Introduction of SmCo Magnets
Samarium-cobalt (SmCo) magnets are a type of rare earth magnet known for their exceptional magnetic strength. These magnets are composed of an alloy of samarium and cobalt. They were developed in the early 1960s by Dr. Karl Strnat and Dr. Alden Rey at Wright-Patterson AFB and the University of Dayton.
Considered among the finest permanent magnet materials available, SmCo magnets offer remanence similar to neodymium magnets but boast higher temperature ratings and superior temperature stability. Two main types of SmCo magnets exist: SmCo 1:5 and SmCo 2:17.
SmCo Magnets 1:5
Samarium-Cobalt 1:5 (SmCo5) magnet alloys consist of one atom of rare earth samarium per five atoms of cobalt. Typically, these alloys contain approximately 36% samarium and the remaining balance is cobalt. In some cases, praseodymium is added to enhance the remanence of the magnet. The energy products of SmCo5 alloys range from 16 MGOe to 25 MGOe (approximately 128 – 200 kJ/m3).
Compared to SmCo 2:17 magnets, the 1:5 magnets are relatively easy to magnetize and can achieve saturation magnetization with a moderate magnetizing field. As a result, 1:5 magnets are more straightforward to calibrate to a specific magnetic field compared to SmCo 2:17 magnets.
SmCo Magnets 2:17
The SmCo Series 2:17 (Sm2Co17) alloys are age-hardened and composed of two atoms of rare-earth samarium per 13-17 atoms of transition metals (TM). The TM content is predominantly cobalt but also includes other metals like iron and copper. Small amounts of elements such as zirconium and hafnium may be added to improve the response to heat treatment. These alloys exhibit maximum energy products ranging from 20 to 32 MGOe (approximately 160-260 kJ/m3).
Among rare-earth alloys, Sm2Co17 has the most favorable reversible temperature coefficient of remanence, typically at -0.03%/°C.
In Sm2Co17 magnets, the coercivity mechanism is based on domain wall pinning, in contrast to SmCo 1:5 and NdFeB magnets where nucleation plays a role. The purpose of alloying and heat treatment is to create a microstructure within the magnets that hinders domain wall motion, thereby resisting the magnetization reversal process.
Surface Treatment for SmCo Magnets
SmCo magnets are inherently resistant to corrosion and typically do not require surface protection. However, in certain cases, a surface coating may be applied to provide protection against chipping. Additionally, in high-vacuum applications, where the sintered material may be porous, surface coating can be beneficial. Although not essential for corrosion resistance, surface coatings can serve as an additional safeguard for specific operational requirements.