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 arguably 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.

Sintered SmCo magnets is a high-performance low-temperature coefficient permanent magnet made of samarium and cobalt and other rare earth elements. Sm-Co magnets can be operated at higher temperatures up to 350 C. Sm-Co materials come in energy products from 16 MGOe up to 33 MGOe. Their high resistance to demagnetizing influences and excellent thermal stability has ensured Sm-Co as the premium choice for the most demanding motor applications.

Advantage 

• Excellent thermal stability
• High magnetic properties
• Good corrosion resistance
• High resistance to demagnetization

Disadvantage

• Difficult to machine
• Expensive material (cobalt is market price sensitive)

Manufacturing process

Both the types of SmCo magnets are manufacture by powder metallurgy. The raw materials are melted in a furnace and cast into thin sheets or ingot. The material is crushed and then pulverized by ball milling or by jet milling. The powder is then oriented with the external magnetic field and pressed into blocks or near-net shape. Final properties and density are achieved in following sintering and heat treatments. The blocks are then cut to shape and magnetized.

SmCo 1:5

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 a 36% samarium with the balance cobalt. Sometimes Praseodymium 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.

SmCo 2:17

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 Zirconium 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, which 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.