MHT-CET Physics · Magnetic Materials

Dia-, Para- and Ferromagnets, Hysteresis and Shielding

Diamagnets have a small negative susceptibility that does not depend on temperature, paramagnets a small positive one that falls as 1/T (Curie's law), and ferromagnets a huge one that disappears above the Curie temperature; only ferromagnets show hysteresis, and soft ferromagnets are used for shielding and electromagnets.

All Magnetic Materials notes MHT-CET Physics strategy

Why this matters

10 PYQs, none HARD. Four classify materials — the sign of χ, a χ–T graph, Curie's law, and a ferromagnet above its Curie temperature. Six are the hysteresis loop and its uses: retentivity and coercivity on a B–H curve, material for electromagnets, and what surrounds an instrument to shield it. Two cards.

Concept 1 of 2: The Three Classes and Curie's Law

Diamagnets are weakly repelled: χ is small and negative and, on a χ–T graph, a flat line just below zero. Paramagnets are weakly attracted: χ is small and positive and falls with temperature as C/T, so M = CB/T. Ferromagnets have aligned domains and an enormous χ; heated above the Curie temperature their domains become random and they behave as paramagnets.

Definition

Diamagnetic: χ small, negative, independent of T.
Paramagnetic: χ small, positive; Curie's law χ = C/T, so M = CB/T.
Ferromagnetic: χ very large; above the Curie temperature the domains become random (paramagnetic).

Class	Susceptibility	Temperature
Diamagnetic	Small, negative	Independent of T
Paramagnetic	Small, positive	χ = C/T (Curie's law)
Ferromagnetic	Very large, positive	Becomes paramagnetic above the Curie temperature

Practice this conceptself-check · 1 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 2nd May Shift 2 · Q1Easy

Example 1 · Magnetic Materials · Classification, Curie's Law, Hysteresis, and Shielding

A ferromagnetic material is heated above its curie temperature. The correct statement from the following is that

Letting a diamagnet's χ change with temperature

Diamagnetism comes from induced orbital moments and does not depend on temperature; the χ–T graph is a flat line below zero. Only paramagnets follow Curie's law.

Writing Curie's law upside down

Magnetisation grows with the applied field and falls with temperature: M = CB/T, so C = MT/B.

Drill 3 more on the three classes and curie's law

Concept 2 of 2: Hysteresis, Electromagnets and Shielding

Only ferromagnets show hysteresis: their B lags behind H round a loop. When H falls to zero, the B that remains is the retentivity; the reverse H needed to bring B to zero is the coercivity. Soft iron has a high retentivity but a low coercivity, so it is easy to magnetise and demagnetise — right for electromagnets. A permanent magnet needs both high. A soft ferromagnet around an instrument draws the field lines into itself and shields the inside.

Definition

Retentivity: B at H = 0 (point on the B-axis). Coercivity: H at B = 0 (point on the H-axis).
Hysteresis: ferromagnets only.
Electromagnets: high retentivity, low coercivity (soft iron). Permanent magnets: both high.
Shielding: surround with a soft ferromagnetic material (soft iron).

Use	Retentivity	Coercivity
Electromagnet core (soft iron)	High	Low
Permanent magnet (steel, alnico)	High	High
Magnetic shield	—	Soft ferromagnet, high permeability

Practice this conceptself-check · 1 quick reps

The same idea in a real exam question:

MHT-CET · 2023 · 10th May Shift 1 · Q38Moderate

Example 2 · Magnetic Materials · Classification, Curie's Law, Hysteresis, and Shielding

In the following graph of flux density versus magnetizing force, coercivity and retentivity are respectively represented by the points

Swapping retentivity and coercivity on the loop

Retentivity is where the loop crosses the B-axis (H = 0); coercivity where it crosses the H-axis (B = 0).

Shielding with a diamagnet

A diamagnet repels field lines only weakly. Shielding needs a soft ferromagnet, whose high permeability carries the field around the protected space.

Drill 5 more on hysteresis, electromagnets and shielding

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Reference tables (2)

The Three Classes and Curie's Law3 rows

Class	Susceptibility	Temperature
Diamagnetic	Small, negative	Independent of T
Paramagnetic	Small, positive	χ = C/T (Curie's law)
Ferromagnetic	Very large, positive	Becomes paramagnetic above the Curie temperature

Hysteresis, Electromagnets and Shielding3 rows

Use	Retentivity	Coercivity
Electromagnet core (soft iron)	High	Low
Permanent magnet (steel, alnico)	High	High
Magnetic shield	—	Soft ferromagnet, high permeability

Watch out for (4)

Letting a diamagnet's χ change with temperature
→ The Three Classes and Curie's Law
Writing Curie's law upside down
→ The Three Classes and Curie's Law
Swapping retentivity and coercivity on the loop
→ Hysteresis, Electromagnets and Shielding
Shielding with a diamagnet
→ Hysteresis, Electromagnets and Shielding

Test yourself on Magnetic Materials

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