The five-minute guide to magnet grades

Short versionN for neo, a number for the energy product class, letters for the coercivity class. The temperatures attached to the letters are a rule of thumb, NOT a rating.

N is neo: sintered neodymium-iron-boron, or NdFeB. 35 is the energy product class, and SH is the coercivity class.

The energy product, (BH)max, is the largest value of B times H on the demagnetization curve: roughly, how much field you get per cubic centimeter of magnet. An N35 carries 33 to 36 MGOe and an N52 50 to 53,^[1] so the number names a band, not a minimum, and the bands overlap. Multiply by 7.96 for metric (SI) units: an N35 is about 263 to 286 kJ/m³.

The letters are coercivity

The letters set the floor on intrinsic coercivity, HcJ: how hard you have to push backward on the magnet before its own magnetization gives up. The Chinese national standard names the classes, from low coercivity N through medium (M), high (H), super-high (SH), ultra-high (UH) and extremely high (EH) to "top high" (TH).^[2] Catalogs usually write that last one AH.^[1] What the A stands for, I have not found anybody willing to commit to in writing.

The temperatures get all the attention, and they are the softest numbers in the table. A thin magnet in a motor, with the windings pushing back on it, runs out of coercivity well below the temperature its letters suggest; a fat block in a sensor shrugs off more. What the letters actually promise is a room-temperature HcJ. The rest is inference, and Choosing a magnet is about doing it properly.

One quirk worth knowing: the top grade of a class often carries a lower coercivity floor than the rest of it. In the same table, N52 guarantees 11 kOe where N35 through N48 guarantee 12, and N50H guarantees 16 where the other H grades guarantee 17.^[1] Remanence and coercivity trade against each other, and the top of each class is where the maker spends a little of one to buy the other.

Where the coercivity comes from, and the G

Mostly from dysprosium and terbium, the heavy rare earths. Alloyed through the whole magnet they raise HcJ and lower remanence (Br), which is why the high-letter classes stop at lower numbers. Grain boundary diffusion (GBD) puts them only at the grain boundaries, which is how those classes got higher numbers (The NdFeB grade chart, explained has the story). Arnold marks it with GB in front (GB48SH),^[3] VAC with a D (VACODYM 801 DTP),^[4] others with a G. It works from the surface in, so for a thick part, ask.

Short versionThe R is Recoma, a trademark that became a habit. The number tracks the energy product loosely, and drifts ahead of it at the top, partly on purpose. The letters after it are real differences with uninformative names.

R is Recoma. Brown Boveri trademarked the name in Switzerland in 1972, short for Rare Earth Cobalt Magnets, and it reached Arnold Magnetic Technologies in 2007 by way of Pechiney, Ugimag, Magnequench and Precision Magnetics.^[5] Other makers have borrowed the R for their own samarium cobalt (SmCo), the way people ask for a Kleenex; others write S before the number (S20 to S32 in one European catalog),^[6] XG and XGS,^[7] or just SmCo.^[8]

Under all of those prefixes are two families, named for their atomic ratios: SmCo₅, the 1:5 type, and Sm₂Co₁₇, the 2:17 type.^[9] The 2:17 type is the stronger one. In Recoma's list, 18 through 25 are 1:5 and 24HE on up are 2:17, and Arnold rates most of the 2:17 grades to 350 °C against 250 °C for the 1:5 ones.^[10]

The number

The number is tied to the energy product in MGOe, loosely. Recoma 26 runs 23 to 26 MGOe and Recoma 30 runs 27 to 29.^[10] By the top of the range the name has run ahead of the material: Recoma 35E's own datasheet gives 32.0 MGOe minimum and 33.3 nominal.^[11] Nobody is lying, exactly. The number is a name, and names drift.

The letters

HE was taken to mean high energy, S select and E enhanced. Recoma 30, 30HE and 30S share the same remanence, 1.09 to 1.12 T. What moves is the floor on normal coercivity (HcB), the catalog's way of saying the curve is squarer; the intrinsic coercivity band; and the rated temperature, 250 °C for Recoma 30 against 350 °C for the other two.^[10] Real differences, and also largely branding: the letter doesn't say which property moved, and nobody else's E means what Arnold's does. Read the table, not the letter.

The Chinese-style tables are more honest about it. Their suffixes name a coercivity class, as the neo letters do: XGS28H guarantees 25 kOe and 350 °C, plain XGS28 18 kOe and 300 °C, and M and L step down from there.^[7]

Short versionVACOMAX numbers are the energy product in kJ/m³; divide by 7.96 to line them up with everybody else's. VACODYM numbers name the alloy, NOT the energy. The letters after both are the pressing method.

VAC names its samarium cobalt for the energy product in SI units: VACOMAX 225 HR is 225 kJ/m³ typical, 240 HR is 240, 262 HR is 262.^[4] Divide by 7.96 and they fall in with everyone else's, as a 28, a 30 and a 33, which puts VACOMAX 262 HR beside Recoma 33E. (The older, lower grades run above their names, with 170 HR at 200 kJ/m³ typical, so treat the number as a floor there.)

The letters are not branding at all. They are how the part was pressed: HR is isostatically pressed, TP transverse-pressed, AP axially pressed, and VAC says the isostatic and transverse parts carry about 5 to 8% more remanence than the axial ones.^[4] Same alloy, three values of Br. That holds for everybody's magnets, and most grade names don't tell you which press you're getting.

Now the trap. VACODYM, VAC's neo, uses the same three-digit format, and its number is NOT the energy product: 688 TP is 250 kJ/m³ typical and 722 HR is 415.^[4] The number names the alloy, so to compare with an N grade you read across the table: VACODYM 745 HR, at 1.44 T, 1,115 kA/m (14 kOe) and 400 kJ/m³ (50 MGOe), sits about where an N50M does.^[12] A D before the pressing letters means grain boundary diffusion.

Short versionThere is no single table behind a grade name. Put the minimum properties on the drawing, write the grade as "or equivalent," and buy the lowest coercivity class that clears your requirement.

There is no one table behind a grade name. The standards define the classes; each maker publishes its own numbers inside them. A "28" samarium cobalt magnet guarantees an HcJ of 1,194 kA/m (15 kOe) in one catalog and 1,433 kA/m (18 kOe) in another, under nearly the same name.^[6][7] For most designs that gap is nothing. For the one running near its knee, it's a failed first article.

So put the properties on the drawing: minimum Br, minimum HcJ, and, where the design depends on them, minimum HcB and (BH)max, all at a stated temperature. Then name the grade as "N38SH or equivalent." That tells the supplier what you had in mind without making their table the spec. And write limits as limits: a nominal with no tolerance is a wish.