Samarium Cobalt
Properties
| State | Solid (sintered ceramic-metallic) |
| Color | Silvery metallic gray |
| Solubility | Insoluble in water; slowly oxidizes in humid air above 300 °C |
| Melting Point | 1320 °C |
About Samarium Cobalt
Samarium cobalt is a hard magnetic intermetallic (SmCo5, 445.025 g/mol) that crystallizes in the hexagonal CaCu5-type structure and, together with the related 2:17 phase Sm2Co17, makes up the second major family of rare-earth permanent magnets. SmCo predates NdFeB by roughly fifteen years — Karl Strnat developed it at the Wright-Patterson Air Force Materials Lab in 1966 and the technology was commercialized through the 1970s. The room-temperature maximum energy product runs 180-240 kJ/m3 for the 1:5 phase and up to 240 kJ/m3 for sintered Sm2Co17 — lower than the 350-400 kJ/m3 of top-grade NdFeB, but that comparison misses the point. What SmCo does that NdFeB cannot is hold its coercivity at temperatures where NdFeB irreversibly demagnetizes: Sm2Co17 operates reliably to 350 C versus roughly 150-200 C for the highest-temperature NdFeB grades like N42UH, and the cobalt-based matrix passivates rather than rusting in humid air, eliminating the need for the nickel-copper-nickel triple plating that NdFeB magnets require. Those two properties — high-temperature stability and corrosion resistance — keep SmCo dominant in aerospace propulsion, satellite reaction wheels, missile guidance gyros, magnetic bearings in turbomolecular vacuum pumps, and any application that combines a tight thermal envelope with a long deployment lifetime. The cost is roughly 2-3x NdFeB per kilogram because cobalt runs ~$30/kg versus iron at under $1/kg, and samarium is 2-3x the price of neodymium.
Where you'll encounter it
If you have ever opened a high-end turbomolecular vacuum pump (the kind on the back of an electron microscope or a UHV chamber), the magnetic-bearing assemblies that levitate the rotor at 60,000 rpm are SmCo — the bake-out cycle hits 150 C for hours and NdFeB would steadily demagnetize. The same calculation drives the magnet selection in every recent spacecraft reaction wheel and momentum-management gimbal: thermal cycling between -100 C in eclipse and +80 C in sunlight over a 15-year mission would degrade NdFeB measurably, while Sm2Co17 holds its remanence within fractions of a percent. Defense electronics and missile fuze designers default to SmCo for the same reason — high-G launch and short-duration thermal soaks above 200 C are routine. The only place NdFeB displaced SmCo over the past three decades is room-temperature consumer applications (hard-drive voice coils, headphone drivers, MRI shim magnets at field strength rather than coercivity), where the cost differential dominates and the temperature ceiling never gets stressed.
Common Uses
- Aerospace reaction wheels, gyroscopes, and star-tracker actuators on long-life satellites
- Magnetic bearings for turbomolecular vacuum pumps operating at 60,000+ rpm
- Missile guidance gyros and defense electronics with high-G and high-T requirements
- High-temperature industrial servo motors and actuators above 200 °C
- Precision instruments where NdFeB demagnetizes (spectrometer ion sources, MEPS systems)
Safety Information
GHS: Skin sensitization Category 1 (cobalt component), Carcinogen Category 1B (cobalt metal and salts in fume/dust form per IARC). Solid magnets are chemically inert in service, but machining, grinding, or laser-cutting generates Co-bearing dust that is the actual hazard. OSHA PEL for cobalt metal dust is 0.05 mg/m3 (8-hour TWA), ACGIH TLV-TWA is 0.02 mg/m3 — both have been recently tightened because cobalt-dust lung disease (hard-metal lung disease, fibrosing alveolitis) is well documented in tool-grinding workers. Use Co-dust local exhaust ventilation, P100 respiratory protection, and impermeable gloves during machining. Strong magnetic fields up to 1 T at the surface — keep pacemakers and credit cards at distance, and beware of pinch hazards on assembly because two SmCo blocks can attract with hundreds of pounds of force at close range.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.