Ferromagnetic Materials

There are various kinds of magnetism, out of which ferromagnetism is the strongest type. Ferromagnetic materials are those materials which exhibit a spontaneous net magnetisation at the atomic level, even in the absence of an external magnetic field.

When placed in an external magnetic field, ferromagnetic materials are strongly magnetised in the direction of the field. Ferromagnetic materials are strongly attracted to a magnet. These materials will retain their magnetisation for some time, even after the external magnetising field is removed. This property is called hysteresis.

Table of Contents

• What Is Ferromagnetism?
• Causes of Ferromagnetism
• Ferromagnetic Materials
• Examples of Ferromagnetic Materials
• Properties of Ferromagnetic Materials
• Hysteresis
• Curie Temperature
• Uses of Ferromagnetic Materials

What Is Ferromagnetism?

Ferromagnetism gets its name from the word ‘ferrous’, which means iron, and it was the first metal known to show attractive properties to magnetic fields. Ferromagnetism is a unique magnetic behaviour that is exhibited by certain materials such as iron, cobalt, alloys, etc. It is a phenomenon where these materials attain permanent magnetism, or they acquire attractive powers. It is also described as a process where some electrically uncharged materials attract each other strongly. Ferromagnetism is a property that considers not only the chemical make-up of a material but also takes into account the microstructure and the crystalline structure.

Causes of Ferromagnetism

In a ferromagnetic material in the unmagnetised state, atomic dipoles in small regions called domains are aligned in the same direction. The domains exhibit a net magnetic moment even in the absence of an external magnetising field.

Also Read: Paramagnetic Materials

However, the magnetic moments of neighbouring domains are oriented in opposite directions. They cancel out, and therefore the net magnetic moment of the material is zero. On applying an external magnetic field, these domains align themselves in the direction of the applied field. In this way, the material is strongly magnetised in a direction parallel to the magnetising field.

Ferromagnetic Materials

Ferromagnetic materials are a certain group of substances that tend to manifest or display strong magnetism in the direction of the field due to the application of a magnetic field.  The cause of magnetism in these materials is mainly due to the alignment patterns of their constituent atoms. These atoms tend to behave as elementary electromagnets.

Also Read: Diamagnetic Materials

Examples of Ferromagnetic Materials

Most of the ferromagnetic materials are metals. Common examples of ferromagnetic substances are Iron, Cobalt, Nickel, etc. Besides, metallic alloys and rare earth magnets are also classified as ferromagnetic materials.

Magnetite is a ferromagnetic material which is formed by the oxidation of iron into an oxide. It has a Curie temperature of 580°C. Earlier, it was recognised as a magnetic substance. Magnetite has the greatest magnetism among all the natural minerals on the Earth.

Properties of Ferromagnetic Materials

1. The atoms of ferromagnetic substances have permanent dipole moments present in domains.
2. Atomic dipoles in ferromagnetic substances are oriented in the same direction as the external magnetic field.
3. The magnetic dipole moment is large and is in the direction of the magnetising field.
4. The intensity of magnetisation (M) is very large and positive and varies linearly with the magnetising field (H). Hence, saturation depends on the nature of the material.
5. The magnetic susceptibility is very large and positive. Magnetic susceptibility X_m = M / H, where M is the intensity of magnetisation, and H is the strength of the applied magnetic field.
6. The magnetic flux density of the material will be very large and positive. Magnetic field lines become very dense inside ferromagnetic materials. Magnetic flux density B = ε₀ (H + M), where ε₀ is the magnetic permittivity of free space, H is the strength of the applied magnetic field, and M is the intensity of magnetisation.
7. The relative permeability is also very large and varies linearly with the magnetising field, and the field inside the material is much stronger than the magnetising field. They tend to pull in a large number of lines of force by the material.
8. Ferromagnetic substances are strongly attracted by the field. So, in a nonuniform field, they tend to stick at the poles where the field is strongest.
9. If a ferromagnetic powder is placed in a watch glass placed on two pole pieces which are sufficiently apart, then powder accumulates at the sides and shows depression in the middle because the field is strongest at poles.
10. When a ferromagnetic substance is liquefied, it loses ferromagnetic properties due to higher temperature.

Hysteresis

On removing the external magnetic field, a ferromagnetic material doesn’t get demagnetised fully. To bring the material back to zero magnetisation, a magnetic field in the opposite direction has to be applied. The property of ferromagnetic materials retaining magnetisation after the external field is removed. This is called hysteresis.

The magnetisation of the material measured in terms of magnetic flux density (B) when plotted against the external applied magnetic field intensity (H) will trace out a loop. This is called the hysteresis loop.

Retentivity is the magnetic flux density that remains when the magnetising force is reduced to zero.

Coercivity is the strength of the reverse magnetising field that must be applied to completely demagnetise the material.

Curie Temperature

Ferromagnetic property depends on temperature. At a high enough temperature, ferromagnetic substances become paramagnetic. The temperature at which this transition occurs is called Curie’s temperature. It is denoted by T_C.

Uses of Ferromagnetic Materials

There are wide applications of ferromagnetic materials in the industry. They are widely used in devices like electric motors, generators, transformers, telephones, loudspeakers, and magnetic stripes on the back of credit cards.