Choose the magnet that meets your required field or force in the complete magnetic circuit—not the one with the most impressive material label. Define the working gap, dimensions, temperature, corrosion exposure, mechanical loads, and sourcing rules first. Ferrite and Alnico are established rare-earth-free options; NdFeB and SmCo contain rare-earth elements. Ferrite can reduce cost when a larger magnet is acceptable, NdFeB can minimize size and mass, and SmCo generally retains resistance to demagnetization better at elevated temperature. The correct choice depends on the specific grade, geometry, and load line.
What “rare-earth-free” means
“Rare-earth-free” describes composition, not a universal performance class. In this comparison, ferrite (ceramic) and Alnico are established rare-earth-free magnet families. Neodymium-iron-boron (NdFeB) and samarium-cobalt (SmCo) are rare-earth magnet families. SmCo is not rare-earth-free simply because it is different from NdFeB.
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Some projects also restrict cobalt, either for supply-chain, environmental, or specification reasons. Alnico and SmCo both contain cobalt. A newer iron-nickel material described by the U.S. Department of Energy (DOE) is presented as rare-earth- and cobalt-free, but its reported performance and development status should not be treated as proof of a catalog product.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchWrite the material rule explicitly before comparing candidates: which rare-earth elements are excluded, whether cobalt is excluded too, and whether a research-stage material is acceptable. That prevents a technically strong but ineligible magnet from entering the design late.
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Start with the project requirement, not a material name
A catalog value such as maximum energy product does not predict the field or force in your assembly by itself. The magnet operates with its pole pieces, air gap, return path, neighboring materials, and load line. A small magnet in an efficient circuit can outperform a larger one in a poor circuit, while a coating, adhesive layer, or mounting gap can materially change the result.
Define the magnetic output
- Required field or force, including the location where it must be achieved.
- Working gap and its tolerance over the full range of motion.
- Pole area, magnet dimensions, and the available return path or yoke.
- Continuous, intermittent, pulsed, or alternating operation.
- Acceptable field variation from unit to unit and over the product’s life.
For a consequential design, have a magnetics engineer check the circuit and load line. A handheld gauss meter can help characterize a prototype, but a field reading alone does not establish energy product, coercivity, temperature capability, or long-term suitability.
Define the physical envelope
Record maximum length, width, height, mass, mounting clearances, coating or plating thickness, and the tolerances that the mating parts can absorb. Higher-energy materials may meet the same operating point in a smaller or lighter component. Lower-cost ferrite may require more volume, and its larger size can affect the yoke, housing, actuator travel, and shipping weight.
Define the environment and duty cycle
List normal and peak temperatures, heating duration, humidity, salt or chemical exposure, vacuum or pressure, shock, vibration, rotational speed, and expected service life. A magnet that works on a bench at room temperature may demagnetize, corrode, crack, or lose retention after assembly and cycling.
Use maximum energy product as a screening metric
Maximum energy product, commonly expressed in megagauss-oersted (MGOe), is a useful first-pass indicator of how much magnetic energy a material can provide per unit volume. It is not a complete design result: the actual operating point depends on grade, temperature, magnet shape, circuit reluctance, and the load line.
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| Magnet family | Typical room-temperature maximum energy product | What the figure means |
|---|---|---|
| Alnico | 1–8 MGOe | Broad typical range for commercially available magnets in the DOE’s 2015 assessment. |
| Ferrite | 3–4 MGOe | Broad typical room-temperature range for commercially available magnets in the same assessment. |
| SmCo | 15–30 MGOe | Broad typical room-temperature range; exact grade and operating conditions still govern performance. |
| NdFeB | 25–50 MGOe | Broad typical room-temperature range; the highest family range in this comparison, not a guarantee for every grade. |
These are family-level ranges reported by the U.S. Department of Energy in 2015, not a substitute for a current manufacturer’s demagnetization curves. Do not compare a low-end grade from one family with a high-end grade from another as though the family ranges were fixed specifications.
