction can be observed by pulling a force from the magnet on the ferromagnetic object. There are three major ferromagnetic elements that we always bring up every time we discuss ferromagnets, as any object that can be pulled by a magnet contains either one or more of these elements. They are Iron (Fe), Nickel (Ni), Cobalt (Co) and some rare earth metals. Ferromagnetic materials can be sometimes combined with other elements to create alloys – mixtures of a metal with another metal or nonmetal. Common ferromagnetic alloys include steel i.e., the combination of iron and carbon and permalloy, a combination of iron and nickel. These alloys though are not from the above given three elements, their combination will still make them ferromagnetic.Ferromagnetism is the strongest type of magnetic behaviour and it is the most common.
By itself iron is not magnetic or magnetized because the magnetic domains are randomly organized within the solid and do not point in a specific direction. Once a magnet is introduced into this system iron will realign the atomic nuclei within the magnetic domain and the direction of the magnetic field produced by the magnet. Therefore, magnetizing iron and producing and an attraction to the magnet.
Domain Theory (Weiss’ Theory) Of Ferromagnetism
French physicist Pierre-Ernest Weiss developed the Magnetic domain theory, who suggested in 1906 that magnetic domains actually do exist in ferromagnets. He also suggested that a huge number of magnetic moments, were aligned parallelly. The direction of alignment varied more or less randomly from domain to domain, even though the magnetic moments may prefer a certain crystallographic axis, called easy axes. To explain the reason for the spontaneous alignment of atomic moments within a ferromagnetic material, Weiss came up with the so-called Weiss mean field. It was assumedthat a material with given magnetic moment experienced a very high effective magnetic field because of the magnetization of its neighbours. The mean field was proportional to the bulk of magnetization M, in the original Weiss theory, so that
= α
Where α – mean field constant. This however, due to the variation of magnetization from domain to domain, is not applicable to ferromagnets. So, now the interaction field is,
= α
Where, – the saturation magnetization at 0K.
According to Weiss a ferromagnetic substance contains atoms with permanent magnetic moments, as a paramagnetic substance. But due to a special form of interaction called as exchange coupling or exchange interaction, occurring between adjacent atoms, there exists some strong forces between neighbouring atomic dipoles which cause groups of them in such a way that all atomic dipoles of a group point in the same direction. Now, such groups are known as domains. This can be applied to the example given above about iron.
When a ferromagnetic substance is in a normal state, the magnetisation vectors in different domains are oriented in different directions such that the resultant magnetism is zero.
When the external magnetic field is applied to the substance, the domains rotate instead of the individual magnetic dipole to align their magnetic moments with the field direction. Hence, the net magnetisation results as it moving towards the magnet.
The spontaneous magnetization of each domain is due to the presence of an exchange field , which tend to produce a parallel alignment of the atomic dipoles. The field is assumed to be proportional to the magnetization M of each domain, that is,
=
Λ – constant called the Weiss field constant and is independent of temperature.
– Molecular field on the Weiss field.
Therefore, effective magnetic field on an atom is,
= + = +
Curie Temperature
The curie temperature is an important part of understanding ferromagnetic substances, as these ferromagnetic substances have properties that depend on temperature. Simply, at a temperature that is high enough, these ferromagnetic substances become paramagnetic. This transition occurs at a certain temperature, that temperature is called Curie’s temperature. It is indicated by .
Conclusion
It is important to note that in case of ferromagnetic materials, after external field is removed and the domain has been aligned in one particular direction, the domain boundaries do not change their orientation. Therefore, the ferromagnetic materials do not get demagnetized and this is the exact reason why they are widely used to make permanent magnets These ferromagnetic materials are always attracted strongly towards the eternal field.
Weiss’ theory of ferromagnetism was later surpassed with a more precise and correct theory, that is the quantum theory of ferromagnetism, this theory made it possible for us to exactly understand the microscopic origins of the Wiess field. The exchanged interaction between an anti-parallel in anti-ferromagnets state of neighbouring magnetic moments or localized spins favoured a parallel in ferromagnets.
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