The Magnet Families, and What Each One Is Actually For
Neodymium magnets are the strongest available, which is not the same as being the right answer. Ferrite outsells them by mass many times over, and one family’s coercivity moves the wrong way with temperature.
Magnets and Motors · Part 2 — The Permanent Magnet · Chapter 5 · 19 min read
Neodymium magnets are the strongest available, which is not the same as being the right answer. Ferrite outsells them by mass many times over, and samarium cobalt holds jobs that neodymium simply cannot do.
398–422
N52 (BH)max, kJ/m³
26–40
Ferrite — a tenth of it
−0.03 %/°C
SmCo reversible loss
+0.40 %/°C
Ferrite H_cJ — the wrong way
5.1 — Energy product against temperature
Nothing occupies the top-right corner. Strong and hot at once remains unsolved, and SmCo buys its temperature capability by giving up half its energy product.
5.2 — The four families
What each family is for, and its disqualifying flaw
| Family | What it is for | The flaw |
|---|---|---|
| Nd₂Fe₁₄B sintered (1983) | (BH)max 240–420 kJ/m³, B_r 1.15–1.48 T, T_C 312 °C. Twice the energy product of anything before it, at a fraction of SmCo’s cost — iron replaced cobalt, and neodymium is far more abundant than samarium. | Corrodes readily and must be nickel- or epoxy-coated. Loses coercivity fast with temperature. Brittle, machined only by grinding. Needs heavy rare earths to run hot. |
| SmCo 1:5 and 2:17 (1966) | (BH)max 120–240 kJ/m³ but T_C 750–920 °C and usable to 350 °C. Reversible loss of only −0.03 %/°C, a quarter of NdFeB’s. Essentially corrosion-proof, so no coating needed. | Cobalt-heavy and expensive, samarium is scarcer than neodymium, energy product roughly half. Aerospace, downhole tools, high-temperature servos — not mass-market traction. |
| Hard ferrite (1952) | (BH)max only 26–40 kJ/m³, but made from iron oxide and barium or strontium carbonate — cheap, abundant, non-strategic, chemically inert, and an electrical insulator, so no eddy-current loss at all. | One-tenth the strength, so the motor must be bigger. And coercivity that falls as it gets colder — ferrite motors demagnetise in winter, not summer. |
| AlNiCo (1931) | B_r up to 1.35 T — higher than many ferrites and comparable to mid-grade NdFeB — with T_C of 860 °C and the flattest temperature coefficient of any family. | Coercivity of only 40–130 kA/m, because its anisotropy is shape-based rather than magnetocrystalline: elongated Fe-Co needles in a non-magnetic matrix. Easy to demagnetise by accident. |
5.3 — The datasheet table
| Family | (BH)max kJ/m³ | B_r T | H_cJ kA/m | T_C °C | Max op °C | α(B_r) %/°C | β(H_cJ) %/°C | Cost |
|---|---|---|---|---|---|---|---|---|
| NdFeB N52 | 398–422 | 1.43–1.48 | ≥876 | 312 | 80 | −0.12 | −0.60 | high |
| NdFeB 42SH | 318–342 | 1.28–1.32 | ≥1,592 | 340 | 150 | −0.11 | −0.55 | higher |
| NdFeB 35UH / 33EH | 247–287 | 1.13–1.21 | ≥2,388 | 350 | 180–200 | −0.10 | −0.50 | highest |
| NdFeB bonded | 40–80 | 0.55–0.70 | 600–800 | 312 | 120 | −0.13 | −0.40 | medium |
| Sm₂Co₁₇ | 200–240 | 1.05–1.12 | 600–2,000 | 920 | 300–350 | −0.030 | −0.20 | very high |
| SmCo₅ | 120–180 | 0.85–1.00 | 1,300–2,400 | 750 | 250 | −0.045 | −0.30 | very high |
| Sr ferrite | 26–40 | 0.38–0.45 | 200–350 | 450 | 250 | −0.20 | +0.40 | very low |
| AlNiCo 5 | 40–52 | 1.25–1.35 | 50–60 | 860 | 500 | −0.02 | −0.02 | medium |
5.4 — Read the ferrite row again
Important
β(H_cJ) is positive — the only positive number in the table. A ferrite magnet is at its most vulnerable to demagnetisation when cold, which inverts every intuition built on neodymium.