What the ranges imply for size
When the circuit and required output are held constant, a higher energy product can allow less magnet volume. That is why NdFeB is often the benchmark for compact or lightweight components. A larger ferrite magnet can still be the better engineering choice when enclosure space, mass, and assembly cost are acceptable. SmCo may justify its higher material cost when temperature margin matters more than minimum volume.
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- Coercivity: resistance to demagnetization from opposing fields, armature reaction, or a poor load line.
- Remanence: the residual flux density available after magnetization.
- Operating temperature: reversible temperature coefficients and irreversible loss vary by grade.
- Geometry: length-to-diameter ratio and pole arrangement affect the working point.
- Assembly: air gaps, steel saturation, adhesives, coatings, and tolerances alter the circuit.
Compare the four established families
Ferrite (ceramic)
Ferrite is a common rare-earth-free choice when cost, corrosion resistance, and adequate performance outweigh the need for minimum size. Its DOE typical room-temperature range is 3–4 MGOe. Because its energy product is below NdFeB and SmCo, the design may need more magnet volume or a different circuit. That extra volume can be acceptable in speakers, sensors, holding devices, and other applications where the housing has room.
Ferrite is ceramic and therefore brittle. Treat shock, edge chipping, clamping loads, and drilled or machined features as design concerns. Use a retention method that does not turn thermal expansion or assembly interference into a tensile load on the magnet.
Alnico
Alnico is an established rare-earth-free family with a broad DOE typical room-temperature range of 1–8 MGOe. Its suitability cannot be inferred from that range alone. Check the grade’s coercivity and the actual magnetic circuit carefully, especially where an external opposing field or unfavorable load line could cause irreversible demagnetization.
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Alnico generally has good corrosion resistance, but the specific grade, surface condition, and assembly still need verification. It also contains cobalt, so it does not satisfy a no-cobalt specification even though it is rare-earth-free.
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NdFeB
NdFeB offers the highest typical energy-product range in the DOE comparison, 25–50 MGOe at room temperature. That energy density can reduce magnet volume and mass, which is valuable when a device must be thin, light, or powerful in a small gap.
Temperature and corrosion are the principal design checks. NdFeB’s demagnetization resistance at elevated temperature is generally less favorable than SmCo’s, and effective surface protection is needed for most applications. Specify the coating or plating, thickness, edge coverage, adhesive compatibility, and exposure conditions; a coating can also change fit and air-gap dimensions.
SmCo
SmCo is a rare-earth magnet family with a DOE typical room-temperature range of 15–30 MGOe. DOE guidance identifies better high-temperature resistance to demagnetization than NdFeB as its central advantage. Consider it when the magnet sees sustained or transient heat and the design can accommodate its material cost, geometry, and rare-earth content.
SmCo is brittle, so shock, clamping, machining, and retention deserve the same attention required for other brittle magnet materials. Confirm the exact grade’s curves and maximum operating temperature rather than applying a family-wide temperature assumption.
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| Family | Rare-earth-free? | Typical room-temperature range | Most useful first consideration | Key cautions |
|---|---|---|---|---|
| Ferrite | Yes | 3–4 MGOe | Lower cost when larger size is acceptable | More volume; ceramic brittleness; verify circuit output |
| Alnico | Yes | 1–8 MGOe | Established rare-earth-free option | Check coercivity and load line; contains cobalt |
| NdFeB | No | 25–50 MGOe | Smallest or lightest practical magnet | Temperature limits, demagnetization, corrosion protection |
| SmCo | No | 15–30 MGOe | Higher-temperature demagnetization resistance | Rare-earth content, cost, brittleness, grade-specific curves |
Temperature can reverse the apparent winner
Magnetic properties change with temperature, and the ranking at room temperature may not hold at the operating temperature. DOE identifies SmCo as more resistant to demagnetization at higher temperatures than NdFeB. That does not mean every SmCo grade is automatically suitable or every NdFeB grade will fail: the exact grade, circuit, duration, and load line determine the result.
