Samarium cobalt magnets are a type of rare-earth magnet. The alloy content varies from about 25% to 36% samarium by weight. These permanent, strong magnets are especially useful for high-temperature applications.
The term rare earth refers not to the scarcity of the minerals in the Earth’s crust, but to the relatively low concentration of the atoms of interest in rare-earth mineral deposits. Because they are less concentrated, extraction of these compounds tends to be more costly. Certain rare-earth minerals exhibit permanent magnetism, like iron and iron compounds. Magnets manufactured from rare-earth minerals are superior to iron based magnets in that their magnetic force is stronger. Smaller parts can be made with the same magnetic field from rare-earth minerals.
Magnetism is due to a slight net charge distribution that is not zero. At the atomic and subatomic levels, the lowest, most stable energy state of electrons and other charged particles is in orbitals or other movements that are not geometrically symmetrical. This off-center characteristic is sufficient to be attracted by the Earth’s magnetic pole. In the case of lodestone, a naturally occurring magnetic mineral, magnetism was introduced when the rock cooled from the molten lava state, giving the atoms time to align themselves with the Earth’s magnetic field before being frozen into solid rock.
Magnetic materials have an upper temperature limit, the Curie temperature, above which the material is no long constrained at the atomic or molecular level. Samarium cobalt magnets are second in strength only to neodymium magnets, but may be used in higher temperature operations. The Curie temperatures of samarium cobalt magnets are between 1,100°F to 1,300°F (600°C to 710°C), with a favorable working range between 500°F to 1,000°F (250°C to 550°C).
The strength of magnets is typically measured in energy products with units of megagauss-oersteds, or MGOe. The theoretical limit is 34MGOe. Samarium cobalt magnets range from 16 to 32 MGOe. Although they have a very high magnetic force, they are extremely brittle and prone to breaking and chipping. Even handling is difficult, so machine finishing with diamond cutting tools is a highly skilled operation.
Alloys used in samarium cobalt magnets are of two types. The first is known as Series 1:5, with a ratio of one samarium atom to five cobalt atoms. Their magnetic strength ranges from 16 to 25 MGOe. Series 2:17 designates a spectrum of compounds with ratios of about two samarium to 17 cobalt atoms. Some cobalt atoms may be replaced with other transition metal atoms such as iron and copper. Magnetic strength of these alloys ranges from 20 to 32 MGOe.
Magnets of samarium cobalt are manufactured by a variety of sintering and pressing steps. Sintering fuses small particles of the samarium cobalt materials together. The pressing operations can entail die pressing, with pressure applied from one direction or from isotatic pressing in rubber molds, where pressure is applied in all directions. Trade-offs among tolerances, Curie temperature, and magnetic strength are made in manufacturing processes.