Rare earth magnets include neodymium magnets and samarium cobalt magnets and are far more powerful than ferrite ceramics or alnico magnets. Rare earth lanthanide elements neodymium and samarium have partially filled outer f-electron shells which are the source of their exceptionally powerful magnetic fields. Neodymium magnets are neodymium, iron and boron composites with more powerful magnetic pull than any other type of magnet. Although they have high magnetic force, neodymium magnets have low heat and corrosion resistance and lose magnetism under 200 degrees Celsius or more. Samarium cobalt magnets are composed of samarium and cobalt and are much more resistant to demagnetization and corrosion than neodymium magnets, with a thermal stability of up to 550 degrees Celsius. Samarium cobalt is therefore used most in high heat applications such as motors and medical tools. Neodymium magnets are harder than ceramic magnets but are still brittle and are therefore most often nickel-coated for protection. Due to the scarcity of rare earth materials and the long process of rare earth extraction from lanthanide ores, rare earth magnets are significantly more costly than non rare earth magnets.