Magnetisation: Diamagnetism, Paramagnetism and Ferromagnetism
Matter in a magnetic field acquires a magnetisation of its own, weakly opposing the field, weakly aligning with it, or aligning strongly and staying that way. Ferromagnetic order comes from an interaction between neighbouring moments and disappears above the Curie temperature.
What a learner can do afterwards
- Sorts materials by how their magnetisation responds to an applied field and gives the microscopic reason for each class
- Explains why an iron core multiplies the field of a solenoid while a copper one does almost nothing
- Reads a hysteresis loop and says what its area and its width cost in a transformer core
1 · Read
Sort matter by what its atomic moments do in a field. Diamagnets have no permanent moments, so the field induces a weak opposing response and they are gently repelled. Every material shows a trace of this. Paramagnets carry permanent but randomly aimed moments that partly line up, giving weak attraction that fades as heat jostles the alignment. Ferromagnets couple neighbouring moments into domains that snap into line together.
Slip an iron core into a solenoid and the field leaps. The core domains align with the coil and add their own field on top, multiplying the response thousands of times. Copper shows almost no gain since it lacks that cooperative alignment. This is why motors, transformers, and lifting magnets build coils around iron or steel. Switch the current and an electromagnet field follows; a permanent magnet keeps its domains frozen.
You can magnetise a nail by stroking it one way with a magnet. Each stroke drags more domains into line, and lifting the magnet at the end of each stroke avoids undoing the work. Stroking back and forth at random does far less. Count the paper clips the nail now lifts, then drop or heat it and watch the strength fade as disorder scrambles the alignment.
Ferromagnetic order dies above the Curie temperature, where heat wins and iron goes paramagnetic. Each trip around a hysteresis loop costs energy as heat, and the loop area measures that cost per cycle. Its width sets the field needed to demagnetise. Transformer cores flip constantly, so they use narrow loops that waste little.
Diamagnets weakly oppose, paramagnets weakly align, and ferromagnets lock domains together until heat past the Curie point scrambles them.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.