Best way to de-magnetize a permanent magnet?

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webgiant
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Best way to de-magnetize a permanent magnet?

Postby webgiant » Fri Oct 04, 2013 10:50 pm UTC

So I've got this class of youngsters, teenagers mostly, who need to get their hands on some ferrous torroidal (ring-shaped) cores to play around with a nifty voltage booster circuit called a "Joule Thief". Rather than just whip out the credit card and buy a bag of cores online, I've decided to add on More Science and have us use demagnetized ring-shaped permanent magnets as our ferrous cores. The other bonus is that we have local access to a lot of ring magnets.

So I'm wondering which science method to use, given that these are kids: (1) Hitting the magnets repeatedly with a hammer, first any direction and then lined up opposite to their poles, or (2) #1 plus heating up the magnets first. I've heard heating the magnets demagnetizes them easier, but I don't know how hot they need to get. If the temperature turns out to be too hot I'm probably going to get in trouble for being the coolest volunteer science guy ever.

Can anyone help me with this? The magnets are a mix of solid metal and ceramic-oxide ring magnets.

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cjameshuff
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Re: Best way to de-magnetize a permanent magnet?

Postby cjameshuff » Sat Oct 05, 2013 3:56 am UTC

You're likely to smash the magnets to bits before demagnetizing them with impacts. Heat will demagnetize them, but it will take a lot of heat to do a thorough job, and there's a fair risk of them fracturing or even exploding if they have trapped moisture.

Which is only a minor problem, because they are entirely the wrong kind of ferrite for a joule thief. The ferrites used in permanent magnets are selected for their ability to hold magnetization permanently, with high coercivity and remanence. You want a very magnetically soft ferrite that retains as little magnetism as possible, with very low coercivity and remanence.

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krogoth
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Re: Best way to de-magnetize a permanent magnet?

Postby krogoth » Sat Oct 05, 2013 12:06 pm UTC

Could you run a copper coil around it, run an electro magnet in the opposite direction of the magnet? idk. the heat proccess I would expect could be done in a metal class if they have an oxy welder, But I can't say about the safety, the metal teacher could assist some I expect.
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yurell
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Re: Best way to de-magnetize a permanent magnet?

Postby yurell » Sat Oct 05, 2013 12:19 pm UTC

Curie temperature of iron is only around 1 kK, and other common metals tend to be lower, so a gas stovetop should be sufficient to demagnetise (from what I can find, the flames can get up to about 1900 K).
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cjameshuff
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Re: Best way to de-magnetize a permanent magnet?

Postby cjameshuff » Sat Oct 05, 2013 1:48 pm UTC

yurell wrote:Curie temperature of iron is only around 1 kK, and other common metals tend to be lower, so a gas stovetop should be sufficient to demagnetise (from what I can find, the flames can get up to about 1900 K).


Iron ferrites have curie temperatures around 1000 K as well, so that's the right ballpark. However, that's visibly glowing red. A stovetop flame is hotter, but you'll have a hard time effectively heating the magnet to that temperature. And you'd probably want to drop it in vermiculite (a kind of fluffy expanded mica, good high-temperature insulator) or something to cool afterward, to avoid thermal shock...brittle ceramic, remember. If you really want to try this, maybe find someone with a pottery kiln.

But again, if it needs to be demagnetized, it's the wrong kind of ferrite, at the opposite end of the spectrum from what you want in terms of magnetic hardness. You want something that magnetizes and demagnetizes freely. You want it to magnetize easily from current in one coil so it induces a voltage in the other coil that pulls the transistor on harder, and then you want it to demagnetize just as easily when current in the first coil starts to drop to produce an opposite voltage in the other coil to pull the transistor hard off. The core needs to magnetize and demagnetize, you don't want it getting stuck partway...and the ferrite these are made of was designed specifically to "get stuck". It's probably possible to get it working, but it'll be more difficult to get oscillating and less efficient, with higher losses in the core.

You might try looking for ferrite EMI filters, particularly the clamshell ones you clamp onto a cord (like this). They're much closer to what you want in terms of magnetic properties, and you can open them up to quickly re-wind coils, they're easy to find, and I've successfully built Joule Thief style oscillators out of them. You could even take out the ferrite chunks and demonstrate the effect of separating the cores, switching the coil polarities, etc without disassembling the circuit.

stianhat
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Re: Best way to de-magnetize a permanent magnet?

Postby stianhat » Sun Oct 06, 2013 7:34 pm UTC

Well, its easier to subject it to an opposing magnetic force untill you reach the coercion point. Or subject it to an alternating magnetic field that is stronger than its coercion strength and increase the frequency untill you pass the speed in which the magnet dipoles cant switch fast enough.

In either case you will randomize the different magnetic dipoles. In most cases there will be some remaining magnitization, but for intents and purposes not very much.

It will depend on how hard or soft your magnet is, though, if it is really hard then it won't be easy any way. If its soft enough, you really dont need to demagnetize it.

Also possible to wait untill entropy does the job for you but it might be in the high 10^a large number years.


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