If you are sourcing permanent magnets for a motor, sensor, coupling or fixture, the first decision is not the size or the coating. It is the material. NdFeB, SmCo, ferrite and AlNiCo each behave very differently once heat, corrosion, demagnetizing fields and cost enter the picture, and picking the wrong family usually shows up later as lost torque, drifting sensor output or a quote that is twice what it needed to be.
This guide walks through how we help buyers and engineers narrow the choice, using typical values for each material family. Exact numbers always depend on the grade, so treat the figures below as a starting point rather than a specification.
The short answer
- Need the most magnetic force in the smallest volume? Start with sintered NdFeB (neodymium), and choose a high-coercivity grade if the part runs hot.
- Running continuously above about 180 °C, or need very stable output over temperature? Look at SmCo (samarium cobalt).
- Cost matters more than size, and the part sees moisture? Ferrite (ceramic) is hard to beat.
- Extreme temperatures and a stable, closed magnetic circuit? AlNiCo is still used in sensors, instruments and guitar pickups for good reasons.
- Complex shape, thin walls or insert moulding? Consider bonded or injection-moulded magnets.
Typical properties side by side
| Property (typical range) | Sintered NdFeB | SmCo | Hard ferrite | Cast AlNiCo |
|---|---|---|---|---|
| Remanence Br | 1.1 – 1.48 T | 0.85 – 1.15 T | 0.2 – 0.46 T | 0.6 – 1.35 T |
| Max. energy product (BH)max | 240 – 440 kJ/m³ (30 – 55 MGOe) | 120 – 255 kJ/m³ (16 – 32 MGOe) | 8 – 40 kJ/m³ (1 – 5 MGOe) | 10 – 85 kJ/m³ (1.3 – 10.5 MGOe) |
| Resistance to demagnetization (Hcj) | High to very high, grade dependent | Very high | Moderate | Low |
| Practical max. working temperature | 80 – 230 °C depending on grade | 250 – 350 °C | about 250 °C | 450 – 550 °C |
| Temperature coefficient of Br | about −0.12 %/°C | about −0.03 %/°C | about −0.2 %/°C | about −0.02 %/°C |
| Corrosion resistance | Poor, coating needed | Good | Excellent | Good |
| Relative cost | Medium to high | High | Low | Medium |
Sintered NdFeB: strongest per gram, but watch the temperature
Neodymium-iron-boron is the default choice for modern motors, linear actuators, robot joints and compact assemblies because it delivers the highest energy product of any commercial magnet. That lets designers shrink the rotor or the whole package.
Two things need attention. First, NdFeB loses strength faster with heat than the other materials, and an N-grade magnet that is perfectly fine at room temperature can partly demagnetize in a hot motor. The letter after the grade number tells you the coercivity class (N, M, H, SH, UH, EH, AH), and each step up allows a higher working temperature. Second, NdFeB corrodes easily, so almost every part is coated. Ni-Cu-Ni is the common all-rounder, zinc is a lower-cost option for dry environments, and epoxy or parylene are used where humidity, salt spray or electrical insulation matter.
Good fit for: servo and BLDC motors, linear motors, elevator traction motors, speakers, magnetic separators, consumer products.
SmCo: when heat and stability win
Samarium cobalt magnets are not quite as strong as the best NdFeB grades, but they keep their properties much better as temperature rises and they resist corrosion without a coating in most environments. Their low temperature coefficient also means output changes very little across the operating range, which is why SmCo is often chosen for sensors, precision instruments, aerospace and high-temperature pumps and motors.
There are two families: SmCo5 (1:5) and Sm2Co17 (2:17). The 2:17 type reaches higher energy products and higher working temperatures. Both are brittle, so chamfers, careful handling and sensible tolerances help a lot with yield and cost.
Good fit for: high-temperature motors, magnetic couplings for sealed pumps, sensors that must stay stable over temperature.
Ferrite: low cost, no corrosion worries
Hard ferrite (strontium or barium ferrite) is made from iron oxide and is by far the most economical permanent magnet. It is weaker, so parts are larger, but it does not rust, works up to around 250 °C and is available in a very wide range of shapes, including multipole rings and arc segments for motors.
One detail that surprises people: ferrite’s resistance to demagnetization drops at low temperature, so a design that works at 20 °C should also be checked at the coldest expected condition.
Good fit for: DC motors, automotive auxiliary motors, loudspeakers, holding magnets, small sensors and relays where cost is critical.
AlNiCo: the high-temperature veteran
AlNiCo (aluminium-nickel-cobalt) has excellent temperature stability and can work at temperatures far beyond what rare earth magnets tolerate. Its weakness is low coercivity: a long, thin AlNiCo magnet in a closed magnetic circuit performs well, but a short magnet exposed to opposing fields can be knocked down easily. Cast AlNiCo allows larger and more complex shapes, while sintered AlNiCo gives tighter tolerances on small parts.
Good fit for: measuring instruments, high-temperature sensors, guitar pickups, holding applications in very hot environments.
Bonded and injection-moulded magnets
When the geometry is complicated, bonded magnets are worth a look. Magnet powder (NdFeB, ferrite or SmCo) is mixed with a binder and either compression-moulded or injection-moulded. You give up some magnetic strength compared with a sintered part, but gain thin walls, tight dimensional control, multipole magnetization and the ability to mould the magnet directly onto a shaft or insert. They are common in small motors, encoders and automotive position sensors.
Five questions to answer before you request a quote
- What is the highest temperature the magnet will see, including short peaks, and what is the lowest?
- What load or demagnetizing field does it face, for example in a motor at peak current, or is it in a closed circuit?
- What is the environment: humidity, salt spray, oil, vacuum or cleanroom?
- Which property really matters: pull force, flux density at a distance, or stability over time and temperature?
- What tolerances and quantities do you need? Tight tolerances on brittle materials add grinding cost, so it pays to tighten only the dimensions that matter.
With these answers and a drawing, a magnet supplier can usually propose a material, grade, coating and magnetization direction in one round instead of several.
How Datoyo can help
Datoyo Magnetics has supplied magnets since 1995 and works across sintered NdFeB, SmCo, ferrite, AlNiCo and bonded magnets, including custom shapes, coatings and multipole magnetization. You can see examples of how our magnets are used in motors, sensors, magnetic couplings and automotive parts on our application cases page. If you are not sure which material fits your design, send us the operating conditions and a sketch, and our team will suggest options.
