Sintered SmCo magnets
A samarium–cobalt (SmCo) magnet, a type of rare earth magnet, is a strong permanent magnet made of an alloy of samarium and cobalt. They were developed in the early 1960s based on work done by Dr. Karl Strnat and Dr. Alden Rey at Wright-Patterson AFB and the University of Dayton, respectively.
SmCo is argueably the best permanent magnet material commercially available, with remanence similar to neodymium magnets, but with higher temperature ratings and with superior temperature stability. There are two types of SmCo magnets, SmCo 1:5 and SmCo 2:17.
Both the types of SmCo magnets are manufacture by powder metallurgy. The raw materials are melted in a furnace and cast into thin sheet or ingot. The material is crushed and then pulverized by ball milling or by jet milling. The powder is then oriented with external magnetic field and pressed into blocks or near net shape. Final properties and density are achivied in following sintering and heat treatments. The blocks are then cut to shape and magnetized.
As the nomination indicates, the samarium–cobalt 1:5 magnet alloys (generally written as SmCo5, or SmCo 1:5) have one atom of rare earth samarium per five atoms of cobalt. By weight this magnet alloy will typically contain 36% samarium with the balance cobalt. Sometimes Prasedymium is alloyed to increase the remanence. The energy products of these samarium–cobalt alloys range from 16 MGOe to 25 MGOe, that is, approx. 128 - 200 kJ/m3. Contrary to SmCo 2:17 magnets, the 1:5 magnets are relatively easy to magnetize and saturation magnetization can be achieved with a moderate magnetizing field. Therefore, 1:5 magnets are easier to calibrate to a specific magnetic field than the SmCo 2:17 magnets.
Magnetic properties of SmCo 1:5
There is a series of grades available (see table 1). The nomination is according to EN 60404-8-1:2015. The first number gives the maximum energy product of the grade (in kJ/m3) and the second number the intrinsic coercivity in kA/m divided by ten. Properties given in the table are typical for each grade at the room temperature. More information, including demagnetisation curves in various temperatures, for each grade can be found in the datasheets (possibility to download). The same information is available in numerical form for magnetic modelling (request for info – e-mail).
Table 1: Typical magnetic properties of SmCo 1:5 magnet grades at 20℃
These alloys (written as Sm2Co17, or SmCo Series 2:17) are age-hardened with a composition of two atoms of rare-earth samarium per 13–17 atoms of transition metals (TM). The TM content is rich in cobalt, but contains also other metals such as iron and copper. Other elements like Zirkonium and Hafnium etc. may be added in small quantities to achieve better heat treatment response. The maximum energy products of these alloys range from 20 to 32 MGOe, what is about 160-260 kJ/m3. These alloys have the best reversible temperature coefficient of remanence of all rare-earth alloys, typically being -0.03%/°C.
In Sm2Co17 magnets, the coercivity mechanism is based on domain wall pinning (in contrary to SmCo 1:5 and NdFeB where it’s nucleation). The purpose of alloying and heat treatment is to create a microstructure inside the magnets which impede the domain wall motion and thereby resist the magnetization reversal process.
Magnetic properties of SmCo 2:17
There is a series of grades available (see table 2). The nomination is according to EN 60404-8-1:2015. The first number gives the maximum energy product of the grade (in kJ/m3) and the second number the intrinsic coercivity in kA/m divided by ten. Properties given in the table are typical for each grade at the room temperature. More information, including demagnetisation curves in various temperatures, for each grade can be found in the datasheets (possibility to download). The same information is available in numerical form for magnetic modelling (request for info – e-mail).
Table 2: Typical magnetic properties of SmCo 2:17 magnet grades at 20℃
The typical physical properties of SmCo magnets are given by the table below. These values must not be understood as quaranteed, as the properties are not controlled in manufacture. SmCo magnets are fragile and must always handled with care and never used as load carrying elements in a design.
SmCo magnets are not prone to corrosion and normally do not need any surface protection. However, in some cases surface coating may be used as protection against chipping. Also, as sintered material is porous, in high-vacuum applications surface coating may be helpful.
The corrosion resistance of these coatings in different environment is influenced by the shape of magnets, e.g. chamfer, inner rings.