NCERT · Class 12 · Physics · Magnetism and MatterDifferentiate between diamagnetic, paramagnetic, and ferromagnetic substances on the basis of their magnetic properties, susceptibility, and effect of temperature.

Introduction\nSubstances exhibit different magnetic behaviors when placed in an external magnetic field due to the arrangement and motion of their constituent electrons. Based on their response, magnetic materials are broadly classified into three categories: Diamagnetic, Paramagnetic, and Ferromagnetic substances.

1. Diamagnetic Substances

• Behavior in Magnetic Field: Diamagnetic substances are weakly repelled by a magnet. When placed in a non-uniform magnetic field, they tend to move from stronger to weaker parts of the field.
• Magnetic Susceptibility ($\chi$): Their magnetic susceptibility is small and negative (e.g., $\chi = -10^{-5}$). It is independent of temperature.
• Relative Permeability ($\mu_r$): Slightly less than 1 ($0 < \mu_r < 1$).
• Origin: This property arises due to orbital motion of electrons creating induced magnetic moments that oppose the external field (Lenz's law at atomic level).
• Examples: Bismuth, copper, water, gold, and lead.

2. Paramagnetic Substances

• Behavior in Magnetic Field: Paramagnetic substances are weakly attracted by a magnet. They tend to move from weaker to stronger parts of a non-uniform magnetic field.
• Magnetic Susceptibility ($\chi$): Their magnetic susceptibility is small and positive (e.g., $\chi = +10^{-4}$).
• Effect of Temperature: According to Curie's Law, susceptibility is inversely proportional to absolute temperature ($\chi \propto \frac{1}{T}$). As temperature increases, thermal agitation disrupts alignment, decreasing susceptibility.
• Relative Permeability ($\mu_r$): Slightly greater than 1 ($\mu_r > 1$).
• Origin: Atoms or molecules possess a net permanent magnetic dipole moment due to uncancelled electron spins.
• Examples: Aluminum, sodium, calcium, and oxygen.

3. Ferromagnetic Substances

• Behavior in Magnetic Field: Ferromagnetic substances are strongly attracted by a magnet and get strongly magnetized in the direction of the external field.
• Magnetic Susceptibility ($\chi$): Their magnetic susceptibility is very large and positive ($\chi \gg 1$).
• Effect of Temperature: Above a specific temperature known as the Curie temperature ($T_C$), ferromagnetic substances become paramagnetic (Curie-Weiss Law: $\chi \propto \frac{1}{T - T_C}$). Below $T_C$, they exist in microscopic regions called domains.
• Relative Permeability ($\mu_r$): Very high, ranging from hundreds to thousands ($\mu_r \gg 1$).
• Origin: Strong exchange coupling between adjacent atomic magnetic moments forces them to align parallel in domains.
• Examples: Iron, cobalt, nickel, and various alloys likealnico.