At What Temperature Does a Magnet Lose Its Magnetism?

A magnet loses its magnetism when it is heated above its Curie temperature, which is fixed for each material: about 770 °C (1,418 °F) for iron, 354 °C (669 °F) for nickel, and 1,115 °C (2,039 °F) for cobalt. Room temperature is far below those numbers, and OpenStax calls the value for iron "well above room temperature."

1. What the Curie temperature is

A ferromagnetic material is made of small regions called domains. Inside a domain the atoms are aligned like tiny bar magnets. In a permanent magnet, most domains point the same way.

OpenStax College Physics explains that a permanent magnet can be demagnetized by hard blows or by heating it in the absence of another magnet, because increased thermal motion at higher temperature can disrupt and randomize the orientation and size of the domains. There is a well-defined temperature for each ferromagnetic material, the Curie temperature, above which it cannot be magnetized.

The how magnets work post introduces the Curie point in one line. This post puts numbers on it.

2. Curie temperatures of the ferromagnetic elements

HyperPhysics, a physics reference from the Georgia State University physics department, lists Curie temperatures in kelvins. Subtracting 273.15 converts them to Celsius, and multiplying by 1.8 and adding 32 gives Fahrenheit.

OpenStax gives the same figure for iron: 1043 K, or 770 °C. NDE-Ed.org, a nondestructive testing teaching site from Iowa State University, gives 770 °C (1390 °F) for low carbon steel. Low carbon steel is an alloy, so its number is not identical to pure iron's 1,418 °F in the table above.

The bottom two rows show how much the value varies. Gadolinium's Curie temperature is about 20 °C, close to room temperature, and dysprosium's is far below freezing. OpenStax notes that several elements and alloys have Curie temperatures much lower than room temperature and are ferromagnetic only below those temperatures.

3. How far is room temperature from the limit?

Take room temperature as 20 °C, or 293 K. Then:

• Iron's Curie temperature is 1,043 / 293 = about 3.6 times room temperature on the kelvin scale, or 750 °C above it (770 - 20).
• Nickel's is 627 / 293 = about 2.1 times, or 334 °C above room temperature (354 - 20).
• Cobalt's is 1,388 / 293 = about 4.7 times, or 1,095 °C above (1,115 - 20).

Nickel has the smallest margin of the three, and even that is more than 300 degrees Celsius.

4. What happens above the Curie temperature

NDE-Ed.org states that heating a material above its Curie temperature disrupts the domain alignment most effectively. When steel is heated above that point it loses its magnetic properties, and when it cools back down it contains no residual magnetic field. So the magnetism does not come back by itself. To restore it, the piece has to be magnetized again, and OpenStax says an induced magnetization can be made permanent if the material is heated and then cooled, or simply tapped, in the presence of other magnets.

5. Heat is not the only way

The same sources name two other routes:

1. Hard blows. OpenStax lists hard blows alongside heating as a way to demagnetize a permanent magnet.
2. A reversing, decreasing magnetic field. NDE-Ed.org says that heating above the Curie temperature is often inconvenient, so a common alternative is to subject the part to a reversing and decreasing magnetic field. That returns the dipoles to a nearly random orientation. In practice this means pulling the part out and away from a coil carrying AC, or using an electromagnetic yoke set to AC. The result is a nearly unmagnetized state, not a perfectly zero one.

6. What these sources do not tell you

The figures above are for pure elements and one steel. Commercial permanent magnets, such as neodymium or ceramic types, are alloys or compounds, and the sources opened for this post do not give their Curie temperatures or the lower temperature at which a particular magnet starts to weaken. If a magnet will be used near an engine or oven, look for the maker's stated maximum operating temperature instead of assuming the Curie numbers apply.

7. Key takeaways

• A magnet loses its magnetism above its Curie temperature, which depends on the material.
• Curie temperatures: iron 770 °C, nickel 354 °C, cobalt 1,115 °C, gadolinium about 20 °C, dysprosium -188 °C.
• Above the Curie point the steel in NDE-Ed.org's example does not regain a field on cooling; it has to be magnetized again.
• Hard blows and a decreasing alternating field also demagnetize a magnet, with no need for extreme heat.
• Element values are not ratings for commercial magnets; check the manufacturer's maximum operating temperature.