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Ferromagnetism – Definition, Causes, Properties, Hysteresis

  • Last Updated : 03 Sep, 2021

There are several types of magnetism, with ferromagnetism being the most powerful. Ferromagnetic materials have a spontaneous net magnetization at the atomic level, even when no external magnetic field is present. When ferromagnetic materials are exposed to an external magnetic field, they become highly magnetised in the field’s direction. A magnet attracts ferromagnetic materials aggressively. Even when the external magnetising field is withdrawn, these materials will keep their magnetization for a period of time. Hysteresis is the name for this characteristic.

What is Ferromagnetism?

Ferromagnetism takes its name from the term “ferrous,” which refers to iron, which was the first metal discovered to have magnetic field-attractive characteristics.

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Ferromagnetism is a magnetic property that some materials, such as iron, cobalt, alloys, and others, have. It’s a phenomenon in which certain materials develop persistent magnetic or attracting properties. It’s also known as a process in which electrically uncharged elements aggressively attract one another. Ferromagnetism is a characteristic that considers not just a material’s chemical makeup, but also its microstructure and crystalline structure.



Causes of Ferromagnetism

Atomic dipoles in tiny areas termed domains in an unmagnetized ferromagnetic material are oriented in the same direction. Even in the absence of an external magnetising field, the domains have a net magnetic moment. The magnetic moments of neighbouring domains, on the other hand, are in opposing directions. They cancel each other out, resulting in the material’s net magnetic moment being zero. When an external magnetic field is applied, all of these domains align in the direction of the applied field. The material is strongly magnetised in a direction parallel to the magnetising field as a result of this process.

Ferromagnetic Materials

Ferromagnetic materials are a class of materials that, when exposed to a magnetic field, tend to express or display significant magnetism in the direction of the field. The alignment patterns of these materials’ component atoms are primarily responsible for their magnetic. These atoms have a tendency to act like basic electromagnets.

Examples of Ferromagnetic Materials

Metals make up the majority of ferromagnetic materials. Iron, Cobalt, Nickel, and other ferromagnetic materials are common examples. In addition, ferromagnetic materials include metallic alloys and rare earth magnets. The oxidation of iron into an oxide produces magnetite, a ferromagnetic substance. It has a 580°C Curie temperature. It was previously known as a magnetic substance. Magnetite is the most magnetic of all-natural minerals on the planet.

Properties of Ferromagnetic Materials

  • In domains, the atoms of ferromagnetic substances have a persistent dipole moment.
  • In ferromagnetic materials, atomic dipoles are orientated in the same direction as the external magnetic field.
  • The magnetic dipole moment is big and oriented in the magnetising field’s direction.
  • The magnetization intensity (M) is extremely high and positive, and it changes linearly with the magnetising field (H). As a result, saturation is determined by the material’s composition.
  • Magnetic susceptibility is quite high and positive. Magnetic susceptibility is defined as Xm = M ⁄ H, where M is the magnetization intensity and H is the applied magnetic field strength.
  • The material’s magnetic flux density will be extremely high and positive. Inside ferromagnetic materials, magnetic field lines become extremely thick. B = ε0 (H + M) magnetic flux density, where ε0 is the magnetic permittivity of free space, H is the applied magnetic field strength, and M is the magnetization intensity.
  • The relative permeability is likewise quite high and changes linearly with the magnetic field. The magnetic field inside the material is significantly greater than the magnetic field outside the material. They have a tendency to pull in a high number of force lines from the material.
  • The field aggressively attracts ferromagnetic materials. They prefer to cling to the poles where the field is stronger in a non-uniform field.
  • If the ferromagnetic powder is placed in a watch glass set on two poles pieces that are sufficiently apart, powder collects on the sides and displays depression in the centre because the field is strongest at poles.
  • A ferromagnetic material loses its ferromagnetic characteristics when liquefied because of the greater temperature.

Hysteresis

A ferromagnetic substance does not entirely demagnetize when the external magnetic field is removed. A magnetic field in the opposite direction must be supplied to bring the material back to zero magnetization. Hysteresis is a characteristic of ferromagnetic materials that allows them to retain their magnetization when an external field is removed.

When the magnetization of the material is measured in terms of magnetic flux density (B) and the external magnetic field strength (H), a loop is formed. This is known as hysteresis loop.

Hysteresis Loop

When the magnetising force is decreased to zero, retentivity is the magnetic flux density that remains. The strength of the reverse magnetising field that must be supplied to entirely demagnetize the material is referred to as coercivity.



Curie Temperature

Temperature affects ferromagnetic properties. Ferromagnetic substances become paramagnetic when heated to a high enough temperature. Curie’s temperature is the temperature at which this transition happens. Tc is the abbreviation for it.

Uses of Ferromagnetic Materials

Ferromagnetic materials have a wide range of uses in industry. Electric motors, telephones, generators, loudspeakers, transformers, and the magnetic stripe on the back of credit cards are all examples of equipment that utilise them.

Sample Problems

Problem 1: What is Ferromagnetism?

Solution:

Ferromagnetism is a physical phenomenon (long-range ordering) in which certain materials, such as iron, are attracted to one another strongly. Rare earth minerals including gadolinium contain ferromagnets. Magnetism in magnets is caused by one of the most prevalent phenomena observed in everyday life.

Problem 2: A domain of ferromagnetic iron is shaped like a cube with a side length of 1 μm. Find the number of iron atoms in the domain. (Molecular mass of iron = 55g mol−1 and density = 7.92 g ⁄ cm3)

Solution:

Given:



Side length, l = 1 μm

The volume of the cubic domain, V = (1 μm)3

= (1 × 10−6 m)3

= 1 × 10−12 cm3 

Mass, M = V × d

= 1 × 10−12 cm3 × 7.92 g ⁄ cm3 

= 7.92 × 10−12 g

Now the Avagadro number (6.023 × 1023) of iron atoms have a mass of 55 g, so the number of atoms in the domain:

N = 7.92 × 10−12 g × 6.023 × 1023 ÷ 55

= 0.867 × 1011

Hence, the number of atoms in the domain is 0.867 × 1011 atoms.

Problem 3: What will be the effect on the magnetic domains if a ferromagnetic material is placed in an external magnetic field?

Solution:

For a ferromagnetic material, the magnetic domains align themselves in the direction of the external magnetic field. Domains are essentially directed tiny regions of atoms. When the domains aligned with the field are retained in an external magnetic field, the domains aligned with the field grow and take over regions formerly inhabited by domains oriented against the field. Hence, the domains increases in size.

Problem 4: What is curie temperature?

Solution:

The Curie temperature, also known as the Curie point, is the temperature at which some materials lose their permanent magnetic characteristics, allowing induced magnetism to take their place.

Problem 5: What Applications Does Ferromagnetism Have?

Solution:

A ferromagnetic material has a wide range of applications. The relevance of the hysteresis curve cannot be overstated. In transformers, electromagnets, and magnetic tape recording, ferromagnetism is used.




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