Separate three effects of heat
| Effect | What happens | How to distinguish it |
|---|---|---|
| Reversible change | Output changes while the material is at a different temperature | Measure at temperature and again at the original reference temperature |
| Irreversible magnetic loss | Some magnetization does not return on cooling | Compare before/after readings under the same controlled conditions |
| Physical or assembly damage | Corrosion, a failed bond or another material change affects function | Inspect the part, coating, joint and retention as well as magnetic output |
What a lower hot-state reading can mean
Reversible change
- What happens
- Output changes while the material is at a different temperature
- How to distinguish it
- Measure at temperature and again at the original reference temperature
Irreversible magnetic loss
- What happens
- Some magnetization does not return on cooling
- How to distinguish it
- Compare before/after readings under the same controlled conditions
Physical or assembly damage
- What happens
- Corrosion, a failed bond or another material change affects function
- How to distinguish it
- Inspect the part, coating, joint and retention as well as magnetic output
Even a fully reversible change can cause a functional failure while the device is hot. A motor, sensor or holding system must meet its requirement during operation, not merely recover a reading after cooling.[4]
Why one grade can have different stated limits
Published maximum-use temperatures are conditional guidelines. Arnold’s standard material table lists N52 at 60°C; other tables and product descriptions can use different values or assumptions. Dura states a geometry condition for its reference groupings. This is a reason to obtain the proposed material sheet and assess the actual part, rather than average the numbers into a universal limit.[1] [2]
The operating point depends on the magnet shape and magnetic circuit. At higher temperature, intrinsic coercivity generally falls, and the knee of the demagnetization curve changes. An opposing field or an unfavourable operating point can produce permanent losses below a headline temperature rating. The Curie point is a different material boundary and is unsuitable as the design limit.[3]
Use temperature coefficients within their stated range
A reversible Br coefficient can estimate a material change over the range for which it is supplied. It is normally a linear approximation; real temperature behaviour is not linear over an unlimited range. Coercivity has a separate coefficient and must be checked independently.[3]
Illustrative calculation
Assume Br = 1.30 T at 20°C and α(Br) = −0.12%/°C, valid for this example from 20°C to 80°C. At 80°C:
Br ≈ 1.30 × [1 − 0.0012 × (80 − 20)] = 1.2064 T
This estimates a 7.2% reduction in material remanence. It does not predict a 7.2% reduction in pull force, establish an allowable temperature, or test whether irreversible demagnetization occurs.
Obtain the material’s actual curves and coefficient interval before using the calculation for a design. Keep the local magnet temperature distinct from ambient air temperature.
Define the thermal validation before ordering
- Minimum, normal, sustained maximum and transient magnet temperatures.
- Duration, cycling and the cooling or heat-flow conditions.
- Magnet geometry, surrounding steel and the largest or most demanding operating gap.
- Opposing fields, including relevant motor or fault conditions.
- Required hot-state output and permitted permanent change after cooling.
- Coating, adhesive and mechanical-retention temperature limits.
Agree the fixture, reference temperature, measurement locations and sample criteria. Manufacturer guidance also distinguishes material properties from the characteristics of the individual product; the agreed delivery specification and validation need to connect the two.[5]
Send the known conditions with your drawing. A thermal class can then be discussed as part of a material and assembly specification, rather than a standalone promise.
Technical sources
Source material has its own scope and conditions. These references explain technical choices; they are not evidence of our stock, certifications or supplied products.
- Arnold Magnetic Technologies: NdFeB material properties and grade data
- Dura Magnetics: NdFeB grades and reference conditions
- Arnold Magnetic Technologies: Magnetic properties and measuring techniques
- Dura Magnetics: Temperature effects on neodymium magnets
- TDK: NEOREC product guide: materials, measurement and precautions