Build a temperature case
- Measure or calculate the magnet’s steady-state temperature, including heat conducted from coils, motors, bearings, electronics, or sunlight.
- Capture the highest transient temperature and its duration, not only the average.
- Obtain the manufacturer’s reversible temperature coefficients, irreversible-loss data, maximum operating temperature, and demagnetization curves for the selected grade.
- Evaluate the curves at the actual load line and opposing fields, with a defined safety margin.
- Repeat the check after accounting for thermal expansion of the magnet, housing, pole pieces, and adhesive layers.
General family recommendations are useful for screening, but engineering guidance warns against relying on them blindly without detailed design analysis.
Corrosion protection and assembly are part of magnet selection
Humidity and chemicals can shorten life, change dimensions, weaken coatings, or attack an adhesive joint. NdFeB generally needs effective surface protection for most applications. Ferrite, Alnico, and SmCo generally have good corrosion resistance, but “generally” is not a qualification for an untested environment.
Specify the complete surface system
- Base material and exact grade.
- Coating or plating type, thickness, adhesion, and edge coverage.
- Exposure to water, salt spray, cleaning agents, fuels, oils, or galvanically dissimilar metals.
- Adhesive chemistry, cure temperature, outgassing, and compatibility with the coating.
- Dimensional allowance for coating thickness and any required electrical insulation.
Check mechanical failure modes
Permanent magnets can be brittle, and high-energy magnets can attract tools or neighboring steel with dangerous force. For shock, vibration, or high-speed rotation, design a positive mechanical containment feature rather than relying on magnetic attraction or adhesive shear alone. Verify press fits, clamps, retaining rings, potting, and cure shrinkage against the magnet’s tensile and edge-load limits. Include thermal-expansion mismatch between the magnet and its holder.
A step-by-step selection workflow
- Write the eligibility rule. State whether all rare-earth elements are excluded, whether cobalt is also excluded, and whether prototype or research materials are allowed.
- Define the operating point. Record required field or force, working gap, pole area, duty cycle, and acceptable variation.
- Set the envelope. Establish maximum magnet volume, mass, mounting space, coating allowance, and tolerances.
- Map the environment. Document steady and peak temperatures, humidity, chemicals, shock, vibration, rotation, and service life.
- Screen families with energy product. Use the DOE ranges only to identify plausible size and mass trade-offs, not to approve a design.
- Shortlist exact grades. Obtain current datasheets with remanence, coercivity, energy product, temperature coefficients, demagnetization curves, tolerances, coating details, and availability.
- Model the magnetic circuit. Include the air gap, pole pieces, leakage, steel saturation, opposing fields, and the grade’s load line. Ask a magnetics engineer to review high-consequence designs.
- Check thermal and mechanical margins. Analyze peak temperature, irreversible loss, expansion, shock, vibration, retention, and containment.
- Prototype under representative conditions. Measure field or force at the real gap and temperature, then repeat after environmental and mechanical cycling.
- Qualify supply and production readiness. Confirm current availability, lot consistency, inspection data, supplier support, and a second-source plan before freezing the design.
How to decide when the trade-off is close
| If your dominant constraint is… | Start by evaluating… | Then verify… |
|---|---|---|
| Strict rare-earth exclusion and generous space | Ferrite, then Alnico where its circuit behavior fits | Required force or field, brittleness, corrosion, and cobalt restrictions |
| Minimum size or mass | NdFeB grades with adequate temperature and coating capability | Demagnetization curve, peak temperature, coating durability, and retention |
| High operating temperature | SmCo grades; compare against temperature-qualified alternatives | Actual load line, transient heat, cost, rare-earth eligibility, and brittleness |
| Lowest material cost | Ferrite when the larger volume fits | Total assembly cost, yoke size, shipping mass, machining, and test requirements |
| No cobalt as well as no rare earths | Ferrite or an explicitly qualified cobalt-free candidate | Composition certificate and production availability |
| Outdoor or chemically harsh exposure | Ferrite, Alnico, or SmCo with verified environmental compatibility | Coating, edges, adhesive, galvanic couples, and life-test data |
Research-stage rare-earth-free materials: promising, not interchangeable
New materials should be separated clearly from established purchase-ready families. Their reported numbers may describe simulations, thin films, powders, laboratory samples, or development targets rather than a magnet you can order in a qualified grade.
