Rare earth magnets can be classified into a samarium-cobalt magnet and a neodymium magnet. A characteristic of a samarium-cobalt magnet is strong magnetic force. Its merit is that it is resistant to rust and high temperature, but it tends to be easily broken. It is popular as the most suitable material for a small and lightweight magnet, like neodymium. A neodymium magnet can produce the strongest magnetic field in the world among those sold in the current market. The shape of a neodymium magnet can be round, square or ring according to what it is used for. Although it is mechanically strong, its surface is processed since it is easily rusted and therefore it must be used below 80°C.