There are several different kinds of magnets which people customarily use to attach to their refrigerators. The most common kind these days is the flat, rubbery kind with a printed advertisement on one side, given away for free far too often as a promotion.
These magnets are made with a magnetizable powder (one company I found on the web uses a powder made of Sr0.6
-- essentially iron rust with some strontium added.) The powder is then mixed with molten vinyl or other flexible plastic, and poured into a sheet mold. The sheet mold is then put in a strong magnetic field which alternates in direction along the length of the magnet. The north and south poles of the magnetized powder bits all turn to line up with the magnetic field, and then the vinyl is cooled off, locking it all together. A design is printed on one side, it is cut into the right shapes, and given away!
Most refrigerators have sheet steel doors and the magnetic field lines pass through the door metal from the north poles of the sheet magnet to the south poles, making the magnet stick to the refrigerator door.
Here's an experiment you can try: Get two of these sheet magnets and see if there is any way you can hold them to make them repel each other. Or better yet, get some iron filings, put the refrigerator sheet magnet under a piece of paper and sprinkle some iron filings on top of the paper. (The paper is there so you can get the filings off the magnet -- otherwise they will stick and be hard to clean off). The filings will point towards the poles of the magnet. How are the poles arranged?
Other kinds of magnets will also stick to refrigerators, like horseshoe magnets and bar magnets, and these are usually made by cooling down hot iron, or another magnetizable metal, in an external magnetic field (either generated by an electric coil or with other permanent magnets). See our description about making magnets
Many refrigerator magnets, even if they are bars, or horseshoes or disks or whatever, are either the plastic kind with magnetic powder inside, or are covered in plastic, so that they do not scratch the refrigerator door.
(republished on 07/13/06)