Permanent magnets in DC motors: neodymium vs. samarium-cobalt

In permanent-magnet DC motors (PMDC) and many brushless drives, the main field is generated not by a current-carrying field winding but by a fixed permanent magnet. This saves excitation power, simplifies the design and makes the motor more compact - which magnet material fits best depends heavily on available space, operating temperature and cost. More on classifying permanent-magnet motors in DC Motor Types.

Alloys and composition

The two dominant high-performance magnet materials in electric motors already differ significantly in their base alloy.

NdFeB blanks are produced by alloying, powdering, pressing in an external magnetic field (to create a preferred orientation) and subsequent sintering. Sm2Co17 reaches its properties partly through targeted precipitation hardening via the copper and zirconium additions.

Shapes and dimensions

Common geometries for electric motors are arc-segment or shell magnets, rings, discs/cylinders, and block or rod magnets. Which shape is used depends on the motor type: in brushed PMDC motors, several arc-segment magnets with alternating polarity are typically glued to the inside of the fixed stator housing, facing the rotating armature as the field magnet. In brushless motors (BLDC), the magnets instead sit on the rotor - either glued on as surface magnets (sometimes additionally secured with a retaining sleeve) or embedded in the rotor lamination stack, which allows a more compact design and higher torque density with electronic commutation.

Real catalogue examples for NdFeB arc segments illustrate the order of magnitude for small and medium motors: roughly 20 mm outer radius, 12.5 mm inner radius, a 90-degree arc angle and 5 mm thickness for smaller sizes, or around 37.5 mm outer radius, 33.5 mm inner radius, 25 mm length and 4 mm thickness for medium sizes. These are standard trade-catalogue values for orientation, not fixed JoMo specifications - we define the right geometry together based on your specific motor design.

Magnetic data (magnetic flux)

For motor design, the most relevant figures are remanence Br (the magnetic flux density at zero applied field), coercivity HcJ (resistance to demagnetisation) and maximum energy product BHmax (a measure of achievable power density per unit volume). The following values are manufacturer-dependent typical figures, not guarantees for any single product:

Special NdFeB high-temperature grades with dysprosium or terbium additions reach a coercivity of up to around 2,785 kA/m and a maximum operating temperature of up to about 220 °C, at the expense of remanence and energy product. For SmCo, manufacturers quote up to 550 °C for special cases.

A difference often underestimated in practice is the reversible temperature coefficient of remanence: for NdFeB it is roughly -0.10 to -0.12 %/K, while for SmCo it is only about -0.03 to -0.04 %/°C. SmCo magnets therefore lose far less field strength with rising temperature, delivering more stable motor performance in fluctuating thermal environments. Density is around 7.5 g/cm³ for NdFeB and around 8.3-8.5 g/cm³ for SmCo.

Corrosion resistance and mechanical properties

Because of its high iron content, uncoated NdFeB corrodes comparatively quickly and is therefore practically always coated: common options are nickel-copper-nickel (around 12 µm, standard), zinc (around 4 µm, less durable), a combination of nickel-copper-nickel and zinc (12-15 µm), or black epoxy resin, which is nearly fully corrosion-tight as long as the layer stays intact but is more prone to chipping on impact. SmCo, by contrast, is naturally far more corrosion-resistant and usually needs no additional coating even in aggressive media.

Mechanically, both materials are hard and brittle and tolerate compression far better than tension, bending or impact: compressive strength for both is roughly on the order of 1,000 N/mm², while bending and tensile strength are considerably lower, roughly 80-270 N/mm² for NdFeB depending on source and grade. SmCo is considered even more brittle than NdFeB and can practically only be machined with diamond tools or wire EDM.

Cost and raw material availability

Samarium-cobalt is usually significantly more expensive than neodymium-iron-boron. The main reason is the cobalt content: cobalt is considered a strategically critical, price-volatile raw material, and samarium production is complex given comparatively low global output. Neodymium magnets, in turn, depend more heavily on dysprosium and terbium for high-temperature grades, which SmCo does not require.

Both materials also depend on a concentrated global supply chain for rare earths: according to Germany's Federal Statistical Office, around 270,000 of the global 390,000 tonnes of rare-earth raw materials were mined in China in 2024 (a roughly 69% share of production in 2023), followed by the United States (12%), Myanmar (11%) and Australia (5%). Processing and refining are even more concentrated: roughly 85-91% of global separation and refining capacity is located in China, and Germany sourced neodymium, praseodymium and samarium almost entirely from Chinese production in 2024. The rare-earth market is correspondingly considered volatile, and trade-policy decisions by individual countries can affect availability and prices at short notice.

Pros and cons at a glance

Neodymium (NdFeB)

• Highest energy density of all common magnet materials - compact, lightweight motor designs at high output.
• Lower purchase cost than samarium-cobalt.
• Very high achievable remanence of up to around 1.5 T.
• Lower Curie temperature and lower maximum operating temperature than SmCo, often only 60-80 °C for standard grades.
• Prone to corrosion and must be reliably coated.
• Greater loss of field strength with rising temperature (higher temperature coefficient).

Samarium-cobalt (SmCo)

• Very high Curie temperature and high maximum operating temperature, over 350 °C depending on the alloy.
• Naturally corrosion-resistant, usually usable without additional coating.
• Very low temperature coefficient of remanence - stable magnetisation across a wide temperature range.
• High coercivity and therefore high resistance to demagnetisation.
• Lower achievable energy density than NdFeB.
• Significantly more expensive and even more brittle and demanding to machine.

Typical applications

NdFeB is preferred where compact design and high power density at moderate operating temperatures are the priority - for example in servo drives, permanent-magnet DC machines and many brushless drives in general mechanical engineering. SmCo, by contrast, is preferred where high temperatures, aggressive environmental media, or especially temperature-stable magnetisation matter more than purchase price: in aerospace (turbines, actuators, sensors), high-precision and high-speed drives running at tens of thousands of RPM, and in the chemical industry and automotive sensors.

For context: where energy density and temperature-stability requirements are lower, the significantly cheaper ferrite magnets often prevail, while AlNiCo magnets have lower coercivity but the highest temperature stability of all four materials. We help determine the most economical magnet material for your specific application based on your actual operating conditions - more on the broader motor choice in Brushed or Brushless Motor?.

Frequently asked questions

Sources (selection): Wikipedia: Neodymium-iron-boron, Wikipedia: Samarium-cobalt, Bomatec: sintered NdFeB magnets, Vacuumschmelze: VACODYM/VACOMAX, Arnold Magnetics: RECOMA SmCo, IBS Magnet: high-energy magnets, radialmagnet.com: magnet material comparison, supermagnete.de: coatings, Federal Statistical Office: rare earths 2024.