A Brief Introduction to High Performance SmCo Magnets

Sm₂Co₁₇-based magnets are still playing an irreplaceable role in the permanent magnet industry due to its unique high temperature magnetic properties and superior magnetic stability, then it always serving to the high-speed motors, electronic communication, and aerospace. High energy product magnets are the important basis to accelerate miniaturization and high efficiency of the device. Thus, obtain high performance SmCo magnets have always been a goal since the advent of Sm₂Co₁₇.

Sm₂Co₁₇-based magnets are composed of Samarium, Cobalt, Iron, Copper, and Zirconium. The content of each element has different influence on the magnetic performance. The microstructure of Sm₂Co₁₇-based magnets is a type of cellular structure. Cellular structure is made up of 2:17R cell phase, 1:5H cell boundary phase, and Zr-rich 1:3R platelet phase. 2:17R cell is a rhomboid which long axis along the easy magnetization axis. Its internal is Fe-rich Th₂Zn₁₇-type rhombohedral Sm₂(Co, Fe)₁₇ main phases. 2:17R main phase gives high saturation magnetization M_s to the magnet which eventually determines the B_r. 1:5H phase is a Cu-rich CaCu₅-type Sm(Co, Cu)₅ cell boundary phases, and it gives higher coercivity to magnets via domain wall pinning. Zr-rich 1:3R platelet phase is perpendicular to the c-axis and running across the cellular structure. Zr-rich 1:3R platelet phase provides diffusion paths for Copper to enter the cell boundary phase, then would be beneficial to expand cellular phase and density variations of cell boundary phase’s domain wall energy, thereby enhance coercivity.

Besides dimension of cellular phase, cell boundary phase’s quantity, thickness, and composition will all affect comprehensive magnetic properties of the magnets. The theoretical value of permanent magnet’s maximum energy product is proportional to the square of M_s. Enhancement of M_s is the prerequisite to obtain high energy product. As a structure sensitive parameter, energy product can be also improved through optimization of cellular structure. That is to say, M_s and energy product of SmCo magnets can be effectively enhanced by composition optimization and heat treatment process modification. Fe in the Sm₂(Co, Fe)₁₇ main phases are mainly served to improve M_s and B_r of the magnets. Saturation magnetization J_s of Sm₂Co₁₇ phase is merely around 12kGs. As the content of Fe increased, J_s of Sm₂(Co_0.8Fe_0.2)₁₇ and Sm₂(Co_0.7Fe_0.3)₁₇ can achieve 13.5kGs and 16.3kGs, respectively. However, cellular structure grew abnormally once the content of Fe in Sm(Co, Fe, Cu, Zr)_z higher than 25wt%. Oversized cellular structure is detrimental to the homogeneity of the cellular structure and result in a sharp deterioration of coercivity and demagnetization curve’s squareness. With the application of jet milling and modification of heat treatment, SDM has already mastered mass production of high performance SmCo magnets. Their magnetic performance is comparable with Electron Energy Corporation (EEC) and Arnold Magnetic Technologies.