Ferromagnetic Effect

FERROMAGNETIC EFFECT

Certain metals like iron (Fe), cobalt (Co), nickel (Ni) and certain alloys exhibit high degree of magnetisation.

These materials show the spontaneous magnetization i.e., they have magnetisation (atomic magnetic moments are aligned) even in the absence of an external magnetic field.

This indicates that there is a strong internal field within the material which makes the atomic magnetic moments align with each other.

This phenomenon is known as ferromagnetism

EXCHANGE INTERACTION AND FERROMAGNETISM

The ferromagnetic property is exhibited by transition elements such as iron, cobalt, and nickel at room temperature and rare earth elements like gadolinium and dysprosium.

The ferromagnetic materials possess parallel alignment of dipoles. This parallel alignment of dipoles is not due to the magnetic force existing between any two dipoles. The reason is that the magnetic potential energy is very small and it is smaller than thermal energy.

The electronic configuration of iron is . For iron, the 3d subshell is an unfilled one. This 3d subshell have five orbitals.

For iron, the six electrons present in the 3d subshell occupy the orbitals such that there are four unpaired electrons and two paired electrons as shown in figure 2.28.

These four unpaired electrons contribute a magnetic moment of 4ẞ. This arrangement shows the parallel alignment of four unpaired electrons.

The parallel alignment of dipoles in iron is not due to the magnetic interaction. It is due to the Pauli's exclusion principle and electrostatic interaction energy.

The Pauli's exclusion principle and electrostatic are combined together and constitute  a new kind of interaction known as exchange interaction. The exchange interaction is a a quantum mechanical concept.

The exchange interaction between any two atoms depends upon the interatomic separation between the two interacting atoms and the relative spins of the two outer electrons. The exchange interaction between any two atoms is given by

where J_e is the numerical value of the exchange integral, S₁ and S₂ are the spin angular momenta of the first and second electrons.

The exchage integral value is negative for a number of elements. Therefore, the exchange energy value is negative (minimum energy energy configuration) when the spin angular momentum S₁ and S₂ are opposite direction.

Hence, antiparallel alignment of dipole is favoured. This explains the antiparallel ben alignment of dipoles in antiferromagnetic materials.

In some materials like iron, cobalt and nickel the exchange integral value is positive. The exchange energy is negative when the spin angular momentum is in the same direction. This will produce a parallel alignment of dipoles.

A plot between the exchange integral and the ratio of the interatomic separation to the radius of 3_d orbital (r/r_d) is shown in figure 2.29.

For the transition metals like iron, cobalt, nickel and gadolinium the exchange integral is positive, whereas for manganese and chromium the exchange integral is negative.

The positive value of the exchange integral represents the material as ferromagnetic and the negative exchange integral value represents the material as antiferromagnetic.

In general, if the ratio, r/r_d >3, the material is ferromagnetic, otherwise the material is antiferromagnetic. It should be noted that manganese is suitably alloyed so that r/r_d >3, and it will become ferromagnetic.

Ferromagnetic materials

The materials which exhibit the ferromagnetism are 18lug called ferromagnetic materials.

Properties

i. All the dipoles are aligned parallel to each other due to the magnetic interaction between the dipoles.

ii. They have ey have permanent dipole pole moment. They are strongly attracted by the magnetic field.

iii. They exhibit magnetisation even in the absence of magnetic field. This property of ferromagnetic materials is called as spontaneous magnetisation.

iv. They exhibit hysteresis (lagging of magnetisation with applied magnetic field).

v. On heating, they lose their magnetisation slowly.

vi. The dipole alignment is as shown in fig. 2.30

Fig. 2.30 Dipole alignment for ferromagnetic materials

vii. The magnetic susceptibility is very high and it depends on temperature.

It is given by

where C is Curie constant and 0 ferromagnetic Curie temperature.