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?
Hi, Thanks for the info. I couldn’t find any video or even picture of the actual production process of these type of magnets! Have you seen any? I appreciate it if you share!
Sorry I looked around but the best I could find is a ‘how its made’ video of regular magnets. I had only read about the process of making halbachs from a physics website. https://www.youtube.com/watch?v=noGGcyPHtdI
[…] Those really thin fridge magnets are a special type of magnet called a Halbach array that has almost no magnetic field on one side – these were originally designed for use in particle accelerators.. Ref: engineerdog.com/2015/03/12/why-do-refrigerator-magnets-only-stick-on-one-side/ […]
Thanks for the really interesting article. I am not able to understand the diagram with the cross section. What do the blue and pink colors mean? What do the arrows (left, right, top and down) mean?
The arrows are indicating the direction of the magnetic poles, from south to north. The colors are just there to help the different arrow directions stand out better.
Is the same thing going on with Hard drive magnets?
HI Bill. I did some googling and as best I can tell, no. Hard drives just use really powerful neodymium magnets but not in a Halbach array.
thank you i was doing a research for school and this is the only thing that helped and for that thank you.
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Wow,These blog is very nice and informative. Thank you for sharing this information it helps me a lot. I really love reading your article very nice and very interesting.
You can see this same phenomenon in a magnet taken out of a computer hard drive — only on a very powerful magnet. It is very strange to have one side super strong, and the other side nothing.
So, if you glue two fridge magnets together, front to front, it will stick on both sides.