Thermal expansion is the dimensional change that occurs when a material is heated or cooled. For solid materials, this behavior is commonly described by the linear thermal expansion coefficient, usually expressed in ×10-6/K or ppm/K. In permanent magnet applications, the importance of thermal expansion is not limited to the dimensional change of the magnets themselves. It also depends on how that change compares with the expansion or contraction of surrounding structural materials.
When magnets are bonded, fitted, sleeved, or otherwise constrained within an assembly, differences in thermal expansion can generate internal stress, relative movement, or changes in dimensional clearance. These effects become increasingly important in large assemblies, precision magnetic systems, and components exposed to repeated temperature changes.
Permanent magnet materials do not always expand uniformly in all directions. Neodymium (NdFeB), Samarium Cobalt (SmCo), and ferrite magnets can exhibit anisotropic thermal expansion, with different coefficients parallel and perpendicular to the easy magnetization direction. Neodymium magnets are particularly unusual because their thermal expansion behavior can vary strongly with crystallographic direction and temperature, and negative expansion may occur in certain directions over specific temperature ranges.
Thermal expansion should also be distinguished from the temperature coefficients of magnetic properties such as remanence and intrinsic coercivity. Thermal expansion describes dimensional behavior, while these temperature coefficients describe changes in magnetic performance. Both can affect the reliability of permanent magnet assemblies, but through different mechanisms.
Thermal Expansion Coefficients of Permanent Magnet Materials
The thermal expansion coefficient of permanent magnets varies with material composition, crystal structure, and measurement direction. For anisotropic sintered magnets, values measured parallel and perpendicular to the easy magnetization direction can differ significantly. The values below are representative rather than universal constants, as exact coefficients may vary with grade, composition, and temperature range.
| Permanent Magnet Material | Parallel to Easy Magnetization Direction | Perpendicular to Easy Magnetization Direction | Typical Characteristics |
| Neodymium magnets | ~5–7 ×10-6/K | ~0 to negative values in some temperature ranges | Strongly anisotropic and temperature-dependent thermal expansion |
| SmCo5 magnets | ~6–7 ×10-6/K | ~14–15 ×10-6/K | Pronounced thermal expansion anisotropy |
| Sm2Co17 magnets | ~10–11 ×10-6/K | ~12–13 ×10-6/K | Lower anisotropy than SmCo₅ |
| Ferrite magnets | ~15 ×10-6/K | ~10 ×10-6/K | Direction-dependent thermal expansion |
| AlNiCo magnets | ~11–12.5 ×10-6/K | – | Typical values vary with grade; directional data are not always specified |
For comparison, common structural materials also show substantially different thermal expansion behavior. Typical linear thermal expansion coefficients are approximately 12 ×10-6/K for steel, 17 ×10-6/K for copper, and 23–24 ×10-6/K for aluminum. These differences become important when magnets are rigidly bonded or mechanically constrained within components made from these materials.
The table also shows why a single thermal expansion value is not always sufficient for permanent magnets. SmCo5 exhibits a much larger difference between the two principal directions than Sm2Co17, while Neodymium magnets can show even more unusual temperature-dependent behavior. These effects are closely related to the crystallographic and magnetic anisotropy of the materials.
Direction-Dependent Thermal Expansion in Permanent Magnets
Permanent magnets do not always expand at the same rate in every direction. In Neodymium, Samarium Cobalt, and ferrite magnets, the thermal expansion coefficient can differ between the direction of easy magnetization and the direction perpendicular to it.
This difference can be significant. SmCo5, for example, expands much less along the easy magnetization direction than across it, while Sm2Co17 shows a smaller directional difference. Ferrite magnets also exhibit different expansion coefficients in the two directions.
For assembly design, this means that the relevant thermal expansion coefficient should be matched to the actual orientation and dimensions of the magnets. This is especially important where magnets are tightly fitted, bonded, or constrained by surrounding components.
Neodymium magnets show even more unusual temperature-dependent behavior, including very low or negative expansion in certain directions and temperature ranges.
Thermal Expansion Mismatch in Permanent Magnet Assemblies
Thermal expansion becomes an assembly issue when magnets and surrounding structural materials expand or contract at different rates. Steel, aluminum, copper, plastics, adhesives, and permanent magnets can all have different thermal expansion coefficients, so temperature changes may alter clearances, contact pressure, or the stress carried by bonded and mechanically constrained interfaces.
The effect depends not only on the difference in thermal expansion coefficients, but also on the size of the components, the temperature range, and how strongly the magnets are constrained.
In bonded assemblies, repeated expansion and contraction can place additional stress on the adhesive layer. In press fits or rigid housings, thermal mismatch can change interference or clearance. Retaining sleeves and other containment structures can also experience changing contact conditions as the assembly heats and cools.
These effects become more important when assemblies operate over a wide temperature range or undergo repeated thermal cycling. A design that remains stable at room temperature may experience different mechanical conditions at elevated or reduced temperatures, especially when materials with significantly different expansion rates are combined.
Thermal expansion should therefore be considered together with dimensional tolerances and assembly constraints. These dimensional effects are separate from temperature-induced changes in magnetic properties, although both can occur simultaneously during service.