A ferrite-magnet motor validated in an Indian summer can fail on a Ladakh winter morning, and the test plan has to be written the other way round.
In plain English
This matters commercially because ferrite-assisted synchronous reluctance is one of the most credible rare-earth-free paths for cost-sensitive vehicles — and anyone adopting it inherits a demagnetisation failure mode that is the temperature mirror image of the one their engineers have spent a decade learning.
5.5 — Why the two coefficients diverge
Technical framing
Coercivity falls roughly five times faster than remanence in NdFeB. That is why a magnet does not gradually fade — it works fine, and then at some temperature and load it crosses the knee and loses a chunk permanently.
SmCo’s flatness is the entire reason it still exists despite half the energy product and several times the price.
Quick check: test yourself
1.Why does ferrite still outsell neodymium by mass?
Show answer
2.A ferrite motor passes demagnetisation testing at 120 °C. Is that reassuring?
Show answer
3.Why does SmCo survive commercially at half the energy product and several times the price?
Show answer
Chapter summary
- ✓Nothing occupies the strong-and-hot corner. SmCo buys temperature by surrendering half its energy product, and that trade has not been beaten.
- ✓NdFeB is the strongest but corrodes, loses coercivity fast with temperature, and needs heavy rare earths to run hot.
- ✓Ferrite is a tenth the strength and costs almost nothing, with no eddy loss because it is an insulating oxide.
- ✓AlNiCo’s anisotropy is shape-based rather than magnetocrystalline, which gives high remanence and very low coercivity.
- ✓Ferrite is the one common magnet whose coercivity falls as it cools — a demagnetisation risk that is the temperature mirror image of NdFeB’s.
Frequently asked questions
Why is ferrite still used when neodymium is ten times stronger?+
Because it is made from iron oxide and barium or strontium carbonate — cheap, abundant, non-strategic, chemically inert, and an electrical insulator, so it has no eddy-current loss at all. Its (BH)max is only 26 to 40 kJ/m³ so the motor must be bigger, but on cost per kilojoule it is the smallest bubble on the chart by a wide margin, and it still outsells everything else by mass.
What makes ferrite dangerous in cold climates?+
Its coercivity temperature coefficient is positive — roughly +0.40 %/°C, the only positive number in the family table. Ferrite’s coercivity falls as it gets colder, so a ferrite-magnet motor is at its most vulnerable to demagnetisation in winter rather than summer. A design validated through an Indian summer can fail on a Ladakh winter morning, and the demagnetisation test plan has to be written the other way round from an NdFeB one.
Why does samarium cobalt still exist if it is half the strength and more expensive?+
Temperature. SmCo has a Curie temperature of 750 to 920 °C and is usable to 350 °C, with a reversible loss of only about −0.03 %/°C — a quarter of NdFeB’s. It is also essentially corrosion-proof, so it needs no coating. That combination keeps it in aerospace, downhole tools and high-temperature servos, where the energy product it gives up is worth less than the temperature headroom it buys.
Why is AlNiCo easy to demagnetise despite its high remanence?+
Because its anisotropy is shape-based rather than magnetocrystalline — elongated needles of Fe-Co in a non-magnetic matrix. That gives remanence up to 1.35 T, comparable to mid-grade NdFeB, but coercivity of only 40 to 130 kA/m. It survives in sensors, guitar pickups and instruments, where its exceptionally flat temperature coefficient matters and stray reverse fields do not.
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Magnets and Motors is an original educational series on permanent magnets and electric machines. Datasheet ranges, temperature coefficients and efficiency figures are representative standard-condition literature values, not measurements of a specific product, and sources differ on several. Always verify against the specific magnet and lamination datasheets in use before making design, procurement or certification decisions.