Ferromagnetic Materials

The ferromagnetic materials are those substances which exhibit strong magnetism in the same direction of the field, when a magnetic field is applied to it. First, we have to know what a domain is. It is actually a tiny area in ferromagnetic materials with a specific overall spin orientation due to quantum mechanical effect. This effect is really exchange interaction. That is; when we consider some unpaired electrons, they will interact with each other between two atoms and they line up themselves in a tiny region with the direction of magnetic field (Figure 1). This mechanism of the ferromagnetic material is ferromagnetism. It can be defined as some materials (cobalt, gadolinium, iron etc) will become permanent magnet with the use of magnetic field.

Properties of Ferromagnetic Materials

• When a rod of this material is placed in a magnetic field, it rapidly aligns itself in the track of the field.
• It is strongly attracted by the magnet.
• The ferromagnetism mechanism is not present in liquids and gases.
• The intensity of magnetization (M), magnetic susceptibility (χ_m), relative permeability (µ_r), and magnetic flux density (B) of this material will be always prominent and positive.

µ₀ → Magnetic permittivity of free space.
H → Applied peripheral magnetic field strength.

Hysteresis Loop

This loop is formed by changing the magnetizing force at the same time as measuring the magnetic flux of the material.
For understanding it, we will consider a ferromagnetic rod. It is placed in a solenoid and the current is given. We can see that when the current is increased, at first numerous domains line up with the field. On the dipoles of the domains which are not aligned, a torque is developed. When the majority dipoles line up with the field, then there is no more increase in M. Thus saturation is reached (figure 2).

Now, if the current is cut back to zero, the magnetization does not track the original curve. That is it lags behind the original curve. This is called hysteresis. The loop obtained as b-c-e-f-b is the hysteresis loop. It is shown below.

a-b → Initial magnetization, saturation at b
b-c → Demagnetization but M not equal to 0, when I = 0
c-d → Reversal of current direction, M not equal to 0 at d, some negative I
d-e → Saturation with all dipoles in reverse direction
At c and f, rod has permanent magnetization with I = 0.
Here; for understanding, we have plot the hysteresis curve as I verses M. But normally, it is a curve obtained by plotting B verses H. It is shown below.

Curie Temperature

There is a temperature, above which the ferromagnetic material will turn into paramagnetic material. This particular temperature is called as Curie temperature. That is, when we increase the temperature beyond the Curie temperature, it will cause the ferromagnetic materials to lose their magnetic property. It is represented by T_C. The magnetic ordering of the dipoles of the ferromagnetic material is interrupted by thermal energy.


k_B → Boltzmann constant
T → Temperature in Kelvin
C → Curie Constant
Curie temperature of some materials are shown below.

When compared with other magnetisms, ferromagnetism is the powerful one. But the materials are only few in numbers. They include cobalt, nickel, and iron. The alloys of these three metals, lodestone (mineral) and some compound of rare earth metals.
These materials have numerous applications in the field of electrical, magnetic storage and electromechanical devices. They are electromagnets, transformers, electric motors, tape recorder, generators etc.