Look at the picture: you see a small magnet, floating freely in the air. There
is nothing concrete hindering it to fall down or slip aside. And still it can
hold its position for month and years, eventually swinging slowly around its
equilibrium position. You can play with it by using another magnet to make it
oscillate. It's even possible to lift the upper half of the cavity in which
it resides. It will still float, but the system will be less stable.
So how does this trick work? The key point are the black disks, which consist of graphite, a material which is diamagnetic. diamagnetic materials have the property, that they act repulsive on magnets. You can imagine, that a virtual mirror magnet is induced inside the diamagnet, when you approach it with a magnet. This mirror magnet has the opposite orientation of the magnet, so that the magnet experiences a repulsive force.
Magnet
Diamagnet with induced virtual Magnet. Note the opposing orientation!
The strongest diamagnets are supraconductors. They are so strong that the repulsive force can overcome gravity and a magnet can float above a supraconductor. The disadvantage is, that all the known supraconductors today have to be kept very cold to be supraconducting. Usualy one uses liquide nitrogen to cool them to -76 degrees Celsius. Good diamagnets at ambiant temperatures are bismuth or graphite. The latter was used for the experiment on the picture. But they are not strong enough to compensate gravity using the best available permanent magnets, neodymium magnets. So one has to use a trick, to reduce gravity. This trick consists in positioning another permanent magnet (a cheap ferrite magnet) above the small magnet that will float. The magnets are orientated the same, so they are attracting themselves. The distance is choosen to compensate gravity nearly, but not completely. The rest is done by the repulsive force of the diamagnet under the neodymium magnet. To understand this better, look at the following picture:
Drawn are the
different potentials which are in play: the gravitational
potential, the potential of the ferrite magnet
and the potential of the diamagnet. The red
dashed line indicates the position of the ferrit magnet, the green dashed
line the position of the diamagnet. You can see, that the sum gives a potential
with a minimum (white line), in which the neodymium magnet can sit. You can
also see, that the diamagnet is crucial: without, the small magnet would either
fall in gravity, or be attracted to the ferrite magnet (pink
line).
Perhaps you think now: why don't I simply use two opposing magnets, one above the other, the upper magnet is repulsed against gravity and should float. If you try this and keep the magnets from flipping, you will see that the floating magnet will at once slip to a side, where the magnetic repulsion is less strong. It has no radial confinement. You can add radial confinement by using concrete objects touching the floating magnet. (Perhaps you have seen a floating ashtray made like this in a "fun with science" shop.) In the configuration described above, the two magnets are attracting themselves. This attractive force is the strongest on the axis of the big magnet, so the small magnet tries to get there, it is confined radially.
The last question to answer is, why the small magnet does not flip. Imagine it would start to flip and incline a bit. Then the magnet and the virtual magnet inside the diamagnet, would have a greater distance at one side of the magnet than at the other. The repulsive force would be less at this side and so the magnet will fall back.
Instead of using only one diamagnet, one can build a diamagnetic cavity around the small magnet (see first picture). This gives more confinement in the upwards direction and thus makes the system more stable. This is especially important if the temperature changes. The magnetic forces are reduced at higher temperatures and potential depth can get insufficient. You can observe that the position of he potential minimum changes, so one can in principle use a diamagnetic trap as a thermometer.
What are the incredients to have your magnet float as high as possible? First the diamagnet should be strong, bismuth is better than graphite. The magnet should be as strong as possible per weight. After my knowledge, neodymium magnets are the best, other rare earth magnets should also do. The upper magnet should produce a magnetic field as homogeneous as possible in the vertical direction. At the high field laboratory at Grenoble, they use a supraconducting magnet which produces magnetic fields of several Tesla. The neodymium magnet can be made floating three meters beyond the supraconducting one. As diamagnets one can use fingers. (They consist mainly of water, which is diamagnetic.) Using this, a one centimeter cube magnet is floating between thumb and forefinger at a distance of one centimeter from each.
To build the diamagnetic trap you can see on the first picture I used the following materials and dimensions:
The construction itself should be clear from the picture.
using two neodymium magnets and a steel ball, you can PierceYou!
Throw the neodymium magnet through a copper tube: it will fall extremely
slowly because
of eddy currents.
Use the diamagnetic trap without the graphit. If the distance between
the aluminium ground plate
and the upper magnet is correct, the neodymium magnet will be sucked
uppwards only on the
axis of the aluminium disk. Put the magnet at the outer side of the
aluminium disk. It will slowly
glide to the axis, slowed down by eddy currents in the aluminium. The
suddenly it is sucked
upwards.
After the creation of this page, I found similar pages for example:
http://www.scitoys.com/scitoys/scitoys/magnets/suspension.html
http://www.eskimo.com/~billb/neodemo.html
SCIENCE NEWS, 7/24/99
NATURE, 7/22/99, author A.K. Geim.
The first diamagnetic trap I saw was presented by Eric
Cornell at the Les Houches summer school
of physics in august 1999.
Look for lots of more interesting stuff on my homepage.
Last changed: 12.3.2000