Demo #13: The Curie Temperature

However, this cannot be the case. Iron is a ferromagnetic material. This means that, when placed within a magnetic field, the electron spins of the iron atoms tend to align, allowing the material to develop a magnetic field of its own. When these materials are heated, their electrons are excited such that, above a certain temperature, their electron spins can no longer align; they are no longer ferromagnetic. This temperature is called the Curie temperature, and for iron it is about 700 degrees C.

This temperature is reached at about 20 km depth in the Earth, whereas at the outer boundary of the Core (about 2900 km depth) the temperature is about 2000 degees C. This is well above the Curie temperature, so although the core is largely iron, that iron is no longer ferromagnetic, and the Earth's core cannot be a permanent magnet.

To illustrate the Curie temperature, use a strip of Nickel. Nickel is a ferromagnetic material with a Curie temperature of 358 degees C, which is easily achieved by heating with a torch or Bunsen burner. Suspend the strip of Nickel by a wire attached to a ring stand or other appropriate holder. Nearby suspend a permanent magnet so that the Nickel and magnet can attach to each other with a sort-or "V" shape of their wires. Now heat the Nickel. Before long, as the temperature exceeds the Curie temperature, it releases itself from the magnet and swings away. Carefully (it will still be hot) re-attach the Nickel and magnet, since as the Nickel cools below the Curie temperature it once again becomes ferromagnetic and is attracted to the magnet. This cycle of heating and cooling, with the Nickel releasing and re-attaching to the magnet, can be repeated as many times as needed.