Question
Classify magnetic materials into diamagnetic, paramagnetic, and ferromagnetic. Explain the origin of each type and give examples. What happens to each when placed in an external magnetic field?
(CBSE Class 12 / JEE Main / NEET pattern)
Solution — Step by Step
flowchart TD
A["Magnetic Materials"] --> B{"Unpaired\nelectrons?"}
B -->|"No"| C["Diamagnetic\n(weakly repelled)\nCu, Bi, H₂O, NaCl"]
B -->|"Yes"| D{"Domains\npresent?"}
D -->|"No"| E["Paramagnetic\n(weakly attracted)\nAl, Na, O₂, CuSO₄"]
D -->|"Yes"| F["Ferromagnetic\n(strongly attracted)\nFe, Co, Ni, CrO₂"]
F --> G["Below Curie temp:\nferromagnetic"]
F --> H["Above Curie temp:\nbecomes paramagnetic"]Origin: All electrons are paired. When an external field is applied, the orbital motion of electrons creates a tiny induced magnetic moment that opposes the applied field (Lenz’s law at the atomic level).
Behaviour: Weakly repelled by the field. Susceptibility \chi < 0 (small negative value). Does not retain magnetism.
Examples: Cu, Bi, Ag, Au, Pb, H₂O, NaCl, diamond.
Origin: Atoms have unpaired electrons, giving each atom a permanent magnetic dipole moment. But these dipoles are randomly oriented, so the net magnetism is zero without a field.
Behaviour: In an external field, dipoles partially align → weakly attracted. Susceptibility (small positive). Alignment increases at lower temperatures (Curie’s law: ). No retention.
Examples: Al, Na, O₂, Mn, CuSO₄, FeCl₃.
Origin: Atoms have unpaired electrons AND a special exchange interaction between neighbouring atoms creates domains — regions where all dipoles are spontaneously aligned in the same direction. Each domain acts as a strong magnet.
Behaviour: In a field, domains aligned with the field grow at the expense of others → very strong attraction. Susceptibility (large positive, ~10³ to 10⁵). Retains magnetism after the field is removed (permanent magnets).
Examples: Fe, Co, Ni, CrO₂ (used in cassette tapes), Fe₃O₄ (lodestone).
Above the Curie temperature (), thermal energy destroys the domain structure, and the material becomes paramagnetic. For iron, .
Why This Works
The classification is fundamentally about electron configuration and inter-atomic interactions. Diamagnetism is universal (present in all materials) but very weak. Paramagnetism is stronger and requires unpaired electrons. Ferromagnetism is the strongest and requires both unpaired electrons and the quantum mechanical exchange interaction that aligns neighbouring dipoles spontaneously.
The key distinguishing test in a lab: a diamagnetic rod aligns perpendicular to the field (repelled from strong field regions), while paramagnetic and ferromagnetic rods align parallel (attracted to strong field regions).
Alternative Method — Using Susceptibility and Permeability
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| (susceptibility) | Small, negative | Small, positive | Large, positive |
| (relative permeability) | Slightly < 1 | Slightly > 1 | Much >> 1 |
| Temperature dependence | Almost none | Complex (domains) | |
| Hysteresis | No | No | Yes |
For NEET MCQs, the most tested fact: “Which of the following is diamagnetic?” Check for paired electrons. All noble gases, Cu (… wait, Cu has one unpaired — it is actually paramagnetic as an atom, but bulk Cu metal is diamagnetic due to band structure). Safest approach: memorise the standard examples for each category rather than deriving from electron configuration for metals.
Common Mistake
Students confuse paramagnetic and ferromagnetic by saying “both are attracted to magnets, so they are the same.” The crucial difference: paramagnetic materials lose their magnetism immediately when the external field is removed. Ferromagnetic materials retain magnetism (hysteresis). Also, ferromagnetism exists only below the Curie temperature — above it, the material is merely paramagnetic. This temperature distinction is a favourite MCQ trap.