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.
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 10/22/2007)
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.)
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.
(published on 10/22/2007)
(published on 05/04/2009)
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(published on 12/01/2011)
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 09/08/2013)
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 02/27/2014)
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.
(published on 06/28/2015)
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.
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)