There are two types of cow magnets that are widely used:
Alnico Cow Magnets
Alnico cow magnets are made primarily from Aluminum, Nickel and Cobalt. They may also contain copper, iron and sometimes titanium.
Ceramic Cow Magnets
The ceramic cow magnets have special magnetization pattern which causes nails and other sharp metal objects to line up along the length of the magnet thereby preventing the cow from the danger.
Used by dairy farmers for treating and preventing hardware disease in cows.
The cow magnets are popular among students for various experiments.
They also have many industrial applications.