Classifying magnetic materials
When a material is placed in an external field of intensity $H$, it acquires a magnetisation $M$, the dipole moment per unit volume. For a linear material these are related by $M = \chi H$, where $\chi$ is the dimensionless magnetic susceptibility. The susceptibility is the single number that decides how a substance responds, and its sign and magnitude define the three classes recognised in NCERT §5.5: diamagnetic, paramagnetic and ferromagnetic.
The susceptibility is tied to the relative permeability through $\mu_r = 1 + \chi$, and the permeability of the material is $\mu = \mu_0\,\mu_r$. A negative $\chi$ therefore gives $\mu_r < 1$ and $\mu < \mu_0$; a small positive $\chi$ gives $\mu_r$ slightly greater than one; and a very large $\chi$ gives $\mu \gg \mu_0$. Table 5.2 of NCERT collects these ranges, reproduced below.
| Property | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| Susceptibility $\chi$ | −1 ≤ χ < 0 |
0 < χ < ε |
χ ≫ 1 |
| Relative permeability $\mu_r$ | 0 ≤ μr < 1 |
1 < μr < 1+ε |
μr ≫ 1 |
| Permeability $\mu$ vs $\mu_0$ | μ < μ₀ |
μ > μ₀ |
μ ≫ μ₀ |
| Field inside material | Reduced | Slightly enhanced | Highly concentrated |
Here $\varepsilon$ is a small positive number used to quantify the weak paramagnetic response. The structure of the table is itself a NEET favourite: the 2024 paper asked candidates to match each class to its $\chi$ band, with the non-magnetic limit being $\chi = 0$. Reading the field behaviour from $\chi$ is the goal of the rest of this note.
Diamagnetism
Diamagnetic substances have a tendency to move from the stronger to the weaker part of an external magnetic field. Where a magnet attracts iron, it would repel a diamagnetic substance. Placed in a uniform field, the field lines are partly expelled and the field inside the material is slightly reduced — typically by about one part in $10^5$. In a non-uniform field the sample drifts from high field towards low field.
The microscopic cause lies in electron orbits. The orbiting electrons act as tiny current loops carrying orbital magnetic moments, but in a diamagnetic atom these moments sum to zero. When a field is switched on, electrons whose moments point with the field slow down while those opposing it speed up, by induced currents obeying Lenz's law. The net effect is a small moment opposite to the applied field, which is why $M$ points against $H$ and $\chi$ comes out negative, of order $-10^{-5}$.
Field lines are pushed out of a diamagnetic bar but crowd into a paramagnetic bar, mirroring NCERT Fig. 5.7.
Common diamagnets include bismuth, copper, lead, silicon, nitrogen (at STP), water and sodium chloride. The most extreme case is a superconductor: cooled below its transition temperature, it expels the field completely, with $\chi = -1$ and $\mu_r = 0$. This perfect diamagnetism is the Meissner effect, and it underlies magnetically levitated trains. Critically, diamagnetism is universal — it is present in every material, but it is so feeble that any paramagnetic or ferromagnetic response masks it entirely.
Paramagnetism and Curie's law
Paramagnetic substances become weakly magnetised in the direction of an applied field and are weakly attracted to a magnet, moving from a weak-field region towards a strong-field region. Each atom, ion or molecule carries a permanent magnetic dipole moment, but ceaseless thermal motion keeps these moments randomly oriented, so there is no net magnetisation in zero field. The field lines concentrate slightly inside the bar, enhancing the internal field by about one part in $10^5$.
Applying a strong field at low temperature aligns the dipoles along $B_0$. As NCERT states, increasing the field or lowering the temperature raises the magnetisation until it reaches a saturation value where every dipole points along the field. This competition between field-driven alignment and thermal disordering is captured by Curie's law, in which the magnetisation is proportional to the applied field and inversely proportional to the absolute temperature:
$$M = C\,\frac{B_0}{T}, \qquad \chi \propto \frac{1}{T}$$$C$ is the Curie constant of the material. A higher temperature randomises the dipoles, so the susceptibility falls as $T$ rises.
Examples are aluminium, sodium, calcium, oxygen (at STP) and copper chloride. For these materials both $\chi$ and $\mu_r$ depend not only on the substance but also, through Curie's law, on the sample temperature — a dependence that does not exist for diamagnets.
A paramagnetic salt has susceptibility $\chi = 3.0 \times 10^{-3}$ at $300\ \text{K}$. Estimate $\chi$ at $150\ \text{K}$, assuming Curie's law and an unchanged field.
By Curie's law $\chi \propto 1/T$, so $\chi_2 = \chi_1 \dfrac{T_1}{T_2} = 3.0\times10^{-3}\times\dfrac{300}{150} = 6.0\times10^{-3}$. Halving the temperature doubles the susceptibility, because thermal disordering is weaker.
