Choosing a Neodymium Magnet

1. Start with the job

Before looking at grades, write down what the magnet must do: hold a part, lift a load, trigger a sensor, drive a motor or couple two shafts. Then note four things: the size you have room for, the force or field you need, the temperature range it will live in, and the environment (dry indoor air, humidity, water, oil, chemicals). These four answers decide almost everything else, and the last two are the ones most often forgotten.

2. What the grade number tells you

In a name such as N42, the number is roughly the maximum energy product in MGOe, so N42 is about 42 MGOe or 334 kJ/m³. A higher number means a stronger magnet for the same size, but it does not mean a better magnet for every job. In manufacturer tables the very high-energy grades are rated for lower temperatures: for example one producer lists N35 to N48 at 80 °C, but N50 and N52 at 70 °C [1]. Another source notes that, especially in thin magnets, N52 can have a lower maximum operating temperature than N42 [2].

3. What the suffix letters tell you

The letters after the number describe intrinsic coercivity, that is, how strongly the material resists demagnetisation. In order they are: none (standard), M, H, SH, UH, EH and AH. A producer's reference maximum operating temperatures are [1]:

Two things stand out in the table. First, higher temperature capability comes with a lower maximum strength: in this producer's range the EH and AH grades are offered only at lower energy products. Second, these temperatures are reference values, not guarantees for your magnet; the producer itself notes that the maximum working temperature depends on the circuit in which the magnet operates [1]. The next two sections explain why.

4. Why shape changes the temperature limit

A magnet's working point on its demagnetisation curve depends on its shape and on what surrounds it. This is described by the permeance coefficient (Pc): tall, narrow magnets, or magnets in a closed steel circuit, have a high Pc; thin, wide magnets in open air have a low one. A low Pc sits lower on the curve and reaches the knee, where permanent loss begins, at a lower temperature [2].

K&J Magnetics gives a clear example: an N42 disc 1/2 in in diameter and 1/8 in thick (Pc 0.61) reaches the knee at 80 °C, so 80 °C is its maximum operating temperature, and at 100 °C its strength falls to roughly 60 % of the original. An N42 cylinder with a Pc of 3.46 showed no temperature losses until above 140 °C [2]. Another author notes that the commonly quoted 120 °C for N42H assumes a length-to-diameter ratio above about 0.7; with a lower working point the usable limit can be around 75 °C [3].

The consequence is simple: the same grade can be safe as a tall cylinder and unsafe as a thin disc at the same temperature. If your magnet is thin and sits in open air, move up at least one suffix.

5. Continuous or peak temperature

Datasheet maximum temperatures do not say for how long a magnet may stay at that temperature. For sintered NdFeB, flux also keeps falling slowly at a constant elevated temperature, roughly with the logarithm of time. In a doctoral study at a permeance coefficient of 1.1, the extrapolated 30-year loss stayed below 1 % only below a critical temperature that rose with coercivity, about 80 to 220 °C for the four materials tested. Above it, losses grew with temperature: one material at 100 °C lost about 5 % after one hour and almost 10 % over 30 years [4].

So a magnet that runs at a temperature 24 hours a day needs more margin than one that only sees that temperature briefly. Our Temperature Selector asks for both values: the highest continuous temperature and the highest short-term peak. It subtracts 20 K from the datasheet maximum for continuous duty and adds a planning margin for shape and opposing fields. These two rules are our own conservative planning choices, not published standards, and the tool says so.

6. Coating and environment

Sintered NdFeB corrodes easily, so it is almost always coated. Nickel-copper-nickel is the most common finish; epoxy, zinc and other coatings are also used. Choose the coating from the environment, not from habit: a dry indoor assembly and a part that sees humidity, condensation or washing are different problems. A coating does not change the magnetic grade, but a chipped coating on a brittle magnet is where corrosion starts, so ask how the magnets are packed and how they should be handled.

Also check the temperature limit of anything attached to the magnet. Adhesives and plastic housings often give up before the magnet does.

