Share
How magnets behave under temperature: the relationship between dimensions and operating point
The behavior of a magnet under temperature conditions does not depend only on the material. The same magnetic alloy can behave in very different ways depending on the component dimensions, the geometry of the magnetic circuit in which it is installed, and the operating point occupied by the magnet on the demagnetization curve. Understanding this relationship is essential for correctly sizing an industrial magnet and avoiding irreversible losses or product malfunctions during operation.
This article explores the relationship between the BH curve, operating point, magnet geometry, and temperature effects, including the role of magnetic simulation in magnetic product development services supporting designers.
The demagnetization curve and the magnet operating point
To understand how a magnet behaves under temperature conditions, it is necessary to start from the demagnetization curve, which corresponds to the second quadrant of the hysteresis loop. It is the graphical representation of the relationship between magnetic induction B and magnetic field H when the magnet is exposed to a field opposite to its own magnetization direction. This curve provides the three main parameters that describe the material.
BH curve: combines three key pieces of information: residual induction Br (the magnetic flux retained by the magnet when the external field is zero), coercive force Hc (the intensity of the opposing magnetic field required to reduce induction to zero), and the maximum energy product BHmax, which measures the maximum magnetic energy stored per unit volume.
Each magnet family has its own curve and reacts differently to temperature variations.
Operating point: the exact point on the demagnetization curve where the magnet operates once integrated into its application. It is the intersection between the BH curve and the load line, a line passing through the origin whose slope depends on the magnet geometry and the magnetic circuit.
Knee point: the critical point of the BH curve. Beyond this point, a small variation in field strength or temperature causes a sharp drop in induction, creating more or less permanent damage.
Effect of temperature on magnetic properties
Temperature is the environmental variable that most strongly affects permanent magnet performance. As temperature increases, thermal agitation disrupts the alignment of magnetic domains, reducing Br and Hc.
Temperature management is one of the key elements in permanent magnet design, where material selection, magnetic characteristics, and operating conditions determine the final performance of the component.
As long as the operating point remains above the knee point, temperature-related variations are reversible: when returning to room temperature, the magnet recovers its original properties. When the operating point falls below the knee point, however, part of the magnetization is lost irreversibly.
Keeping the operating point far away from the knee point is one of the primary objectives of magnetic sizing.
Curie temperature and permanent losses
Above a critical threshold called the Curie temperature, the magnet completely loses its magnetic properties due to structural changes in the material. At this point, remagnetization is no longer possible.
For NdFeB, the Curie temperature is around 310°C; for SmCo, it exceeds 700°C; for Ferrite, it reaches approximately 450°C; and for AlNiCo, it reaches around 850°C.
This represents an absolute material limit, but in real applications the actual operating constraint is the maximum working temperature, which is always significantly lower.
Proper temperature management therefore requires an integrated evaluation of the magnetic material, operating conditions, and component characteristics. For applications where reliability and long-term stability are critical requirements, a preliminary analysis of magnetic behavior helps identify the most suitable solution from the early stages of the design process.
To further explore these aspects and assess the correct magnet sizing, it is possible to get in touch with CIBAS Group specialists.
Dimensions and geometry: how they influence the operating point
This is the aspect that is often overlooked. The maximum operating temperature stated in a material datasheet assumes a specific magnet geometry and magnetic circuit. Changing these conditions means changing the operating point, and therefore also the actual temperature at which the magnet experiences irreversible losses.
Two magnets made from the same material can behave completely differently if their geometries are different.
The L/D ratio and permeance factor
The ratio between the magnet length in the magnetization direction (L) and its diameter or cross-sectional dimension (D) determines the magnet permeance factor, meaning the slope of the load line.
High L/D ratio means an elongated magnet, a steep load line, and an operating point close to Br: the magnet uses a large portion of its potential and is less sensitive to thermal variations.
Low L/D ratio means a flat or thin magnet, a lower load line, and an operating point farther from Br: the magnet operates in a region closer to the knee point of the curve, with a higher risk of irreversible losses.
At the same ambient temperature, a thin NdFeB disc may begin losing performance at temperatures lower than those indicated by the material grade, while the same material in an elongated magnet operates safely within its reversible region.
This principle influences the choice between sintered NdFeB magnets and sintered SmCo magnets: in unfavorable geometries (thin magnets, large air gaps), SmCo recovers additional margin thanks to its significantly lower knee point.
The role of the magnetic circuit and air gap
The external magnetic circuit also affects the operating point. A magnet installed in a closed ferromagnetic circuit experiences a steep load line and a high operating point; the same magnet with a large air gap or without a ferromagnetic return path experiences a flatter load line and a lower operating point.
In the presence of external demagnetizing fields, typical in permanent magnet motors during current transients, the load line shifts further, lowering the effective operating point.
Correct sizing must include the worst-case scenario: maximum temperature, maximum opposing magnetic field, and the application’s effective air gap.
Choosing the correct grade: thermal grades and stabilization
Once the geometry has been defined and the operating point calculated, material grade selection completes the design process.
For example, sintered NdFeB magnets are available in different thermal classes, identified by the letters following the BHmax value: N (up to 80°C), M (100°C), H (120°C), SH (150°C), UH (180°C), EH (200°C), AH (230°C).
Moving to a higher class means increasing coercivity and therefore shifting the knee point downward, ensuring safe operation at higher temperatures.
However, moving to higher grades also increases costs: it requires larger quantities of heavy rare earth elements such as dysprosium or terbium. Solutions such as HRE-Free sintered NdFeB magnets make it possible to achieve high thermal performance while reducing the use of these critical raw materials.
When the thermal design margin is limited, thermal stabilization is used: the magnet is exposed to a temperature slightly higher than the maximum operating temperature to induce in advance the irreversible losses that would occur during operation. Once stabilized, the magnet operates in a purely reversible regime and enters production with known and repeatable performance.
Temperature, dimensions, and operating point: a system to be designed together
As we have seen, magnet behavior under temperature conditions is not a property of the material alone: it is the result of the interaction between magnetic grade, component geometry, magnetic circuit, and application thermal profile.
Reducing the choice to a single parameter leads to sizing errors that become evident during series production, when corrective actions become expensive.
The correct approach involves starting from the material demagnetization curve, calculating the operating point in the real geometry, verifying the distance from the knee point under worst-case conditions (maximum temperature, maximum opposing field, largest expected air gap), and selecting the grade that provides the required safety margin.
When requirements are demanding, thermal stabilization and magnetic simulation should be added to validate the design before prototyping.
Magnetic simulation software such as Comsol Multiphysics allows engineers to predict the operating point under real conditions and select geometry, grade, and stabilization parameters before prototyping, reducing costly development iterations.
Do you have a project with critical temperature requirements?
For projects where temperature and reliability are critical variables, you can contact our team for a technical evaluation of the magnetic design and to define the most suitable combination of geometry, material, and stabilization for the application.
