Heating Magnet

What happens if you heat a magnet?
- anon
england

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 they're 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 won't have a net magnetized permanent magnet.

Mike W.

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.

Tom

(published on 10/22/2007)

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)
Harahan, LA

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:, 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.)

It's not hard to make materials (alloys, for example) with Curie temperatures right around room temperature or a little above. I've 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 don't absorb much more energy, avoiding some potential risks.

Mike W.

Lee H

(published on 10/22/2007)

Hi.does this mean that if the curie temperature isnt reached the magnet remains?
- Lasse Egelund (age 15)
denmark

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.

Mike W.

(published on 05/04/2009)

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.

Mike W.

(published on 05/16/2009)

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.

Mike W.

(published on 10/14/2011)

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.

Mike W.

(published on 12/01/2011)

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? Thank you.
- Abhilash Javaji (age 17)
Oxford

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.

Mike W.

(published on 09/08/2013)

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)
mankato mn

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!

Mike W.

(published on 02/27/2014)

Hi!I wonder, is this also true for a diamagnetic element such as pure bismuth? Since I can't seem to find the Curie temperature for that.
- Michael (age 28)
Sweden

No, the diamagnetism doesn't come from some sort of special ordered state, so it doesn't have a melting temperature.

Antiferromagnetism, like ferromagnetism, does melt at a specific temperature. The semantic convention is to call that the Neel temperature, not the Curie temperature.

Bismuth, however, is not antiferromagnetic. The magnetic properties of bismuth do change at the actual melting temperature where it turns liquid.

Mike W. 

(published on 06/28/2015)

Hi there, extremely interesting Q & As. Totally unfamiliar with magnet knowledge, Please let me know if you heat neodymium magnet at a cure temperature will it loose magnetism and when calling down will it be magnetic again? If yes can it be performed more then once. Many thanks Maritta
- Maritta Nemsadze (age 29)
United Kingdom

Yes, the neodymium magnet will lose its magnetism when heated above its Curie point. When you cool it back down, small domains will again become magnetized. Unless it's held in a strong field while it's cooling, however, the magnetic directions of those little domains will point all different directions. Their fields will mostly cancel, so it won't act like much of a magnet any more.

Mike W.

p.s. To be a little more technical about it, there's a reason that the material can't hold a memory of some sort about which way the magnetism should point when it re-forms. The choice of magnetic direction (say North vs. South) is switched if the direction of spins is reversed. That means the direction changes if you switch forward and backward in time. No material structure breaks forward-backward time symmetry, so no special shape or composition can pick the magnetic direction, For the electrical analog (ferroelectric) of a magnet, time-reversal does not change the field direction, so materials can be built that line up predictably on cooling.

(published on 10/27/2015)

All very interesting, thanks. this ha been slightly covered but i'm looking for slightly more practical advice. im a jewellery student tasked currently with designing brooches, and want to use magnets as an attachment method. the best way to attach the design to the magnet would be soldering, using a torch, not a soldering iron. How will the effect the magnet? if I put a stronger magnet next to it as it cools will it retain its magnetic properties? is the curie point of neodymium magnets high enough to melt solder? thanks.
- douglas craig (age 19)
glasgow scotland

There are some grades of neodymium magnets that are supposed to do well up to 200º C. () The Curie point of the magnet is much higher, but as we wrote up-thread,  the magnetism can mostly get lost well below the Curie point. 

Typical soft solders melt around 190º C. () So with a lot of care, you should be able to manage this. You might also consider usingy epoxy rather than solder.

Mike W.

(published on 12/03/2015)

I had a 1/2 inch diameter (3/8 inch thick) refrigerator magnet get stuck to the bottom of a small 2 cup capacity pot. The pot was placed on a standard stove coil burner and was used to boil water. The magnet is stuck to the bottom of the pot and we can't get it off. We tried sliding, hammer and chisel with no luck. It is messy with what appears to be black soot rub off on finger whenever touched. Can you provide suggestions on how to remove it?
- Rio (age 19)
Charlotte, NC, USA

I would suggest heating your pot. As also explained in  elevated temperatures induce demagnetization. This is because magnetism is caused by preferential alignment of spins in the material, and their orientations keep switching due to thermal motions. The energy barrier is easier to overcome at high temperatures, so magnetic strength gradually decays as you increase the temperature. The temperature at which magnetism is totally destroyed is material-dependent and can be quite high, but it may still worth.

Tunc

(published on 12/13/2015)

Fairly related to the above questions, I am trying to find out what happens to a magnet that is heated while still within an applied magnetic field. I understand that the coercivity and remanence both drop as you approach the Curie temperature, and magnets naturally have close to a relative permeability of 1. If I have a magnet, say within the field of an MRI, and I heat that magnet, will the force on the magnet drop to zero as well? I know the magnet will get re-magnetized once the heat is removed if the applied field remains, but if you could pull the magnet out of the MRI while heated, would it feel no force from the magnetic field? FYI for those reading, this situation is highly dangerous for any unsecured magnet larger than 5 mm, but illustrates my point. I actually use a research MRI to magnetize magnets I machine (easiest access I have to the high field necessary to magnetize my SmCo).
- Brock (age 30)
Philadelphia PA

The magnet still has high susceptibility above the Curie point, so it will have a large magnetic moment in the big field. The moment will always align to lower the energy, so the magnet will still be sucked in to the high-field regions.

Mike W.

(published on 08/10/2016)

i was wondering if the weight of iron changes when heated above curie temperature? i had a theory about gravity and magnetism 🤷�♂�
- Donald Sleightholme (age 35)
United Kingdom

Changes in the mass due to loss of magnetization have never been detected. In principle, special relativity says that if temperature (rather tha energy) is held constant as a material demagnetizes, then the change of magnetic energy should cause a mass change  following m=E/c². The mass change for iron would be about one part in 10¹⁶.

Mike W.

(published on 04/27/2018)