Magnets are allowed in checked bags, as well as carry-on bags.
Source: TSA (https://www.tsa.gov/travel/security-screening/whatcanibring/items/magnets)
We have been bringing magnetic hooks and magnetic clips on cruises for a few years, and find them invaluable. I prefer the hooks, as they can be used to hang hats, clothes, cords, and many other items. And they can be used as a regular magnet (not using the hook part) to post printed items to the wall. On our last World Cruise, I think we brought a dozen or so, and used them all.
Amazon has a good selection. I error on the larger side, and have the Strongman 50-pound magnetic hooks, but there are smaller ones available.
Most come with a thin adhesive pad that can be applied to the circular magnet. I put these on to protect the wall from scratches, and make them slide around easier. I haven't found that the pad interferes with the magnetic strength at all.