candukid neodymium--Rare Earth magnet. Strongest there is.
I bought 2 different sizes for a project a few weeks ago off Amazon. Very cheap, in quantities. I have to use pliers to separate the damn things and take care where I lay them down. That is to say after I get them off the plier's jaws.😄
Learned about them back in my scrapping days.....when I took stainless steel to the scrap yard. And their magnets stuck fast! (they will not accept non-magnetic SS unless you can show proof of U.S. made, btw)