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Choose a magnet by the field or force it must produce in its actual magnetic circuit—not by the words “rare-earth” or a single strength number. Define the working gap, available volume and mass, temperature, corrosion exposure, mechanical loads, and sourcing restrictions first. Then compare specific grades using manufacturer demagnetization curves and energy-product data.
Ferrite and Alnico are established rare-earth-free families. NdFeB and SmCo contain rare-earth elements. Ferrite is often the economical option when a larger magnet is acceptable; NdFeB can deliver the smallest or lightest design; and SmCo is usually the safer choice than NdFeB when high temperature threatens demagnetization. The correct decision depends on the complete circuit, grade, geometry, and operating conditions.
What “rare-earth-free” means—and what it does not
“Rare-earth-free” describes composition, not a 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 has different properties from NdFeB.
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Rare-earth avoidance can be a hard requirement, a supply-chain preference, or a way to reduce cost and sourcing risk. Write the rule down before screening materials. If the project also excludes cobalt, say so separately: both Alnico and SmCo contain cobalt, while the iron-nickel material discussed by Ames Laboratory is described as rare-earth- and cobalt-free. A design that is “free of rare earths” is not necessarily cobalt-free.
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Material family is only the first filter. Magnet grade, dimensions, pole arrangement, air gap, steel parts, temperature, and the load line determine the field that reaches your mechanism. A catalog maximum energy product is useful for narrowing candidates, but it is not a prediction of force or flux in your finished assembly.
Use energy product as a screening number, not a final answer
Maximum energy product, commonly written in megagauss-oersteds (MGOe), combines a magnet’s flux density and resistance to demagnetization. A higher value can allow a smaller or lighter magnet for a comparable operating point. It does not by itself tell you the field at your gap, the pull force on your target, the temperature margin, or whether the magnet will survive assembly.
The U.S. Department of Energy’s 2015 assessment gives these broad, typical room-temperature ranges for commercially available magnets:
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| Family | Typical room-temperature maximum energy product | How to interpret it |
|---|---|---|
| Alnico | 1–8 MGOe | Established rare-earth-free family; the broad range spans different grades. |
| Ferrite | 3–4 MGOe | Established ceramic family; generally requires more volume than a higher-energy material for the same magnetic task. |
| SmCo | 15–30 MGOe | Rare-earth family; high energy product and strong high-temperature demagnetization resistance relative to NdFeB. |
| NdFeB | 25–50 MGOe | Rare-earth family with the highest typical range in the assessment; often the compact-design benchmark. |
These are family-level ranges reported by the DOE, not guaranteed values for every grade or shape. Compare the manufacturer’s B-H and demagnetization curves at the intended temperature and load line. If a supplier gives only a surface-field number, ask for the grade, test geometry, pole configuration, and measurement distance before using it in calculations.
A project-by-project selection workflow
1. Specify the required field or force at the working gap
Start with the result your mechanism needs: field at a sensor, holding force at a specified separation, torque in a motor, or flux through a core. Record the gap, pole-face area, magnet dimensions, steel path, and any nonmagnetic layers such as paint, plastic, adhesive, or an air-filled clearance. The same magnet can produce very different results when the gap or return path changes.
For a consequential design, have a magnetics engineer model the complete circuit. The magnet’s operating point is set by the intersection of its demagnetization curve and the circuit’s load line. A larger magnet is not automatically safer if the circuit leaves it vulnerable to a reverse field.
2. Set the envelope for size and mass
Measure the maximum length, width, thickness, and mass that the assembly can accept. NdFeB’s higher energy product can reduce volume and weight, which matters in compact sensors, actuators, and rotating equipment. Ferrite can be the better economic choice when the enclosure has room for a larger magnet and added mass is acceptable.
Compare candidates at the required operating point, not by comparing equal-size blocks. A small NdFeB part and a large ferrite part may provide the same useful field, while differing in retention, cost, shock tolerance, and thermal behavior.
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3. Define normal, peak, and transient temperatures
List the continuous temperature, expected peaks, heating during startup or overload, and the temperature of adjacent steel, adhesive, and housing materials. Magnetic properties change with temperature, and the ranking of materials can change with it. The DOE notes that SmCo resists demagnetization at higher temperatures better than NdFeB, while NdFeB’s higher energy product can reduce size and weight.
Do not substitute a generic “maximum temperature” for a design check. Obtain the exact grade’s reversible temperature coefficients, irreversible-loss data, maximum operating temperature, and demagnetization curves. Apply those curves at the actual load line and include a safety margin for hot spots and transients. Engineering guidance warns that a general temperature recommendation without detailed circuit analysis can be misleading.
4. Account for humidity, chemicals, and corrosion
Ask where the magnet will operate: dry indoor air, condensation, salt spray, process chemicals, outdoor exposure, or immersion. Corrosion can reduce dimensions, weaken a coating, contaminate an adhesive joint, and change the magnetic circuit.
