I have a couple of ceramic magnets, and I need to cut one in half so I have two squares instead of one rectangle. They’re about ¼" thick. What’s a good way to do it? I thought maybe a Dremel tool with a cutting wheel, or a hacksaw.
I’d go with the dremel. I think a hacksaw will turn it into iron flakes.
ETA, Also, since it’s only a quarter of an inch thick, I think I would tape it to a piece of wood so it doesn’t move and I don’t have to hold it. It’s far too small to put in a clamp/vice and I’ve seen videos where people put the dremel in a vice, but I think it’s to thin to not be totally secured where it’s going to be cut (that is, I’d be worried it would shatter if you tried to clamp the Dremel and hold the magnet).
Dremel.
I’m not even sure if a normal hacksaw blade is hard enough to cut a Neodymium magnet.
I think a ceramic magnet is just those cheap non-shiny Fe ones. Like the kind that you find inside of things where they don’t have to look nice, just do their job (motors, toys, etc)
Be careful when you cut it, lest you end up with two monopoles.
Pull the other one. It has bells on. 
Dremel and a ceramic tile cutoff wheel
It might be harder to cut than you think and thay are not that strong structurally (think a dense and hard clay pot) and some tend to fracture pretty easily so a clean cut might be difficult. Also after being cut they are very susceptible to fracturing at the cut point.
Just be aware that structurally they are not that rugged if exposed to any serious impacts. Don’t be surprised if little chips of material start coming off the edges where it was cut.
Yeah, I figured they were pretty fragile. I’ll use the Dremel to carefully cut the magnet. I have two, so I can screw up once. I’ll sand the cut ends or otherwise get bits off. The plan is to super-glue the squares onto a plastic gate cover (for a motion picture camera). I’ll paint ‘something’ over them to keep magnetic dust from coming off.
You can buy rare-earth magnets of virtually any size on eBay, you know…
I do have ten small (5/16" diameter), rare-earth button magnets. Five of them would fit in the spaces where I want to put each square.
You may want to experiment with smaller magnets first before committing. If the magnetic lock is too strong it could damage or jar something loose something in the camera if you have to yank on cover to remove it. The magnet size you are intending on using per your OP description will clamp with a large amount of force.
I ended up using the rare-earth magnets. I used super glue to affix three in a horizontal line above the plastic ‘stud’ (for pulling off the cover), and three similarly affixed below it. One more went on each side of the stud. I covered them with bathroom sealer that supposedly dries clear. So the super glue will hold them to the plastic, the three-in-line magnets will hold each other together in case the super glue fails, the the sealer will help them adhere to the plastic and also keep them all together.
Not that the cover is subject to any stress. It sits quietly over the gate, and is removed when you put the magazine in. I think the rare-earth magnates will be handy-dander (salamander).
Oh, I did manage to break one of those little 5/16" rare-earth button magnets. I had three in a row, just sitting on the cover. I was going to hold them up against the fridge to ensure they held. The problem was that I was also holding another row-of-three, and I dropped one row. One of the magnates broke when it hit the floor. Did not expect that. Taking them to the counter, I dropped them again, and the two-and-two-halves magnates rolled under the front. Fortunately I had six more in a stack, so it was easy to find them! 
One thing to keep in mind is that magnets can lose their magnetism when heated. Neodymium magnets are more sensitive than ceramic ones, so if you use a grinding disc to cut a magnet it would be a good idea to do it slowly or use a coolant to keep the temperature down.
I considered cautioning him about that earlier, but a quick google showed that the Curie temperature for a ceramic magnet is 860 degrees Fahrenheit, and I figured that a quick cut on a small magnet wasn’t going to reach that temperature. (Excellent FAQ on magnets here.)