A magnet that lifts a handful of paper clips at room temperature may lift fewer when it is hot. That small change points to a contest inside the material: interactions between electrons favour magnetic order, while heat makes that order harder to maintain. Heat does not simply “switch off” magnetism at one universal temperature. What happens depends on the material, how hot it gets and whether the change survives cooling.
What gives a permanent magnet its strength?
Electrons have a magnetic property called spin. In a ferromagnetic material, interactions between neighbouring electrons favour a shared orientation. It is tempting to picture each atom as a tiny compass needle, although that picture hides the quantum mechanics responsible for the alignment. Groups of aligned magnetic moments form domains. If the domains in a whole object point in different directions, their effects can largely cancel. In a magnetised object, enough of them contribute in the same direction to produce a useful external field.¹,²
A permanent magnet also needs to keep that alignment after the magnetising field has gone. Its composition and internal structure resist a return to a less ordered arrangement. This resistance to demagnetisation, called coercivity, helps explain why two magnets can respond very differently to the same heat. A magnet's pull is therefore not just a property of its constituent atoms: it reflects how their moments are arranged and how securely that arrangement is held.³
Imagine two crowds instructed to face the same way. In one, people can turn easily; in the other, barriers make changing direction difficult. The barriers are a useful analogy for a magnet's resistance to reorientation. They are not literal walls around electrons, and heat affects more than domain boundaries, but the comparison captures why a magnet's existing order may survive one temperature and fail at another.
Heat competes with order
Temperature measures, in part, how much thermal energy is available to jostle a material's microscopic state. As a ferromagnet warms, thermal fluctuations increasingly compete with the interactions that align its magnetic moments. Its ordered magnetisation generally falls. Close to a particular temperature, the fluctuations become strong enough that the material can no longer sustain spontaneous ferromagnetic order. That boundary is its Curie temperature.²,⁴
For iron the Curie temperature is about 770°C. That does not mean an iron magnet keeps exactly the same strength until it reaches 769°C and abruptly fails at 770°C. The magnetisation changes with temperature on the way there. Nor is 770°C a rule for every magnetic material: composition changes the temperature of the transition.¹,⁴
Above its Curie temperature, a ferromagnet becomes paramagnetic. An applied magnetic field can still produce a response, but the material no longer maintains the same self-sustaining magnetic order. This is why saying a hot magnet “has no magnetism at all” is too sweeping. The important loss is the organised, persistent magnetisation that made it useful as a permanent magnet.¹,⁴
Does it become strong again when it cools?
There are two separate questions: how strong is the magnet while hot, and how strong is it after it returns to room temperature? Confusing them makes heat damage seem simpler than it is.
A modest rise in temperature can reduce magnetic output reversibly. Once the magnet cools, its original output returns. Magnet makers describe this with a reversible temperature coefficient: an estimate of how much a particular property changes per degree within a specified range. These coefficients differ across materials, so “magnets lose one per cent per ten degrees” would be a poor general rule.³
If heating pushes a magnet beyond the conditions in which its alignment remains stable, some of its magnetisation can be lost irreversibly. Cooling alone then does not restore the former strength. In many cases the material can still be remagnetised with a sufficiently strong external field; that is different from physical damage to its structure. Manufacturer guidance distinguishes reversible loss, irreversible loss that remagnetisation can recover, and loss caused by structural change.³
The Curie point is therefore not the same as a safe operating limit. Standard grades of neodymium magnets, for example, are commonly given a maximum operating temperature of about 80°C, while their Curie temperature is roughly 310°C. The operating figure is a guide, not a guarantee for every shape or installation. Some neodymium grades tolerate higher temperatures. A magnet can be permanently weakened well before it reaches the temperature at which ferromagnetic order disappears altogether.⁵
Consider a magnet that seems weaker after sitting in a hot device. Measuring it only while hot would not tell you whether it has been damaged. Letting it return to its original temperature separates the reversible part of the change from any strength it has actually lost. Even then, a change in the pull you observe is not always a direct measurement of the material's magnetisation: the distance to the steel surface, the magnet's shape and the magnetic circuit also affect the result.⁵
Why the particular magnet matters
A fridge magnet, a small neodymium disc and a specialised motor magnet may all attract steel, but they do not share a single thermal specification. Their magnetic materials differ, and the same material can be made in grades designed for different levels of coercivity. In a motor or measuring instrument, engineers must consider both the magnet's behaviour while operating and the possibility that a heat excursion will leave a lasting change.³,⁵
The geometry and surroundings matter too. A thin magnet in open space can face a different demagnetising condition from one incorporated into a well-designed magnetic assembly. This is why a listed maximum operating temperature should be treated as a starting point for design, with performance checked under the conditions in which the magnet will actually work.⁵
The short answer is that heating gives disorder more opportunity to compete with magnetic alignment. Sometimes the effect is only a temporary dip in output. At higher temperatures, the magnet's stored alignment may be partly lost; at the Curie temperature, the material itself can no longer sustain ferromagnetic order. Cooling reverses the first effect, but it does not automatically undo the second.