Paramagnetic Materials

Class 12 Physics • Magnetism and Matter

Interactive Simulation

Observe how atomic magnetic dipoles respond when an external magnetic field is applied to a paramagnetic material.

65%
60%
Magnetic Field: OFF — Dipoles are randomly oriented
Your browser does not support HTML Canvas.
Magnetic Dipole External Magnet Net Magnetisation
Field OFF

Dipoles are randomly oriented because of thermal motion.

Field ON

Dipoles show partial alignment with the applied magnetic field.

Temperature Effect

Higher temperature produces stronger thermal agitation and reduces alignment.

1. What are Paramagnetic Materials?

Paramagnetic materials are those materials which are weakly attracted by an external magnetic field.

These materials generally contain atoms or ions having one or more unpaired electrons.

An unpaired electron possesses a magnetic moment due to its spin and orbital motion. Therefore, the atoms or ions can behave approximately like tiny magnetic dipoles.

Key Idea: Paramagnetic materials are weakly attracted towards a region of stronger magnetic field.

2. Origin of Paramagnetism

The main reason for paramagnetism is the presence of unpaired electrons.

  • Electrons possess intrinsic spin angular momentum.
  • A spinning electron has a magnetic moment.
  • Unpaired electrons therefore produce a net atomic magnetic moment.
  • These atomic magnetic moments interact with an external magnetic field.

Thus, an atom containing unpaired electrons can be considered as a small magnetic dipole.

3. In the Absence of External Magnetic Field

When no external magnetic field is applied, the magnetic dipoles are oriented approximately in random directions because of thermal motion.

Net magnetic moment ≈ 0

Therefore, the material does not show appreciable magnetisation in the absence of an external field.

4. When an External Magnetic Field is Applied

When an external magnetic field is applied, the magnetic dipoles tend to orient themselves in the direction of the applied field.

However, complete alignment does not occur because thermal agitation continuously tries to randomise their orientations.

Applied Magnetic Field → Partial Alignment of Dipoles

As a result, the material develops a small positive magnetisation and is weakly attracted towards the stronger magnetic field.

5. When the Magnetic Field is Removed

When the external magnetic field is removed, the aligning effect of the field disappears.

Thermal motion again makes the dipoles randomly oriented. Therefore, paramagnetic materials generally do not retain strong permanent magnetisation.

Important: Paramagnetic materials show induced magnetisation in an external magnetic field but generally do not behave as permanent magnets.

6. Magnetic Properties

Magnetic Susceptibility

For a paramagnetic material, magnetic susceptibility is positive.

χm > 0

For a linear magnetic material:

M = χm H

where:

  • M = Magnetisation
  • H = Magnetising field
  • χm = Magnetic susceptibility

Magnetic Field Relation

B = μ0(H + M)

Using M = χmH:

B = μ0(1 + χm)H

Therefore:

μr = 1 + χm

For paramagnetic materials:

μr > 1

But μr is only slightly greater than 1.

7. Curie's Law

For many paramagnetic materials, magnetic susceptibility follows Curie's law.

χm = C / T

where:

  • C = Curie constant
  • T = Absolute temperature in kelvin

Hence:

χm ∝ 1/T

Therefore, when temperature increases, paramagnetism generally decreases.

Higher Temperature → More Thermal Agitation → Less Alignment

8. Examples of Paramagnetic Materials

  • Aluminium (Al)
  • Platinum (Pt)
  • Oxygen (O2)
  • Manganese salts
  • Chromium compounds

9. Important Characteristics

Property Paramagnetic Material
Response to magnetic field Weak attraction
Unpaired electrons Generally present
Magnetic susceptibility Positive
Relative permeability Slightly greater than 1
Permanent magnetisation Generally absent
Temperature effect Paramagnetism generally decreases as temperature increases

10. Quick Revision

  • Paramagnetic materials are weakly attracted by an external magnetic field.
  • They generally contain unpaired electrons.
  • Unpaired electrons produce atomic magnetic moments.
  • Without an external field, dipoles are approximately randomly oriented.
  • In an external field, dipoles become partially aligned.
  • Magnetic susceptibility is positive.
  • Relative permeability is slightly greater than 1.
  • For many paramagnetic materials, χm = C/T.
  • Increasing temperature generally decreases paramagnetism.
Physics Advisor • Class 12 Physics