Overview: Magnetic Classification of Substances

Some substances, when placed in a magnetic field, are feebly magnetised opposite to the direction of the magnetising field. When brought close to a pole of a powerful magnet, they are somewhat repelled away from the magnet. These substances are called 'diamagnetic' substances and their magnetism is called 'diamagnetism'.

Some substances when placed in a magnetic field, are feebly magnetised in the direction of the magnetising field. When brought close to a pole of a powerful magnet, they are attracted towards the magnet. These substances are called 'paramagnetic' substances and their magnetism is called 'paramagnetism'.

Some substances, when placed in a magnetic field are strongly magnetised in the direction of the magnetising field. They are attracted fast towards a magnet when brought close to either of the poles of the magnet. These substances are called 'ferromagnetic' substances and their magnetism is called 'ferromagnetism'.

When a piece of any substance is placed in an external magnetic field, the substance becomes magnetised. The magnetism so produced in the substance is called 'induced magnetism' and this phenomenon is called 'magnetic induction'.

The magnetic lines of force inside the magnetised bar are called 'magnetic lines of induction'.

The number of magnetic lines of induction inside a magnetised substance crossing unit area normal to their direction is called the magnitude of magnetic induction or magnetic flux density, inside the substance.

The intensity of magnetisation, or simply magnetisation of a magnetised substance represents the extent to which the substance is magnetised. It is defined as the magnetic moment per unit volume of the magnetised substance and is denoted by \[\vec M\].

Numerically, \[\vec M\] = \[\frac {\vec m}{V}\]

SI unit: (A m^-1)

The magnetic intensity \[\vec H\] is defined through the vector relation \[\vec H\] = \[\frac{\overrightarrow{B}}{\mu_{0}}-\overrightarrow{M}\], where \[\vec B\] is magnetic field induction inside the substance and \[\vec M\] is the intensity of magnetisation. μ₀ is permeability of free space.

It is defined as the ratio of the magnetic induction \[\vec B\] inside the magnetised substance to the magnetic intensity \[\vec H\] of the magnetising field.
Numerically, μ = \[\frac {B}{H}\]
SI unit: newton/ampere² (NA²), or tesla-metre/ampere (TmA^-1), or weber/ampere-metre (Wb A^-1m^-1).

The relative magnetic permeability of a substance is the ratio of the magnetic permeability u of the substance to the permeability of free space μo, that is,
μ_r = \[\frac{\mu}{\mu_0}\]

The relative permeability of a substance is defined as the ratio of the magnetic flux density B in the substance when placed in a magnetic field and the flux density B₀ in vacuum in the same field,

\[\mu_r=\frac{B}{B_0}\]

It may be defined as the ratio of the intensity of magnetisation to the magnetic intensity of the magnetising field,
\[\chi_m=\frac{M}{H}\]

The temperature above which a ferromagnetic substance becomes paramagnetic is called the 'Curie temperature' of the substance. The Curie temperature of iron is 770°C and that of nickel is 358°C.

The magnetisation remaining in the substance when the magnetising field is reduced to zero is called the "residual magnetism".

The retentivity of a substance is a measure of the magnetisation remaining in the substance when the magnetising field is removed.

The coercivity of a substance is a measure of the reverse magnetising field required to destroy the residual magnetism of the substance.

The energy lost per unit volume of a substance in a complete cycle of magnetisation is equal to the area of the hysteresis loop (M-H curve).

Statement

In 1895, Curie discovered experimentally that the magnetisation M (magnetic moment per unit volume) of a paramagnetic substance is directly proportional to the magnetic intensity H of the magnetising field and inversely proportional to the Kelvin temperature T, i.e., 

M = C (H/T)

where C is a constant called the Curie constant.

Explanation

The law states that magnetisation depends on both the applied magnetic field and temperature. It holds so long as the ratio H/T does not become too large. Magnetisation cannot increase indefinitely and approaches a maximum value corresponding to the complete alignment of all the atomic magnets in the substance.

Since magnetic susceptibility is defined as

χ_m = M/H,

eliminating M from the above equations gives

χ_m = C/T,

or

χ_m ∝ 1/T.

Conclusion

For paramagnetic substances, the magnetic susceptibility varies inversely with the absolute temperature, and this relation is known as Curie’s Law.

For ferromagnetic materials, the variation of X_m with T is very peculiar and follows Curie-Weis law, X_m = \[\left(\frac{C}{T-T_{c}}\right)\] according to which the variation of X_m at low
temperatures less than T_c is very complex but above it the variation becomes as simple as the paramagnetic susceptibility.

• Diamagnetic substances are weakly magnetised opposite to the applied magnetic field.
• In a magnetic field, a diamagnetic rod aligns perpendicular, while a paramagnetic and ferromagnetic rod aligns parallel to the field.
• In a non-uniform magnetic field, diamagnetic substances move from stronger to weaker regions, whereas paramagnetic substances move from weaker to stronger regions.
• Diamagnetic liquids are depressed in regions of strong magnetic fields, while paramagnetic liquids rise in regions of strong magnetic fields.
• Diamagnetic gases spread across the magnetic field, whereas paramagnetic gases spread along the field.
• Ferromagnetic substances exhibit strong magnetisation even in weak magnetic fields and have high permeability.
• Ferromagnetic substances exhibit magnetic hysteresis and lose their ferromagnetic nature above a certain temperature.

• The retentivity of soft iron is greater than that of steel.
• The coercivity of soft iron is less than the coercivity of steel.
• The hysteresis loss in soft iron is smaller than in steel because the area of its hysteresis loop is smaller.
• Steel has a larger hysteresis loop area than soft iron, indicating greater energy loss per cycle.
• The permeability of soft iron is greater than that of steel, as shown by the B ⁣− ⁣H curves.

• Permanent magnets need high retentivity and high coercivity, so steel is used.
• Electromagnets need high permeability and low retentivity, so soft iron is used.
• Transformer cores and telephone diaphragms must have low hysteresis loss to reduce heating.
• Soft iron and special alloys (permalloys, µ-metals) are preferred for efficient magnetic performance.