Rare earth magnets are the strongest type of permanent magnet currently made. They are amazingly powerful for their size and have innumerable uses. These block magnets are 0.25 inch (6.4mm) long, 0.25 inch (6.4mm) wide, and 0.1 inch (2.5mm) thick. They are composed of grade 40 neodymium iron boron magnetic material and are plated in nickel-copper-nickel for a shiny corrosion resistant finish. Max Working Temperature - 176 F (80 C).