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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallFerrite, Alnico, and SmCo generally have good corrosion resistance, but confirm the chosen grade and finish. NdFeB usually needs effective surface protection for most applications. Check coating type and thickness, edge coverage, pores, machining damage, cleaning chemicals, and adhesive compatibility. A coating also consumes part of a tight gap, so include its tolerance in the mechanical drawing.
5. Check mechanical loads and mounting
List shock, vibration, centrifugal loads, repeated impacts, clamp forces, and thermal expansion. Many permanent magnets are brittle even when their magnetic properties are excellent. A ferrite or rare-earth block can chip under a point load; a high-speed rotor can require a nonmagnetic sleeve or other positive containment instead of adhesive alone.
Design the retention method with the magnet’s failure mode in mind. Control fit, corner clearance, adhesive cure temperature, surface preparation, and cure shrinkage. Check the coefficient of thermal expansion of the magnet, adhesive, housing, and steel return path. A joint that is safe at room temperature can crack or preload the magnet when the assembly cycles hot and cold.
6. Decide whether rare-earth avoidance is mandatory
If rare earths are prohibited, remove NdFeB and SmCo from the eligible set before optimizing performance. If avoidance is only a preference, keep a rare-earth candidate as a benchmark so that the size, mass, and cost of the restriction are visible. Document which elements are excluded and whether cobalt is restricted as well.
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Before freezing the design, obtain a current manufacturer datasheet, grade designation, tolerance information, coating specification, lot consistency, and availability statement. Ask whether the supplier can provide samples from the same production route and whether the grade is qualified for the temperature and environment you specified.
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Separate established catalog families from research materials. A simulation, laboratory sample, or prototype plan is not evidence of a drop-in commercial magnet. Production-readiness evaluation is a distinct engineering task, not an assumption.
How the established magnet families compare
Ferrite: economical when space is available
Ferrite is a ceramic, rare-earth-free family with a DOE typical room-temperature energy-product range of 3–4 MGOe. Its usual advantage is cost and broad corrosion resistance, making it a practical first candidate for holders, speakers, sensors, and other assemblies that can accept more volume.
The trade-off is size and brittleness. For the same magnetic requirement, a ferrite design may need a larger pole area or thicker magnet than an NdFeB design. Check the ceramic part for chipping, crushing, and impact during insertion. Specify the actual grade and geometry rather than assuming every ferrite magnet has the same coercivity or temperature behavior.
Alnico: established and rare-earth-free, but circuit-sensitive
Alnico is an established rare-earth-free family. The DOE lists a broad typical room-temperature range of 1–8 MGOe. That lower family range does not automatically disqualify it: a suitable Alnico grade can work when the magnetic circuit, dimensions, and operating conditions fit.
Pay particular attention to coercivity and the actual load line. A lower energy-product figure alone cannot decide suitability. Have the supplier provide the relevant demagnetization curves and verify that nearby fields, assembly steps, and temperature excursions will not push the operating point into an irreversible-loss region.
NdFeB: the compact, lightweight benchmark
NdFeB has the highest typical room-temperature energy-product range in the DOE comparison, 25–50 MGOe. That makes it the natural benchmark when a project is constrained by volume or mass. A smaller magnet can simplify packaging, reduce moving inertia, or leave more room for electronics.
Its advantages come with design obligations. Temperature can reduce available margin, and the material generally needs effective corrosion protection. Specify the grade, coating, maximum operating temperature, reversible and irreversible temperature behavior, and demagnetization curve. Check coating thickness and chipped edges in the final gap, and do not treat a room-temperature surface-field reading as proof that the assembled part will retain its field.
SmCo: a high-temperature rare-earth option
SmCo is a rare-earth family with a DOE typical room-temperature energy-product range of 15–30 MGOe. It generally resists demagnetization at high temperature better than NdFeB, so it is worth considering when the magnet sits near a hot motor, furnace, engine, or other heat source.
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SmCo still requires grade-specific analysis. Compare its dimensions, mass, cost, mechanical fragility, and supply constraints with the alternatives. Verify the full temperature and load-line curves rather than selecting it solely because it is labeled “high temperature.”
Research-stage rare-earth-free alternatives
New materials can change the design space, but their evidence status must remain explicit. The figures below are research results in particular forms, not general commercial ratings:
| Material or sample | Reported result | Status and limitation |
|---|---|---|
| MnBi-Co aligned powder | 12.1 MGOe at room temperature | DOE 2015 assessment; powder form and development context matter. |
| MnBi-Co sintered bulk magnet | 8.6 MGOe at room temperature | Different sample form from the aligned powder; not interchangeable with that figure. |
| MnBi-Co double-layer thin film | About 25 MGOe | Thin-film result, not a rating for a bulk purchase-ready magnet. |
| Fe-Co-B simulated polycrystalline bulk magnet | 26 MGOe | Ames Laboratory 2024 micromagnetic simulation; potential development result. |
| Fe-Co-B bonded nanowire magnet | 18 MGOe | Ames Laboratory 2024 simulation; not a standard catalog grade. |
| Iron-nickel-nitrogen material | 6 MG-Oe energy density | Ames Laboratory 2020 report, described as comparable to Alnico; current catalog availability is not established. |
The DOE’s MnBi discussion also notes that the material’s theoretical single-phase maximum is higher than ferrite and Alnico but about half that of NdFeB and SmCo. That is a theoretical comparison, not a guarantee for a commercial grade. Treat Fe-Co-B and iron-nickel-nitrogen as development options until a supplier can document reproducible production, tolerances, qualification, and availability.
