If there is one thing I know about magnets it’s that they have poles. That’s basic magnet theory; you simply can’t have a north side without a south side. So you can imagine my confusion when I encountered a magnet that only attracted metal on one side.
If you’ve ever played with a refrigerator magnet you may have noticed how you can’t stick the side with the printed label against the fridge. In fact, that face has almost no detectable magnetic field at all.
It also displays another interesting quirk: When you rub the magnetic faces together the right way the surfaces jump from position to position, but only on one axis. See the quick video I made below for a visual:
So what’s going on here? How does this little thing even exist? Turns out that a refrigerator magnet is a Halbach array, which is a special arrangement of magnets that uses superposition to strengthen the magnetic field on one side of the array while cancelling the field on the other side.
The lines I drew on the magnet face in the video show the direction of small magnet strips, each oriented in a deliberate pattern so that the field directions supplement each other on one side and cancel each other on the back.
Here’s how magnetic superposition works:
A cross section of a fridge magnet looks like this:
How it’s made:
A refrigerator magnet is made of powdered ferrite (iron rust) and a liquid binding agent of plastic or rubber. The two are heated, mixed together, and poured into a sheet mold which is then exposed to a strong magnetic field. This field alternates in direction along the length of the magnet orienting the poles of the powder mix to line up with the magnetic field. That field is permanently locked in place when the mixture is cooled off.
Why bother making a fridge magnet into a Halbach? Doing so allows manufacturers to use less material to make a functioning magnet and also ensures that the information on the label side of the magnet is never covered up.
Outside the world of refrigerator magnets, Halbach arrays are used in AC motors, generators, and magnetic couplings. The first Halbach was originally designed for focusing the beams of particle accelerators.
Pretty neat huh?