Most recent answer: 10/22/2007
The strongest available permanent magnets consist of compounds of neodymium, a rare earth metal with atomic number of 60 and symbol of Nd. The commercial magnets often are coated with nickel, another familiar magnetic metal, which is less likely to chip or corrode.
These magnets are actually made of an alloy of neodymium, iron, and boron. Alloys of different elements make stronger, longer-lasting magnets because pure magnetic materials usually demagnetize quickly. The reason is that the magnetic forces favor breaking up the domains into ones whose magnetizations point different ways and cancel out. When there are enough impurities in the material, the boundaries between the domains get stuck, keeping most of the domains from losing their alignment. That's why good permanent magnets are often made of alloys, like Alnico, in which one of the components (like aluminum) isn't even magnetic.
More about neodymium:
You can buy here.
If we find out more, we'll update this answer.
anonymous and Mike W.
(published on 10/22/2007)
Follow-Up #1: Neodymium Magnets
- Wu Fan and Qihan
Neodymium magnets are the kind of magnets used in computer hard-drives, among other things. If you ever manage to get your hands on an old broken computer hard-drive, try opening it up and check out the magnet inside. (Be very careful when doing this - there may be some very sharp edges around the magnet.) Also be careful about handling the magnets themselves. While it is possible to pull two of them apart using your bare hands (if you pull really hard), it really hurts if your fingers get pinched in between.
So how strong are they? Well, using 1 small hard-drive magnet, I can attach a small notepad almost 1 cm thick or a normal sized notebook about 1/2 cm thick to my refrigerator door securely. And it does sometimes take some tugging to get them off again.
(published on 10/22/2007)
Follow-Up #2: Nd magnet info
- Ryan (age 22)
(published on 09/09/2009)
Follow-Up #3: Field strength of Nd magnets
- Sheng Xu
The useful field strength called remanence in the trade can vary from 0.8 to 1.4 Tesla depending on composition. Different manufacturers can give you specific values.
To find suppliers, just type neodymium magnets into your favorite search engine. My favorite supplier K&J showed up.
(published on 11/22/2012)
Follow-Up #4: levitating graphite
1. I just took put graphite lubricant powder on a white piece of paper, and slid two different types of strong magnet under it. In both cases I could see some of the larger flakes flipping around, apparently in response to the magnetic field. I used two types of magnet, because one had sort of rough surfaces and the other was smooth, so I wanted to check if mechanical jiggling of the paper was important. It didn't seem to be. Also, I tried running a non-magnetic stick under the paper, it didn't flip the graphite flakes.
2. There's this nice image online:
The magnets used there look just like one type I used, probably gold-plated neodymium-cobalt. The other magnet I used was pulled from an old head of a Phillips Sonicare toothbrush. These are remarkably strong, probably also a rare-earth type. The hard drive magnets should also be quite strong.
(published on 12/26/2012)