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Build a decision matrix before requesting quotes
Put the requirements in a one-page matrix and score only what can be supported by a datasheet or calculation. A useful layout is:
| Requirement | Record | Why it can eliminate a candidate |
|---|---|---|
| Field or force | Value at the specified gap, with tolerance | A catalog rating may not meet the assembled geometry. |
| Envelope | Maximum dimensions and mass | Ferrite or Alnico may need more volume; NdFeB may offer a smaller design. |
| Temperature | Continuous, peak, transient, and margin | Demagnetization risk differs by grade and family. |
| Environment | Humidity, chemicals, salt, immersion, cleaning | Corrosion or coating failure can shorten life and alter fit. |
| Mechanical duty | Shock, vibration, rotation, thermal cycling | Brittle magnets and adhesive joints can fail even when magnetic data is adequate. |
| Composition | Rare-earth and cobalt restrictions | Determines which families are eligible. |
| Supply and qualification | Current datasheet, lot control, lead time, production evidence | Research samples and obsolete grades cannot support a production design. |
Use “pass,” “fail,” or “needs analysis” instead of a vague overall score. A candidate that passes field and size but needs a coating study is not equivalent to one that passes every requirement. Keep the unresolved item visible and assign an owner before release.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Prototype measurements that answer the right question
A handheld gauss meter can be useful for checking a prototype, matching poles, or finding assembly mistakes. Measure at a defined location and orientation, and record the fixture, gap, temperature, and instrument uncertainty. A field reading alone does not reveal energy product, coercivity, corrosion life, operating temperature, or complete-circuit suitability.
For force measurements, reproduce the real mating material, contact area, surface finish, and gap. For temperature validation, measure the magnet itself rather than only the housing. Compare before-and-after readings after the planned thermal and mechanical cycles, and investigate any permanent loss instead of accepting a lower value as normal variation.
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Common comparison mistakes and their fixes
Choosing by the strongest label
Problem: “Super strong” or a high surface-field number is treated as a complete specification.
Fix: Request the grade’s energy product, coercivity, remanence, demagnetization curves, dimensions, and test conditions. Calculate or simulate the operating point in the actual circuit.
Comparing equal-size samples instead of equal performance
Problem: A small rare-earth magnet is compared with a same-size ferrite part even though the project needs a fixed field at a specific gap.
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Fix: Compare candidate volumes and masses that meet the same field or force requirement, then add cost, temperature, corrosion, and retention constraints.
Ignoring the hottest part of the cycle
Problem: Room-temperature data is used for a motor, actuator, or enclosure that heats during operation.
Fix: Use peak and transient temperatures, the exact grade’s curves, and a safety margin. Consider SmCo when high-temperature demagnetization resistance is decisive, but still verify the full design.
Forgetting the coating and adhesive
Problem: A coated NdFeB part is modeled at its bare dimensions, or an adhesive is assumed to survive without checking cure and thermal cycling.
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Calling a research result a product
Problem: A simulated or laboratory energy-product value is used as though it were a stocked, qualified grade.
Fix: Require current supplier documentation, production route, tolerances, lot consistency, and qualification evidence. If those are unavailable, label the material as a development option and do not base a production commitment on it.
When each family is the sensible starting point
- Start with ferrite when cost and corrosion resistance matter, the design can accept a larger magnet, and ceramic brittleness can be managed.
- Start with Alnico when a proven rare-earth-free family fits the geometry and the circuit can be checked carefully for demagnetization.
- Use NdFeB as the compact-design benchmark when volume or mass is the dominant constraint, provided temperature and corrosion protection are engineered.
- Evaluate SmCo when high-temperature resistance to demagnetization is more important than the smallest size, lowest cost, or avoidance of rare-earth elements.
- Investigate MnBi, Fe-Co-B, or iron-nickel-nitrogen only with an explicit development plan and evidence that the exact material form can be produced, qualified, and supplied.
Final selection rule
The best magnet is the one that meets the required field or force in the complete circuit with margin against temperature, corrosion, demagnetization, and mechanical failure. Use the DOE family ranges to screen options, then select a specific grade from current manufacturer data. If rare-earth avoidance is mandatory, begin with ferrite or Alnico and document any size, mass, or circuit changes. If compactness dominates, benchmark NdFeB; if high temperature dominates, compare SmCo; and if a research material is attractive, treat production readiness as a separate requirement rather than an assumption.
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