Question
How do we classify materials as diamagnetic, paramagnetic, or ferromagnetic? What causes each type, and how do their properties differ?
Solution — Step by Step
Cause: All electrons are paired. The material has no permanent magnetic dipole moment.
When placed in an external magnetic field, the orbital motion of electrons adjusts slightly to oppose the field (Lenz’s law at the atomic level). This creates a weak induced magnetisation opposite to the applied field.
Properties:
- Weakly repelled by a magnet
- Susceptibility is small and negative ()
- Independent of temperature
- Examples: Bi, Cu, Ag, Au, water, NaCl, diamond
Cause: Atoms have unpaired electrons, giving them a permanent magnetic dipole moment. But these dipoles are randomly oriented due to thermal agitation.
In an external field, the dipoles partially align with the field, creating a weak net magnetisation in the same direction as the field.
Properties:
- Weakly attracted by a magnet
- Susceptibility is small and positive ( to )
- Decreases with temperature: (Curie’s law)
- Examples: Al, Na, Ca, O, CuSO, liquid oxygen
where is the Curie constant.
Cause: Like paramagnets, atoms have unpaired electrons. But here, neighbouring atomic dipoles interact strongly through exchange interaction and align parallel within regions called domains.
Properties:
- Strongly attracted by a magnet
- Susceptibility is very large and positive ( to )
- Shows hysteresis (magnetisation depends on history)
- Above the Curie temperature (), becomes paramagnetic
- Examples: Fe (C), Co (C), Ni (C), Gd
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| Unpaired electrons | None | Present | Present |
| sign | Negative | Positive (small) | Positive (large) |
| Temperature dependence | None | ||
| In non-uniform field | Moves to weaker region | Moves to stronger region | Strongly to stronger region |
| Hysteresis | No | No | Yes |
flowchart TD
A["Magnetic Material Classification"] --> B{"Unpaired electrons?"}
B -->|"No"| C["Diamagnetic"]
B -->|"Yes"| D{"Strong exchange interaction between neighbours?"}
D -->|"No"| E["Paramagnetic"]
D -->|"Yes"| F["Ferromagnetic"]
C --> G["chi negative, repelled, temp independent"]
E --> H["chi small positive, attracted, chi proportional to 1/T"]
F --> I["chi large positive, strongly attracted, hysteresis, Curie temp"]Why This Works
The classification traces directly to electronic structure. Paired electrons have zero net magnetic moment (their spins cancel), leading to the weak induced effect of diamagnetism. Unpaired electrons provide permanent dipoles. Whether those dipoles interact strongly with neighbours (ferromagnetic) or act independently (paramagnetic) determines the strength of the magnetic response.
Temperature fights magnetic order — thermal energy randomises dipole orientations. This is why paramagnetism follows Curie’s law () and ferromagnets lose their special properties above the Curie temperature.
Alternative Method
For exam purposes, remember the mnemonic: “DiPaFe” = Dia-Para-Ferro in order of increasing magnetic strength. Diamagnetic materials are the “default” — every material shows diamagnetism, but it is overwhelmed if unpaired electrons exist (paramagnetic) or if domains form (ferromagnetic).
Common Mistake
Students confuse the Curie law (, for paramagnets) with the Curie-Weiss law (, for ferromagnets above ). The key difference is that ferromagnets have a characteristic Curie temperature where a phase transition occurs. Below they are ferromagnetic; above they obey the Curie-Weiss law. Paramagnets have no such transition. JEE Main 2023 had a graph-based question testing which law applies to which material.