Can I weaken a neodymium super-magnet?

February 10, 2013 8:37 PM

I got a bunch of neodymium super-magnets to put up heavy stuff on the side of my refrigerator and hold music to a steel music stand. I made the mistake of ordering the maximum-strength (N50) ones, which are so strong I can't handle them, even in the small size (1/4" x 1/2").

Is there a way to reduce the strength to the "standard" of around N35? Perhaps heating, freezing, pounding them with a hammer on an anvil?

I also have a high-capacity demangnitizer made to erase VHF video cassettes. Will that work?

Suggestions will be appreciated.

posted by KRS to technology (19 answers total)

Pounding on an anvil will break it and you will find that neodymium is powdery on the inside. Once you get the powder on something, since it is charged, it is almost impossible to remove.
posted by Nanukthedog at 8:39 PM on February 10 [3 favorites]

How about wrapping it in tape? Keep adding layers until you've reduced the strength to the desired level.
posted by alms at 8:41 PM on February 10 [3 favorites]

Rare-Earth magnetic products should be handled with care. These magnets are very powerful and can accelerate a great speeds toward each other and toward ferrous material. When these magnets come together quickly, they can shatter and break sending particles at high speed. These magnets can also pinch strongly if allowed to come together against the skin. You should always wear gloves and eye protection when handling strong rare-earth magnets.

- http://www.rareearth.org/magnets_safety.htm
Keep them away from kids and pets too.

Tape would work, as would drilling out wooden or plastic blocks and gluing them in with a dollop of epoxy.

I'd keep them around for something esoteric and get some weaker ones for fridge/music stand duty. And enrobe the latter in tool dip/heat-shrink wire wrap/tape so I wouldn't worry about them shattering.
posted by sebastienbailard at 8:46 PM on February 10

You can weaken magnets by putting them in a string enough opposing magetic field (not sure how easy it would be to do it with rare-earth magnets in a controlled fashion, I'm guessing not easy), but I think alms has the better idea— encase the magnet in something until it has the right strength at the surface.
posted by hattifattener at 8:46 PM on February 10

Heat will demagnetize them. I'm not sure if it's controllable. You could glue them to something so that they're easier to grip and remove. I glued mine to small rocks and clay figurines I made for fridge magnets.
posted by sanka at 8:50 PM on February 10 [1 favorite]

You need to just order some reasonably sized magnets, those can easily take your finger off.
posted by T.D. Strange at 8:51 PM on February 10 [2 favorites]

(Coercivity of some magnetic materials. Assuming you have neodymium-iron-boron magnets you'd need fields on the order of a Tesla to demagnetise them. So maybe that's not the easiest route.)
posted by hattifattener at 8:52 PM on February 10 [2 favorites]

IIUC, rare-earth magnets are made by heating the base material in a strong magnetic field, then letting them cool (while still in the field). Once they cool below a critical temperature, they stay strongly magnetized. I suppose you could heat and cool them in a weaker magnetic field, but that's a lot more work than just wrapping them with tape or embedding them in a lump of epoxy or something.
posted by spacewrench at 8:54 PM on February 10

Neodymium magnets are extraordinarily brittle. Hitting them will give you a bunch of magnetic shards. Freezing won't help.

As it turns out, boiling water ought to cause some irreversible loss of magnetism. How much depends on a bunch of factors.

Note, this is lower than the Curie temperature, which would cause permanent loss of magnetism, all the way.

This is why neodymium magnets aren't used everywhere — they don't have industrial level operating temperatures.
posted by adipocere at 8:55 PM on February 10 [1 favorite]

Heat will work, but it'll probably take a while. I use some rare-earth magnets to hold together a dodgy oven door, and they need replacing every year or so. Chuck a few in the bottom of your oven (or top, I suppose, them being magnets), and they'll weaken eventually.
posted by pompomtom at 8:56 PM on February 10

I've got some encased in a thick rubber shell. They're very strong but still handle-able. I recommend the 'wrap in tape' suggestion.
posted by unixrat at 10:12 PM on February 10

A magnet's attractive force decreases roughly exponentially with distance, so creating a relatively small buffer around the surface will offer relatively large reductions in force. Could you Plasti-Dip them? You could modulate the degree of attenuation with multiple coats.
posted by pullayup at 10:22 PM on February 10

Where "roughly exponentially" means "inverse cube, like any dipole field".

And if anyone tries to make the kneejerk-antiprescriptivist argument that "exponentially" no longer means something related to exponentials, I swear I WILL CUT YOU I WILL CUT YOU AND YOUR PAIN WILL INCREASE LINEARLY OVER TIME
posted by hattifattener at 11:23 PM on February 10 [15 favorites]

Could you get some balls from a sports shop, say squash or handball, cut a slit and put a magnet inside. Bigger softer shape to handle . A toy ball would work as well. Throw in some padding to secure.
posted by stuartmm at 11:39 PM on February 10

Yes, wrapping them in tape would probably be easiest.

But yes, heating those kinds of magnets up will reduce their magnetism. I've accidentally ruined magnets that way.
posted by gjc at 1:53 AM on February 11

I use a bunch of magnets I've salvage from hard drives... they are VERY strong but almost impossible to handle. So I added a handle. Just a three inch piece of duct tape folded over the magnet acts as a great handle. Ugly but very useful refrigerator magnets. They're also great as stud finders, they'll stick to any nails or screws used to hang drywall.
posted by Marky at 7:32 AM on February 11

Heating will certainly do it. Wikipedia tells me that sintered neodymium has a Curie temperature of 320°C, meaning that if you heat it to that temperature it will demagnetize completely. (A bit higher than most household ovens can manage, but you don't want to demagnetize completely anyway.) To weaken the magnetization, I would bake the magnet for 15 minutes at your lowest oven setting. Then check if it has demagnetized sufficiently. If not, increase oven setting by 25°C and bake for another 15 minutes. Repeat until desired results achieved.

Wrapping them in tape or coating them in, say, Plasti Dip would also work.
posted by pont at 9:51 AM on February 11

Where "roughly exponentially" means "inverse cube, like any dipole field".

Well, yes, but because of the complex shape/orientation of the magnetic field, it won't be uniform in every direction. So, "roughly".
posted by pullayup at 8:04 PM on February 11

So, "roughly"

My point is that an inverse cube is really not roughly exponential. They both asymptotically approach zero with distance, but that's about as far as the similarity goes.
posted by hattifattener at 12:27 AM on February 12 [1 favorite]