Magnet Temperature Selector

How the selector decides

For each material the tool checks your highest continuous temperature against the continuous limit and, if you enter one, your short-term peak against the datasheet maximum. A material is marked Suitable when the headroom is at least the planning margin in both checks, Check when it is above your temperature but with less margin than recommended, and Not suitable when the limit is below your temperature. The same test is applied at the cold end, using the low-temperature behaviour of each material.

The planning margin is our own engineering rule of thumb, not a standard: 10 K for a favourable magnet (thick, or in a closed steel circuit), 20 K for a typical free magnet, 40 K for a thin, flat magnet in open air, and a further 20 K when an opposing field is present. A magnet with a low permeance coefficient works lower on its demagnetisation curve, so it loses more at the same temperature. Manufacturers also state that the maximum working temperature depends on the circuit in which the magnet operates [1].

Datasheet maximum versus continuous operation

Manufacturers state a single maximum operating temperature per grade suffix. It is a general guideline, not an exact limit: it depends on the shape of the magnet and its magnetic circuit, and the figure alone does not say for how long the magnet can stay at that temperature [9], [10]. For example, an N42H magnet is commonly quoted at 120 °C, but this assumes a length-to-diameter ratio above about 0.7; with a lower working point the usable limit can fall to about 75 °C [10].

Sintered NdFeB also shows time-dependent losses: at a constant elevated temperature the flux keeps falling slowly, roughly with the logarithm of time. In a doctoral study of four sintered NdFeB materials at a permeance coefficient of 1.1, the extrapolated 30-year loss stayed below 1 % only below a critical temperature of about 80, 130, 160 and 220 °C for materials of increasing coercivity (1,240, 1,540, 1,900 and above 3,000 kA/m). Above it the losses grew with temperature: one material at 100 °C lost about 5 % after one hour and almost 10 % over 30 years [8]. Matching those coercivities to typical suffix values by interpolation (our own, indicative only) puts the critical temperatures roughly 10 to 40 K below the suffix rating.

For this reason the selector asks for two temperatures. The highest short-term peak is compared with the datasheet maximum. The highest continuous (24/7) temperature is compared with the datasheet maximum reduced by 20 K, and the same planning margin is added on top. The 20 K reduction is our conservative planning rule, not a published standard. It is applied to every material for simplicity, although long-term data for materials other than NdFeB are less documented. For continuous service close to the limit, choose a higher-coercivity suffix and ask the manufacturer for demagnetisation curves and long-term data at your temperature. Example: for a typical free magnet at a continuous 120 °C the datasheet maximum should be at least 160 °C (120 + 20 + 20), so SH (150) shows Check and UH (180) shows Suitable.

Data used

α(Br) is the reversible temperature coefficient of remanence. Values are typical and differ between grades and suppliers. Ratings are manufacturer figures for reference and can differ by supplier: for example, one ferrite supplier states 225 °C for all of its grades [5], and the highest NdFeB suffix (AH) is only offered by some producers.

Why the cold end matters too

Sintered NdFeB undergoes a spin reorientation at about −138 °C (135 K); below it the flux can fall by up to about 14 %, reversibly on warming [7]. Ferrite behaves the opposite way to other magnets: its coercivity falls as it gets colder, so at about −60 °C it has lost roughly a third of its room-temperature value and mild irreversible flux loss is reported [7]. Flexible strips and tapes are tested down to −40 °C [6].

Starting points for continuous duty with a typical free magnet and the 20 K planning margin: 60 °C points to M, 85 °C to SH, 120 °C to UH and 150 °C to EH; around 250 °C continuous points to Sm₂Co₁₇, and furnace-side parts at 400 °C to AlNiCo. These are starting points, not a substitute for the datasheet.

References

1. [1]  Eclipse Magnetics Ltd. (n.d.). Sintered Neodymium Iron Boron (NdFeB) Magnets [Technical data sheet]. Sheffield, England. https://www.eclipsemagnetics.com/site/assets/files/2399/neodymium_grades_data.pdf
2. [2]  Arnold Magnetic Technologies. (2015). Temperature effects on magnetic output (TECHNotes TN 0303, rev. 2015a). https://www.arnoldmagnetics.com/wp-content/uploads/2017/10/TN_0303_rev_150715.pdf
3. [3]  Eclipse Magnetics Ltd. (n.d.). Technical data sheet: Samarium cobalt magnets, SmCo magnets. https://www.eclipsemagnetics.com/site/assets/files/19544/eclipsemagnetics_smco_datasheet.pdf
4. [4]  Eclipse Magnetics Ltd. (n.d.). Alnico magnets: Data sheet. https://www.eclipsemagnetics.com/site/assets/files/22210/eclipsemagnetics_alnico_data_sheet.pdf
5. [5]  Goudsmit Magnetics. (2025). Goudsmit ferrite grade system [Grades available at Goudsmit, 22 August 2025]. https://www.goudsmitmagnetics.com/uploads/pdf/ferrite_grades_available_at_goudsmit_20250822_4120.pdf
6. [6]  Arnold Magnetic Technologies. (2025). Plastiform datasheet (US). https://www.arnoldmagnetics.com/wp-content/uploads/2025/05/Plastiform-Datasheet-US-2025.pdf
7. [7]  Trout, S. R., & Constantinides, S. (2015). Using permanent magnets at low temperature (TECHNotes TN 0302, rev. 2015a; original work 2003). Arnold Magnetic Technologies. https://www.arnoldmagnetics.com/wp-content/uploads/2017/10/TN_0302_rev_150715.pdf
8. [8]  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
9. [9]  K&J Magnetics, Inc. (n.d.). Temperature and neodymium magnets. https://www.kjmagnetics.com/blog/temperature-and-neodymium-magnets
10. [10]  Liu, J. (2016). Some design considerations using permanent magnets. Magnetics Magazine. https://magneticsmag.com/some-design-considerations-using-permanent-magnets/