You probably know that magnets don't stick to nonmetal (diamagnetic) materials such as wood, plastic, glass, cotton and wool, but you may not know that magnets don't stick to all metals. In fact, most metals are not magnetic.
TL;DR (Too Long; Didn't Read)
Magnets stick to metals that have strong magnetic properties themselves, such as iron and nickel. Metals with weak magnetic properties include aluminum, brass, copper and lead.
How Magnets Work
A magnet is a piece of metal with the ability to attract other metals. The Earth itself is a huge magnet. It has two ends called poles, a north-seeking pole and a south-seeking pole, and an invisible area of magnetism surrounding it called a magnetic field.
Billions of positively charged atoms produce negatively charged electrons, which spin around the core of an atom and create a magnetic force, transforming the atom into a minuscule magnet. In most materials, the atoms are organized in a way that the magnetic forces point in haphazard directions. However, in some materials, the atoms are organized in a way in which most of the magnetic forces point in the same direction. These forces merge, and the object acts like a magnet. The north pole of one magnet attracts the south pole but repels the north pole of another magnet – unlike poles attract and like poles repel. A metal is a magnet if it repels a known magnet.
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Metals That Attract Magnets
Metals that attract magnets are known as ferromagnetic metals. These metals are made up of billions of individual atoms that have magnetic properties, meaning magnets stick to them firmly. Some examples are iron, cobalt, nickel, steel (because it is mostly iron), manganese, gadolinium and lodestone. Some metals, such as iron, are referred to as magnetically soft because they become strong temporary magnets when you hold a magnet near them but lose some or all their magnetism when you remove the magnet. Alloys of iron and rare-earth metals such as samarium and neodymium maintain most of their magnetism even when they are not within a magnetic field, so they are called magnetically hard and make good permanent magnets.
Metals That Don't Attract Magnets
In their natural states, metals such as aluminum, brass, copper, gold, lead and silver don't attract magnets because they are weak metals. However, you can add properties such as iron or steel to the weak metals to make them stronger. Adding even a small quantity of iron to a metal such as silver makes it magnetic. You may think this would make stainless steel magnetic because it contains iron, a magnetic metal, but when nickel is added during the manufacturing process, the physical structure is changed, creating a nonmagnetic form of stainless steel called austenitic stainless steel.