Heat a magnet past its Curie point and it stops working

A magnet stops being a magnet when it is heated past its Curie temperature, the point at which heat scrambles the atomic alignment behind ferromagnetism: about 770 °C for iron. The threshold is named after Pierre Curie, who showed that a substance’s magnetic properties change at a particular temperature.

In iron, nickel and cobalt, each atom behaves like a tiny compass needle. A quantum-mechanical interaction locks neighbouring atoms into parallel order across regions called domains, from about a tenth of a millimetre to a few millimetres across. Heat works against that order. At iron’s Curie point, around 1,043 K, the thermal energy is about 0.135 electronvolts, more than three times the roughly 0.04 eV available at room temperature, and enough to break the long-range alignment. The metal turns paramagnetic, only weakly magnetic.

The threshold runs from a warm room to white heat. Gadolinium gives up at about 293 K (20 °C), so it can lose its ferromagnetism on a warm day. Neodymium-iron-boron, a common permanent-magnet alloy, lasts to about 320 °C, nickel to about 358 °C and iron to about 770 °C, while cobalt holds out above 1,100 °C.

Cooling back below the Curie point restores ferromagnetic order, but the domains settle in random directions, so an overheated permanent magnet stays largely demagnetised until it is magnetised again.

Nature runs the same threshold in reverse. As new ocean crust forms at mid-ocean ridges and cools below about 700 °C, Earth’s magnetic field at that moment is imprinted on the rock. The slowly moving plates act, in NOAA’s words, as “a kind of tape recorder” of the strength and direction of past fields, which is how geologists have reconstructed the history of Earth’s magnetic field.