NCERT · Class 12 · Physics · Magnetism and MatterClassify magnetic materials into diamagnetic, paramagnetic, and ferromagnetic substances on the basis of their magnetic properties. Give at least three distinct properties and two examples for each.

Introduction to Magnetic Classification\nBased on their response to an external magnetic field and their internal atomic structure, magnetic materials are broadly classified into three categories: Diamagnetic, Paramagnetic, and Ferromagnetic substances.

1. Diamagnetic Substances\nDiamagnetic materials are those substances which are feebly repelled by a magnet and tend to move from stronger to weaker parts of a magnetic field.

• Properties:
  • Magnetic susceptibility ($\chi_m$) is small and negative.
  • Relative permeability ($\mu_r$) is slightly less than 1 ($0 \le \mu_r < 1$).
  • They do not obey Curie's Law; their magnetic susceptibility is independent of temperature.
• Examples: Bismuth, Copper, Water, Gold.

2. Paramagnetic Substances\nParamagnetic materials are those substances which are feebly attracted by a magnet and tend to move from weaker to stronger parts of a magnetic field.

• Properties:
  • Magnetic susceptibility ($\chi_m$) is small and positive.
  • Relative permeability ($\mu_r$) is slightly greater than 1 ($\mu_r > 1$).
  • They obey Curie's Law: Magnetic susceptibility is inversely proportional to absolute temperature ($\chi_m \propto 1/T$).
• Examples: Aluminum, Platinum, Sodium, Oxygen.

3. Ferromagnetic Substances\nFerromagnetic materials are those substances which are strongly attracted by a magnet and get strongly magnetized in the direction of the applied field.

• Properties:
  • Magnetic susceptibility ($\chi_m$) is very large and positive.
  • Relative permeability ($\mu_r$) is very large (much greater than 1).
  • They obey Curie-Weiss Law at temperatures above the Curie temperature ($T_c$), where $\chi_m \propto \frac{1}{T - T_c}$.
• Examples: Iron, Cobalt, Nickel, Alnico.

Summary\nUnderstanding these distinctions is crucial for applications ranging from transformer cores to permanent magnets and magnetic shielding.