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MnBi
The DOE’s 2015 assessment described MnBi-Co aligned powder at 12.1 MGOe and a sintered bulk magnet at 8.6 MGOe at room temperature. A MnBi-Co double-layer thin film was reported at about 25 MGOe. These values refer to different sample forms and development contexts; they are not interchangeable commercial ratings. The same assessment discussed MnBi development challenges and compared its theoretical single-phase maximum with other families. Treat that work as research evidence, not a drop-in product specification.
Fe-Co-B
An Ames Laboratory research highlight from 2024 reported a maximum energy product of 26 MGOe for a simulated polycrystalline bulk magnet and 18 MGOe for a bonded nanowire magnet. Those are micromagnetic simulation results and potential material-development targets, not standard commercial grades. A simulation does not establish manufacturability, coating durability, lot consistency, or a usable demagnetization curve.
Iron-nickel-nitrogen
A 2020 DOE Ames Laboratory account reported an energy density of 6 MG-Oe for an iron-nickel-nitrogen material described as comparable to Alnico. The account characterized the material as rare-earth- and cobalt-free and quoted scientist Vitalij Pecharsky: “The iron-nickel magnetic alloys found in meteors are rare-earth and cobalt-free, but are highly ordered and take millions of years to produce through natural means.” The reported result does not prove present catalog availability. Pecharsky’s statement that “We see this gaining wide adoption in that gap magnet space” is an expectation, not evidence that widespread adoption has already occurred.
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Before selecting any emerging material, require a current manufacturer datasheet, repeatable production samples, qualification data, environmental results, and a supply plan. DOE project descriptions explicitly include production-readiness evaluation for this reason.
Common comparison mistakes and their fixes
- Choosing by the strongest headline number: Recalculate the operating point in the complete circuit and include temperature and opposing fields.
- Calling SmCo rare-earth-free: Keep composition and performance categories separate; SmCo contains rare-earth elements.
- Ignoring size penalties: Compare total magnet volume, yoke and housing changes, mass, and assembly—not just material price per kilogram.
- Using room-temperature data for a hot assembly: Obtain grade-specific curves at the actual peak temperature and load line.
- Treating a coating as cosmetic: Check thickness, edge coverage, chemical compatibility, and dimensional impact.
- Relying on adhesive alone under shock or rotation: Add mechanical containment and verify cure and thermal-expansion loads.
- Assuming a research result is orderable: Confirm commercial grade, production volume, qualification, and lot consistency.
- Measuring only surface gauss: Measure field or force at the real gap and operating temperature; use circuit analysis for design decisions.
Decision checklist before release
- Material eligibility, including a cobalt decision, is documented.
- The exact grade and current datasheet are identified.
- Field or force is specified at the real working gap and load line.
- Magnet volume, mass, tolerances, coating thickness, and assembly clearances fit.
- Peak and continuous temperatures include a justified demagnetization margin.
- Humidity, chemicals, coating, adhesive, and galvanic interactions are addressed.
- Shock, vibration, rotation, brittleness, and mechanical containment are tested.
- Prototype measurements reproduce the intended geometry and temperature.
- Supplier availability, lot consistency, qualification records, and contingency sourcing are confirmed.
Bottom line
Use ferrite when a larger, lower-cost rare-earth-free magnet fits; evaluate Alnico when its established material behavior and circuit characteristics meet the requirement; use NdFeB when compact size or low mass dominates; and consider SmCo when elevated-temperature demagnetization resistance is worth its cost and rare-earth content. Make the final decision from the complete magnetic circuit, exact grade, temperature, environment, and mechanical design—not from a single MGOe value or the words “rare-earth-free.”
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