medium difficulty10 min

Diamagnetic, Paramagnetic and Ferromagnetic Materials

Diamagnetic, paramagnetic, and ferromagnetic materials are three classes of magnetic materials distinguished by the direction and strength of magnetisation produced when they are placed in an external magnetic field.

In one line: Dia disagrees with the field, para partly follows it, ferro follows strongly.

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Understand it

Diamagnetic materials develop weak magnetisation opposite to the applied magnetic field, so they are weakly repelled.
Their susceptibility is small and negative, and relative permeability is slightly less than 1.
Paramagnetic materials develop weak magnetisation in the direction of the applied field, so they are weakly attracted.
Their susceptibility is small and positive, and relative permeability is slightly greater than 1.
Ferromagnetic materials show very strong magnetisation in the direction of the applied field due to domain alignment.
Their susceptibility is large and positive, and relative permeability is much greater than 1.

Example

Bismuth and copper are diamagnetic, aluminium and oxygen are paramagnetic, and iron, cobalt, and nickel are ferromagnetic. A soft iron core is used in electromagnets because it becomes strongly magnetised in an applied field.

Formula and rule

Qualitative rules: diamagnetic materials have chi < 0 and μ_r slightly less than 1; paramagnetic materials have chi > 0 and μ_r slightly greater than 1; ferromagnetic materials have large positive chi and μ_r much greater than 1.

For school-level linear comparison, μ_r = 1 + chi is often used as a guide, but ferromagnetic materials can show non-linear behaviour and hysteresis.

See it worked out

Follow the example line by line, then use the same method on new questions.

Worked example

Question: Classify three materials with susceptibilities -1.5 × 10⁻⁵, 2.2 × 10⁻⁵, and very large positive value.

1Answer: The material with negative susceptibility is diamagnetic.
2The material with small positive susceptibility is paramagnetic.
3The material with very large positive susceptibility is ferromagnetic.
4Interpretation: the sign gives direction of magnetisation, and the magnitude gives strength of response.

Method you can reuse

To answer a comparison question: define the basis of classification; write the sign of chi; state whether μ_r is less than, slightly greater than, or much greater than 1; describe attraction or repulsion; mention magnetisation direction; give examples.

Visual tip

A comparison diagram should show three specimens placed in a magnetic field. Diamagnetic response has induced magnetisation opposite to B, paramagnetic response has weak magnetisation along B, and ferromagnetic response has strong domain alignment along B.

Check yourself

Answer the question in your head before opening the answer.

Try it yourself

How can susceptibility help distinguish diamagnetic and paramagnetic materials?

Think first, then show answer

Diamagnetic materials have small negative susceptibility, so their magnetisation is opposite to the applied field. Paramagnetic materials have small positive susceptibility, so their magnetisation is in the direction of the applied field.

Common wrong answer

A common wrong answer is saying diamagnetic materials are not affected by magnetic fields. They are affected weakly and develop magnetisation opposite to the applied field.

Don't mix this up

Students often write that paramagnetic and ferromagnetic materials are the same because both are attracted by a magnet. The strength and cause of attraction are very different: paramagnetism is weak, while ferromagnetism is strong due to magnetic domains.

Exam tip

In comparison questions, include susceptibility sign, relative permeability, field behaviour, and one example for each type. This gives a complete scoring answer.

Write it in the exam

Each point is worth about one mark. Write point 1 for a 1-mark question and keep adding points as the marks go up.

How CBSE asks it

This concept is frequently asked as a table, reason-based classification, assertion-reason question, or application question on why soft iron is used in electromagnets and why diamagnetic materials are weakly repelled.

1
Enough for 1 mark
Diamagnetic, paramagnetic, and ferromagnetic materials are three classes of magnetic materials distinguished by the direction and strength of magnetisation produced when they are placed in an external magnetic field.
2
Add for 2 marks
Example: Bismuth and copper are diamagnetic, aluminium and oxygen are paramagnetic, and iron, cobalt, and nickel are ferromagnetic. A soft iron core is used in electromagnets because it becomes strongly magnetised in an applied field.
3
Add for 3 marks
Diamagnetic materials develop weak magnetisation opposite to the applied magnetic field, so they are weakly repelled.

Practice

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