Temperature dependence: diamagnetic vs paramagnetic
A miniscule difference in $\chi$ — about $-10^{-5}$ versus $+10^{-5}$ — produces radically different behaviour. The trap is the temperature response: a diamagnetic susceptibility is essentially independent of temperature, whereas a paramagnetic susceptibility obeys Curie's law and falls as $T$ rises. Candidates often wrongly apply Curie's law to diamagnets.
Diamagnetic: $\chi < 0$, $T$-independent. Paramagnetic: $\chi > 0$, $\chi \propto 1/T$.
The relation $M = \chi H$ used throughout this note is developed in Magnetisation and Magnetic Intensity.
Ferromagnetism and domains
Ferromagnetic substances become strongly magnetised in a field and are strongly attracted to a magnet. Their atoms carry permanent dipole moments as in a paramagnet, but here the moments interact and spontaneously align over a macroscopic volume called a domain. A typical domain is about $1\ \text{mm}$ across and contains roughly $10^{11}$ atoms, each domain fully magnetised in some direction.
With no external field, the domains point randomly, their moments cancel, and the bulk magnetisation is zero. When a field $B_0$ is applied, domains aligned with $B_0$ grow at the expense of others and the domain orientations rotate toward the field. Under a strong field the sample behaves as a single giant domain — the saturated state. The elements iron, cobalt, nickel and gadolinium are ferromagnetic, with relative permeability exceeding 1000.
Domains start randomly oriented (NCERT Fig. 5.8a) and amalgamate into one aligned giant domain in a strong field (Fig. 5.8b).
Hysteresis, soft and hard magnets
Because domain alignment is not fully reversible, when the external field is removed some ferromagnets retain their magnetisation. As the field is cycled, the magnetisation lags behind it, tracing a closed loop — the hysteresis loop. The magnetisation remaining at zero applied field is the retentivity, and the reverse field needed to drive it back to zero is the coercivity.
A ferromagnet's M–H curve forms a hysteresis loop; the enclosed area is the energy dissipated per cycle.
The width of the loop separates the two technological families. In hard ferromagnets the magnetisation persists after the field is removed, giving a wide loop and a permanent magnet; alnico (iron, aluminium, nickel, cobalt, copper) and naturally occurring lodestone are examples used in compass needles. In soft ferromagnets the magnetisation disappears once the field is removed, giving a thin loop; soft iron is the standard example, ideal for electromagnet cores and transformers.
Comparison and temperature behaviour
Ferromagnetism is itself temperature-limited. Above a critical Curie temperature $T_c$ the thermal energy disrupts the domain structure, the spontaneous alignment is destroyed, and the ferromagnet becomes an ordinary paramagnet. NCERT notes this loss of magnetisation is gradual; the NIOS supplement records $T_c \approx 1043\ \text{K}$ for iron. The three classes are contrasted below.
| Feature | Diamagnetic | Paramagnetic | Ferromagnetic |
|---|---|---|---|
| Typical $\chi$ | $\approx -10^{-5}$ | $\approx +10^{-5}$ | $\gg 1$ (hundreds–thousands) |
| Atomic moment | Zero (induced only) | Permanent, randomly aligned | Permanent, domain-aligned |
| Behaviour in field | Weakly repelled | Weakly attracted | Strongly attracted |
| Temperature dependence | Essentially none | $\chi \propto 1/T$ (Curie) | Para above $T_c$ |
| Examples | Bi, Cu, Pb, water, NaCl | Al, Na, Ca, O₂, CuCl₂ | Fe, Co, Ni, Gd |
Only diamagnets have negative susceptibility
The 2016 NEET item asked which class has negative susceptibility; the answer is diamagnetic material only. Both paramagnetic and ferromagnetic materials have positive $\chi$. Do not be tempted to call a ferromagnet's $\chi$ negative simply because its loop dips below the axis — that dip is the reverse field, not the susceptibility sign.
$\chi < 0$ ⇒ diamagnetic. $\chi > 0$ small ⇒ paramagnetic. $\chi \gg 1$ ⇒ ferromagnetic.
Five lines before the exam
- Susceptibility sorts materials: $-1 \le \chi < 0$ diamagnetic, $0 < \chi < \varepsilon$ paramagnetic, $\chi \gg 1$ ferromagnetic; $\mu_r = 1 + \chi$.
- Diamagnets have zero atomic moment, develop an induced moment opposing the field (Lenz), are repelled, and are $T$-independent; diamagnetism is universal.
- Paramagnets have permanent moments; they obey Curie's law $M = C\,B_0/T$, so $\chi \propto 1/T$, and saturate when fully aligned.
- Ferromagnets align in domains, give huge $\chi$, show hysteresis (retentivity, coercivity), and turn paramagnetic above the Curie temperature ($T_c \approx 1043\ \text{K}$ for iron).
- Hard ferromagnets (alnico, lodestone) keep magnetisation as permanent magnets; soft ferromagnets (soft iron) lose it for cores and electromagnets.