Effect of temperature on magnetic properties
Heat jumbles atomic magnets. Paramagnets get weaker as they warm, a ferromagnet turns paramagnetic when heated enough, and diamagnets hardly change.
Why heat weakens magnetism
Heat makes atoms jiggle randomly. This thermal agitation knocks atomic magnetic moments out of line. In a paramagnet, the applied field tries to line the permanent moments up while heat scatters them. So the hotter the sample, the smaller its magnetisation and its susceptibility. Cooling has the opposite effect. At low temperature and in a strong field, nearly all the moments line up, and M reaches its largest value, called saturation. In a diamagnet, the moments are induced by the field; they are not permanent moments waiting to be lined up. So heat hardly affects them, and χ is almost independent of temperature.
What happens as a ferromagnet is heated
- 1At room temperature, iron has domains, with the moments lined up inside each domain.
- 2As it is heated, thermal agitation grows and the alignment inside the domains weakens. The magnetisation falls gradually, not all at once.
- 3★At a high enough temperature, the domain structure breaks up completely.
- 4★Above that temperature, the material behaves as a paramagnet: it is only weakly attracted by a magnet.
- 5On cooling, domains form again and the material is ferromagnetic once more.
★ key step
How heating affects each class
| Class | Effect of heating | Reason |
|---|---|---|
| Diamagnetic | Hardly any change | Its moments are induced by the field, not permanent moments that need lining up |
| Paramagnetic | Magnetisation falls as the temperature rises | Thermal agitation scatters the permanent atomic moments |
| Ferromagnetic | Magnetisation falls gradually; heated enough, it becomes paramagnetic | Thermal agitation breaks up the domains |
Activity: watch an iron nail lose its pull (teacher demonstration)
This is done by the teacher, with care, as the nail gets red hot.
- 1Hang a small iron nail from a stand by a thin wire. Place a strong magnet beside it, so that the nail is pulled to one side but does not touch the magnet.
- 2Heat the nail with a burner flame until it glows bright red.
- 3Observe: the nail swings away from the magnet and hangs straight down.
- 4Remove the flame and let the nail cool. Observe: it swings back towards the magnet.
- 5Result: when strongly heated, iron loses its ferromagnetism and behaves as a paramagnet, so the magnet's pull becomes very weak. On cooling, it is ferromagnetic again.
Keep magnets and speakers away from the gas stove: strong heating weakens a permanent magnet. Many electric rice cookers switch off by themselves: a magnetic part in the base stops being strongly magnetic once it gets just above 100 °C, after the water is used up.
2 marks: “How does the magnetisation of a paramagnetic substance change when it is heated? Give the reason.” It decreases. Heating increases the thermal agitation of the atoms, which upsets the alignment of their permanent magnetic moments with the field. Cooling has the opposite effect, up to saturation.
1 marks: “What happens to a ferromagnetic substance when it is heated to a high enough temperature?” Its domains break up and it becomes paramagnetic. The loss of magnetisation is gradual.
2 marks: “Why is the susceptibility of a diamagnetic substance almost independent of temperature, while that of a paramagnetic substance falls as temperature rises?” Diamagnetic moments are induced by the field, not lined-up permanent dipoles, so heat has little effect. In a paramagnet, thermal agitation upsets the alignment of permanent dipoles, so χ falls as T rises.
Hot magnets forget. Para: weaker when warmer. Ferro: heated enough, it turns para. Dia: hardly cares.