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 magnetization at the atomic level, even in the absence of an external magnetic field.

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

Table of Content

What is Ferromagnetism?

Ferromagnetism gets its name from the word ‘ferrous’ which means iron which 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 of the electrically uncharged materials attract each other strongly. Ferromagnetism is a property that considers not only the chemical make-up of a material but it also takes into account the microstructure and the crystalline structure.

Causes of Ferromagnetism

In a ferromagnetic material in the unmagnetized 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 magnetizing 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 external magnetic field these domains all align themselves in the direction of the applied field. In this way, the material is strongly magnetized in a direction parallel to the magnetizing field.Ferromagnetism

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 recognized as a magnetic substance. Magnetite has the greatest magnetism among all the natural minerals on earth.

Properties of Ferromagnetic Materials

  1. The atoms of ferromagnetic substances have permanent dipole moment 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 magnetizing field.
  4. The intensity of magnetization (M) is very large and positive and varies linearly with the magnetizing field (H). Hence saturation depends on the nature of the material.
  5. The magnetic susceptibility is very large and positive. Magnetic susceptibility Xm = M / H, where M is the intensity of magnetization 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 = 0 (H + M), where 0 is the magnetic permittivity of free space, H is the strength of the applied magnetic field and M is the intensity of magnetization.
  7. The relative permeability is also very large and varies linearly with the magnetizing field the field inside the material is much stronger than the magnetizing field. They tend to pull in a large number of lines of force by the material. Relative permeability of material r = 1 + m
  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 poles pieces which are sufficient apart then powder accumulates at 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 demagnetized fully. To bring the material back to zero magnetization, a magnetic field in the opposite direction has to be applied. The property of ferromagnetic materials retaining magnetization after the external field is removed is called hysteresis.

The magnetization 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 magnetizing force is reduced to zero.

Coercivity is the strength reverse magnetizing field that must be applied to completely demagnetize 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 TC.

Uses of Ferromagnetic Materials

There are wide applications of ferromagnetic materials in the industry. They are widely used in devices like an electric motor, generators, transformers, telephone, loudspeakers, magnetic stripe at the back of credit cards.

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