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What happens if you heat a magnet?
As you heat a magnet you supply it with more thermal energy, so the individual electron spins (like tiny magnets themselves) become more likely to be in high-energy states, pointing oppositely to their neighbors. That means that theyíre less lined up so the total magnetism is reduced. At some point, in between the weakening of the overall magnetism and the availability of extra thermal energy, it becomes easy for domain walls- the boundaries between regions that are lined up pointing different directions- to slide around. Then the domains will rearrange so that they reduce the large-scale field energy by pointing different directions. That means that your permanent magnet is no longer overall magnetized. As you heat further, individual spins within domains become more likely to point opposite to their neighbors, and that reduces the average alignment seen by their neighbors too, reducing the effect which favors their having lined up in the first place. At a well-defined temperature, called the Curie temperature, the whole tendency to align into domains collapses, and the material ceases to be a ferromagnet at all. Cooling the material will cause magnetic domains to form again at the Curie temperature, but unless an external field is applied as the material cools, the domains will point all different directions, so you wonít have a net magnetized permanent magnet.
Heat a magnet even more and it'll go through another phase transition from order to disorder -- it will melt, and heat it more, it will vaporize.
(published on 09/25/06)
Follow-Up #1: heating magnets
Does the Curie Temperature change depending on the type of metal a magnet is constructed out of? If so, what is the average Curie Temperature for common magnets? Would the Curie Temperature(s) be attainable in normal weather conditions? Obviously "normal" temperatures for me will be different from "normal" temperatures for you. By nomal I mean, oh, say 80 degrees to about 90 degrees.
- Justin (age 12)
Nice questions. Curie temperatures have an enormous range, from far below room temperature to far above it. Obviously for permanent magnets we choose materials with high Curie temperatures. There's a nice table of some common Curie temperatures in Wikipedia:http://en.wikipedia.org/wiki/Ferromagnetism
, A lot of the ones for materials used in magnets are above 700 K, not typical weather on earth! (The Farenheit temperatures you mention are around 300 K.)
Itís not hard to make materials (alloys, for example) with Curie temperatures right around room temperature or a little above. Iíve heard there are even some schemes to use those materials in medicine. Energy can be dumped into magnetic beads by changing magnetic fields. If the beads are bound to special sites (cancer cells?) they can help kill the nearby cells.However, if the beads get too hot, they cease to be magnetic and donít absorb much more energy, avoiding some potential risks.
(published on 03/03/07)
Follow-Up #2: heating magnets
Hi.does this mean that if the curie temperature isnt reached the magnet remains?
- Lasse Egelund (age 15)
Little regions (domains) would remain partially magnetized. However, a little below the Curie temperature it becomes much easier for their magnetic directions to change. Then even if the magnet is cooled back down, it will have lost most of its net magnetism, because the domains now point in nearly random directions.
(published on 05/05/09)
Follow-Up #3: cooling magnets
So, if a permanent magnet was cooled, would it have a stronger magnetic force? Or would it remain the same?
- Eric (age 12)
Littleton, CO, USA
Usually the strength of a permanent magnet does grow slightly as it is cooled. For ordinary permanent magnets, which lose their magnetism only well above room temperature, the effect is quite small.
(published on 05/18/09)
Follow-Up #4: reducing magnetism with heat
I just baked bead magnets at 275 for 15 minutes. The magnetism was greatly reduced. :( Is this normal for store-bought magnets?
- Virginia (age 31)
College Station, TX, US
That sounds very normal. As the magnets get hot, the domain walls come loose, letting the magnetism rotate. They end up much less aligned than they were to begin with.
(published on 10/16/11)
Follow-Up #5: cooling magnets
I need to do research and no trustworthy place is showing how cold affect a magnet. Does it increase it or decrease and how much? Thanks
- Brandon (age 14)
Manchester, MO, USA
I've marked this as a follow-up to an answer that was admittedly hard to find.
(published on 12/01/11)
Follow-Up #6: heating electromagnet cores
The set of questions and answers here have been very interesting.
I wanted to know if all these characteristics of the magnet losing its magnetic power when heated, and of the domains re-arranging in opposite direction, apply to the core of an electromagnet.
I have been trying to understand if heating the core of an electromagnet can help increase the saturation point and therefore enable us to produce electromagnets with smaller cores.
In short, can heating a ferromagnetic substance increase the time it takes to get saturated with flux?
- Abhilash Javaji (age 17)
Generally speaking, heating the magnet core will slightly weaken the magnetism in the domains and will make it easier for domain walls to move around. That will not increase the saturation magnetization. In fact, for ordinary field strengths the "saturation magnetization" is simply what you get when all the domains are aligned, with the applied field doing very little to increase the magnetization within each domain. So heating the core up will reduce the effective saturation magnetization.
The warmer core may be better in one regard. Since the domain walls are less stuck, it will have lower remnant magnetization, the magnetization left-over as a memory of the previously applied fields. The response to small fields may be smoother, faster, and more linear. If your goal is to reach very large fields, however, you don't want a warm core.
(published on 09/08/13)
Follow-Up #7: heated iron losing magnetism
I watched a video on the restoration of a blacksmith anvil. He heated it till a magnet would not stick anymore. Is that true? What tempeture? Why?
- matt (age 31)
Yes, iron ceases to be strongly magnetic (i.e.is not a ferromagnet) when heated to 1043 K, called the Curie temperature. It doesn't melt until it reaches 1811 K. So there's a range where it's not melted but heated enough to be non-magnetic where it's softened some. The magnet test is pretty cute!
(published on 03/01/14)
Follow-up on this answer.