Ferromagnetic Materials

Ferromagnetic materials strongly magnetize in the direction of an applied magnetic field. A key concept here is a domain, which is a small area within these materials where electrons align due to an exchange interaction—essentially, unpaired electrons between atoms aligning with the magnetic field. This process, known as ferromagnetism, allows certain materials like cobalt, iron, and gadolinium to become permanent magnets.

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

The hysteresis loop is created by varying the magnetizing force while simultaneously measuring the material’s magnetic flux.
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.
For clarity, we’ve plotted the hysteresis curve as current (I) versus magnetization (M), although it is typically shown as magnetic flux density (B) versus applied magnetic field strength (H).

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.

Compared to other types of magnetism, ferromagnetism is the strongest, though only a few materials, including cobalt, nickel, and iron, and their alloys like lodestone and some rare earth metal compounds, exhibit it.
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.