7. Pull force in real life

The pull force on a datasheet is measured by pulling the magnet straight off a thick, flat, clean steel plate under ideal conditions. In your assembly it will be lower, because of air gaps, coatings, paint, rough, thin or curved steel, the steel grade and temperature. The force needed to slide the magnet sideways (shear) is usually much lower than the direct pull. Design with a safety factor and test the real assembly. Details are in the glossary entry on pull force.

8. What to send us when you ask for a quote

The more of this list you send, the faster and more accurate the quote:

• Shape and dimensions with tolerances (for example diameter, thickness, length, width, inner diameter).
• Grade and suffix, or the force and temperature if you want us to suggest one.
• Direction of magnetisation (through the thickness, across the diameter, radial, multi-pole).
• Highest continuous temperature and highest short-term peak, and whether an opposing field is present.
• Coating and the environment the part will see.
• Quantity per order and per year, packaging needs and any drawings or samples.

9. Common mistakes

• Choosing the highest N number and ignoring temperature.
• Using the datasheet maximum temperature as a continuous operating temperature.
• Ignoring shape: a thin disc loses its margin much faster than a tall cylinder.
• Forgetting an opposing field in motors, generators and magnetic couplings.
• Expecting datasheet pull force in a real assembly with a gap, paint or thin steel.
• Skipping a sample test under real conditions.

Frequently asked questions

Is a higher N number always better for a neodymium magnet?

No. A higher number means more strength for the same size, but the very high-energy grades are rated for lower temperatures: one producer lists N35 to N48 at 80 °C, but N50 and N52 at 70 °C. Choose the lowest grade that gives the force you need with a reserve, then spend the margin on temperature and coating.

What do the letters M, H, SH, UH, EH and AH mean?

They show intrinsic coercivity, that is, how strongly the material resists demagnetisation. One producer’s reference maximum temperatures are 80 °C for standard grades, 100 °C for M, 120 °C for H, 150 °C for SH, 180 °C for UH, 200 °C for EH and 220 °C for AH. These are reference values, not guarantees, and depend on the circuit the magnet works in.

Why does a thin magnet lose strength at a lower temperature than a tall one?

A thin magnet in open air has a low working point on its demagnetisation curve, so it reaches the knee, where permanent loss begins, at a lower temperature. K&J Magnetics gives an N42 disc 1/2 in in diameter and 1/8 in thick that reaches the knee at 80 °C, while a tall N42 cylinder showed no losses until above 140 °C. For a thin magnet in open air, move up at least one suffix.

References

1. [1]  Eclipse Magnetics Ltd. (n.d.). Sintered Neodymium Iron Boron (NdFeB) Magnets [Technical data sheet]. https://www.eclipsemagnetics.com/site/assets/files/2399/neodymium_grades_data.pdf
2. [2]  K&J Magnetics, Inc. (n.d.). Temperature and neodymium magnets. https://www.kjmagnetics.com/blog/temperature-and-neodymium-magnets
3. [3]  Liu, J. (2016). Some design considerations using permanent magnets. Magnetics Magazine. https://magneticsmag.com/some-design-considerations-using-permanent-magnets/
4. [4]  Haavisto, M. (2013). Studies on the time-dependent demagnetization of sintered NdFeB permanent magnets (Publication 1180) [Doctoral dissertation]. Tampere University of Technology. https://neorem.fi/wp-content/uploads/2023/01/Studies-on-the-Time-Dependent-Demagnetization-of-Sintered-NdFeB-Permanent-Magnets.pdf
5. [5]  Coey, J. M. D. (2010). Magnetism and magnetic materials. Cambridge University Press.
6. [6]  Cullity, B. D., & Graham, C. D. (2009). Introduction to magnetic materials (2nd ed.). Wiley-IEEE Press.

Important notice: This guide is general technical information. Values are typical figures taken from the cited sources and simplified for readability; they differ between manufacturers and are not a specification, an offer or a performance guarantee. They do not bind Rami Technics. Confirm every design decision against the datasheet and demagnetisation curves of the exact grade and shape, and test a sample under real operating conditions.