The everyday magnet, explained
A paramagnet only acts magnetic while you hold a field on it. But a fridge magnet stays magnetic with nothing pushing it — pull it off, it is still a magnet tomorrow. That is a completely different and far stronger phenomenon called ferromagnetism (named after *ferrum*, Latin for iron). The whole difference comes down to one question: do the tiny atomic compasses agree with each other on their own, with no outside field telling them to?
In a ferromagnet, they do. Vast numbers of atomic magnetic moments spontaneously line up parallel to one another, all pointing the same way without being told. Add up trillions of little arrows pointing in unison and you get a powerful net magnet — strong enough to lift nails and snap onto a steel door. This self-made alignment is called spontaneous magnetization, and it is the secret of every permanent magnet.
The hidden force that makes spins cooperate
Here is the puzzle. Two ordinary bar magnets actually prefer to point *opposite* ways when laid side by side — try it with two fridge magnets. So why would atomic magnets choose to line up the *same* way? The ordinary magnetic force between them is far too weak to do it against the constant battering of heat. Something much stronger must be at work.
That something is the exchange interaction. It is purely quantum-mechanical and has no everyday counterpart, so be honest with yourself: it is genuinely strange. It comes from two deep rules. First, electrons are forbidden from being in exactly the same state (the Pauli exclusion principle). Second, electrons repel each other electrically. Put those together and a material can lower its energy when neighbouring electrons keep their spins pointing the same way, because that arrangement keeps them politely apart.
When the exchange interaction favours parallel spins, you get a ferromagnet. Iron, cobalt, and nickel are the famous trio at room temperature. In most other materials exchange is either too weak or actually prefers *anti*-alignment — which is exactly the story of the next guide.
Heat wins in the end: the Curie temperature
Exchange forces order; heat fights it. As you warm a ferromagnet, thermal motion jostles the spins harder and harder, knocking some out of step. The magnetism weakens. At a sharp, special temperature it collapses entirely: the spins can no longer hold their agreement, the order melts away, and the material becomes an ordinary paramagnet. That tipping point is the Curie temperature, named for Pierre Curie.
This vanishing of order at a precise temperature is a genuine phase transition, every bit as real as ice melting into water. Above the Curie temperature the magnetism is simply gone; cool back below it and the spontaneous magnetization returns. For iron the Curie temperature is a glowing 770 degrees Celsius, which is why your fridge magnets are perfectly safe in a warm kitchen.
A rule for the warm region: Curie-Weiss
Even above the Curie temperature, where the order is gone, the exchange interaction has not vanished — it is just losing to heat. Its lingering influence shows up in how the material responds to a field. The susceptibility no longer follows the simple paramagnet rule; instead it obeys the Curie-Weiss law. In plain terms, the law says the material gets dramatically *more* responsive as you cool it toward the Curie point, as if the spins are straining to order and just need the temperature to drop.
susceptibility ≈ C / (T − Θ) T = temperature now Θ = a temperature near the Curie point C = a constant for the material As T falls toward Θ, the bottom shrinks → the response blows up.
This is wonderfully useful in the lab. By measuring susceptibility across a range of temperatures and seeing where the Curie-Weiss curve points, a scientist can read off the strength and even the *sign* of the exchange interaction — long before the material actually orders. The shape of one curve tells you what kind of magnet you are holding.
Putting it together
A real magnet is a democracy of atomic compasses that have all agreed to point the same way. The agreement is enforced by the quantum exchange interaction, paid for by electrical energy, and it survives only as long as heat stays below the Curie temperature. Cool, ordered, and aligned: that is a ferromagnet. Warm it past its Curie point and it forgets, dissolving into an ordinary paramagnet — until you let it cool and remember again.
We assumed all along that the exchange interaction wants neighbours to point the *same* way. But that was a choice, not a law — the quantum bookkeeping can just as easily favour the *opposite*. When it does, the spins still order beautifully, but into an alternating pattern that hides its magnetism from the world. That surprising possibility is exactly where the next guide begins.