Rare earth magnets are strong permanent magnets made up of rare earth alloy elements. Research on rare earth magnets began in the 1970s and 1980s. Among permanent magnets, rare earth magnets can generate the largest magnetic field, much larger than the magnetic field generated by aluminum-nickel-cobalt alloy magnets or ferrite magnets. Rare earth magnets can generally produce a magnetic field of over 1.4 Tesla, while ferrite or ceramic magnets have only about 0.5 to 1 Tesla.
Rare earth magnets are commonly divided into two types: neodymium magnets (also known as neodymium iron boron magnets) and samarium cobalt magnets, which contain the rare earth elements neodymium and samarium, respectively. Rare earth magnet materials are very brittle and susceptible to corrosion, so they are usually coated with other metals to protect the magnets.
The term "rare earth" in rare earth magnets is often misunderstood. In fact, rare earth elements are not rare, and their abundance in the earth's crust is about the same as tin and lead. Research on rare earth magnets began in 1966 when scientists at the US Air Force Materials Laboratory discovered an alloy of yttrium and cobalt, which had a higher magnetic anisotropy constant than any known material.
Samarium cobalt magnets are magnetic tool materials made by mixing samarium, cobalt, and other rare earth materials, melting them into an alloy, and then crushing, molding, and sintering them. They were the earliest rare earth magnets discovered. There are two composition ratios for samarium cobalt magnets, namely (samarium atoms:cobalt atoms) 1:5 and 2:17.
Samarium cobalt magnets have a high magnetic energy product, extremely low temperature coefficient, and a maximum operating temperature of up to 350°C. They are not limited by negative temperatures. When the working temperature is above 180°C, their maximum magnetic energy product, coercive force, temperature stability, and chemical stability all exceed those of neodymium iron boron permanent magnet materials. They have strong corrosion and oxidation resistance. The maximum energy product of samarium cobalt magnets ranges from 16 MGOe to 32 MGOe, and their theoretical limit is 34 MGOe.
Since samarium cobalt magnets are expensive and have weaker magnetic strength, their applications are limited. Because the Curie temperature of samarium cobalt magnets is high, they can be used in situations that require a larger magnetic field at high temperatures. Samarium cobalt magnets are not easily oxidized, but sintered samarium cobalt magnets are brittle, prone to delamination and cracking, and are easily damaged by thermal shock.
A cube-shaped neodymium iron boron magnet after nickel plating. Neodymium magnets were invented in the 1980s and are the strongest and most easily obtainable rare earth magnets. They are made of an alloy of neodymium, iron, and boron, and are therefore also called neodymium iron boron magnets. Neodymium magnets can be used in many situations that require a large magnetic field or a smaller magnet, such as motors in battery-powered tools, hard drives, and jewelry clasps. The magnetic field strength of neodymium magnets is the largest among permanent magnets. Compared with samarium cobalt magnets, neodymium magnets have higher coercive force, but a lower Curie temperature, and are more susceptible to oxidation. To avoid damage from corrosion, neodymium magnets need to be protected on their surface, such as by nickel, zinc, tin plating, and surface coating with epoxy resin, etc. Early on, due to the high cost of raw materials and patent licensing fees, the price of neodymium magnets was high and they could only be used in some applications that required high magnetic field strength. However, since 1990, the cost of neodymium magnets has continued to decrease, so some magnetic toys also use neodymium magnets.
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