Properties of Ferromagnetic Materials

When ferromagnetic materials are exposed to a magnetic field, they become strongly magnetised in the direction of the field. Each atom of ferromagnetic material has a permanent magnetic dipole moment before an external magnetic field is applied. In the absence of an external field, the domains may randomly synchronise, resulting in a magnetic dipole moment of zero in any direction. The magnetic dipole moment increases in the presence of an external field due to domain boundary displacement and domain rotation.

Properties of ferromagnetic materials

• The dipole moment of atoms is permanent and exists in domains.

• The alignment of atomic dipoles and external magnetic fields are in the same direction.

• There is a significant strength of magnetic dipole moment.

• The magnetisation intensity varies linearly with the magnetising field. Moreover, the magnetisation intensity is quite high and positive.

• Magnetic susceptibility is relatively high and positive. 

• The magnetic flux density is also high and positive. The magnetic field lines inside ferromagnetic materials are dense. 

• The relative permeability is also relatively high. It varies linearly with the magnetic field. The magnetic field inside the material is substantially more vital than outside the material. They tend to pull in many force lines from the material.

• The field aggressively attracts ferromagnetic materials. They tend to adhere to the poles where the field is more potent in a non-uniform field.

• Because the field is more vital at poles, if the ferromagnetic powder is placed in a watch glass between two properly faraway poles, powder accumulates on the sides, and the centre is depressed.

• At high temperatures, a ferromagnetic substance loses its ferromagnetic characteristics.

• Crystalline solids exhibit ferromagnetism.

Magnetic hysteresis

Ferromagnetic materials mainly show magnetic hysteresis. B grows non-linearly with H, when placed in an external magnetic field for magnetisation. If H is set to zero again, it declines along path ab. This curve is known as the hysteresis curve because it lags behind B with H. (Hysteresis is the lag between B and H.)

Causes of hysteresis

When an external magnetising field (H = 0) is withdrawn, the magnetic moment of some domains stays aligned in the preceding magnetising field’s applied direction, resulting in residual magnetism.

Residual magnetism = Br ☰ retentivity ☰ remanence 

When a magnetic field is removed from a ferromagnetic specimen, its retentivity is the amount of magnetic field left in the sample.

Coercivity 

The coercivity of a ferromagnetic specimen is the amount of magnetising field required to eliminate the residual magnetism.

Ferromagnetic materials

Hysteresis loss

• The energy loss per cycle per unit volume equals the area of the hysteresis loop. Area of hysteresis loop is B.dH = ∮B.dH = μ₀∮I.dH
• It’s worth varies depending on the substance.
• The hysteresis loop area is equal to the work done every cycle per unit volume of material.

* total energy material loss = Wₕ = VA Joule = VA/J calorie,

I.e. Wₕ = volume of material x area of hysteresis curve.

Conclusion

When put in a magnetic field, ferromagnetic substances become strongly magnetised in the direction of the magnetising field. The magnetic dipole moment is considerable and is orientated in the direction of the magnetising field. Ferromagnetism can be found in crystalline materials. Hysteresis is the term for when B lags behind H. When a magnetic field gets removed from a ferromagnetic specimen, the quantity of magnetic field that remains in the sample is called retentivity. The amount of magnetising field required to remove residual magnetism from a ferromagnetic specimen is coercivity.