NdFeB Thermal Demagnetization: Temperature Limits and Grade Selection for EV and Robotics Motors

Thermal demagnetization is not the same as heating a magnet and watching it cool down. Reversible demagnetization happens every time a magnet heats up: Br drops, you cool it, Br returns to its original value. The temperature coefficient of Br for NdFeB is approximately -0.11 to -0.13 percent per degree C, and this effect is fully recoverable. Irreversible demagnetization is different. When the magnet temperature rises high enough that the operating point drops below the knee on the second-quadrant B-H curve, some magnetic domains flip permanently. Cooling the magnet back to room temperature does not recover the lost flux. The magnet must be re-magnetized in a saturating field to restore its original performance, which is not practical when it is bonded inside a rotor assembly. The knee position depends on intrinsic coercivity (Hcj), which itself drops with temperature. This creates a feedback loop: higher temperature lowers coercivity, which makes the magnet more vulnerable to demagnetizing fields from the stator, which can cause partial demagnetization even at fields that would be safe at room temperature.

NdFeB grades are grouped into thermal families defined by their maximum continuous operating temperature. Standard N grades (N35 through N52) are rated to 80 C. H grades reach 120 C. SH grades reach 150 C. UH grades reach 180 C. EH grades reach 200 C. AH grades, used primarily in aerospace and downhole applications, reach 220 to 230 C. These ratings assume a specific minimum Hcj at room temperature that, after applying the temperature coefficient, still keeps the knee below the magnet's operating point at the rated temperature. An N42H has a minimum Hcj of 17 kOe at 20 C. After the temperature coefficient of approximately -0.55 percent per degree C, it retains roughly 10.6 kOe at 120 C. An N42SH has a minimum Hcj of 20 kOe at 20 C, retaining roughly 11.7 kOe at 150 C.

• •N grades (no suffix): 80 C max, Hcj >= 12 kOe at 20 C
• •H grades: 120 C max, Hcj >= 17 kOe at 20 C
• •SH grades: 150 C max, Hcj >= 20 kOe at 20 C
• •UH grades: 180 C max, Hcj >= 25 kOe at 20 C
• •EH grades: 200 C max, Hcj >= 30 kOe at 20 C

Magnet datasheets typically show the B-H curve at 20 C, sometimes at 80 C and 150 C. The critical question is whether the motor's load line intersects the linear portion of the curve or the nonlinear knee region at operating temperature. The load line is set by the magnet geometry and the magnetic circuit reluctance; it slopes from Br on the vertical axis through a point determined by the ratio of magnet length to air gap plus steel path reluctance. In a well-designed IPM motor, the load line crosses the B-H curve in the upper half of the second quadrant at 20 C, well above the knee. As temperature rises, two things happen simultaneously: the B-H curve shrinks (lower Br, less wide Hcj), and the knee moves upward. The load line stays roughly constant because the geometry has not changed. At some temperature, the load line clips the knee. That temperature is your thermal ceiling, regardless of what the grade suffix says, because the suffix rating assumes a generic load line that may not match your motor. Always plot your specific load line against the B-H curve at your worst-case operating temperature.

Continuous operating temperature ratings assume normal motor operation where the rotor is spinning and the magnets see the time-averaged demagnetizing field from the stator. The worst-case scenario is a stalled rotor: maximum current flowing through the stator at standstill, producing a static demagnetizing field that hits one or two magnets at full intensity rather than being distributed across all poles. In a six-pole motor, a locked-rotor event can produce 3 to 5 times the normal demagnetizing field on the worst-positioned magnet, while the rotor simultaneously heats rapidly because there is no forced convection. Automotive Tier 1 suppliers test for this by running a locked-rotor test at maximum battery voltage and rated coolant temperature. The magnets must survive this event without permanent flux loss. This is why EV traction motors rarely use anything below SH grade, and why the high-speed internal rotor in a humanoid hip actuator (which can stall under leg-contact events) often specifies UH despite lower average operating temperatures.

Higher coercivity costs money because it requires heavy rare earth elements. In the H-grade range, the Nd2Fe14B lattice is lightly substituted with Dy or Tb, which raises the magnetocrystalline anisotropy field and thereby the coercivity. The tradeoff is that Dy and Tb reduce Br: every 1 weight percent of Dy substitution drops Br by roughly 0.12 T. This means an N42SH has lower Br than an N42, not just higher Hcj. Motor designers pay for thermal margin with torque density. Grain boundary diffusion (GBD) changes the economics. Instead of bulk-substituting Dy or Tb throughout the grain, GBD deposits a thin heavy-rare-earth layer at the grain boundaries through vacuum diffusion at 800 to 900 C. The result is a 5 to 8 kOe coercivity boost with only 20 to 40 percent of the Dy/Tb consumption and roughly half the Br penalty of conventional substitution. A GBD-treated N48H can reach 20 kOe Hcj (normally SH territory) while retaining a Br above 1.36 T. For motor designers optimizing torque-per-kilogram, GBD is increasingly the default specification for EV and robotics grades.

Selecting the right grade starts with three numbers: your worst-case magnet temperature (including locked-rotor transients), the demagnetizing field at the worst operating point, and the minimum Br you need at operating temperature to meet your torque spec. Start by determining the maximum magnet temperature. For a liquid-cooled EV traction motor, expect 130 to 160 C at the magnet surface during peak duty. For an air-cooled humanoid actuator, expect 90 to 120 C during continuous operation but potentially 140 to 160 C during stall events. For an industrial servo, expect 80 to 110 C. Match the temperature to the grade family: if your worst-case magnet temperature is 140 C, SH is the minimum grade family. Add 20 to 30 C of safety margin to account for hot spots, manufacturing variation, and aging. Then check that the remaining Br at that temperature still meets your torque requirement. If not, move to a higher energy product within the same thermal family (e.g., N45SH instead of N42SH) or consider GBD-treated grades that offer higher Br at equivalent coercivity.

• •Worst-case magnet temp below 100 C: H grade is sufficient for most designs
• •Worst-case 100 to 140 C: SH grade, verify load line against B-H at 150 C
• •Worst-case 140 to 170 C: UH grade, consider GBD for Br optimization
• •Worst-case above 170 C: EH or AH grade, or switch to SmCo

The price premium across grade families reflects heavy rare earth content and processing complexity. Moving from N42 to N42H adds approximately 8 to 12 percent to per-piece magnet cost. N42H to N42SH adds another 10 to 15 percent. N42SH to N42UH can add 15 to 25 percent because UH grades require significantly more Dy or Tb. EH grades carry the steepest premium, often 30 to 40 percent above the equivalent H grade, and typically have longer lead times because fewer production lines are set up for the higher diffusion temperatures required. GBD-treated grades sit between conventional and full-substitution pricing for a given coercivity level: a GBD N48H with SH-level coercivity typically costs 5 to 10 percent less than a conventional N42SH while delivering higher Br. Lead times for GBD are comparable to standard grades at established producers but can be 2 to 4 weeks longer at suppliers running GBD as a secondary process rather than an integrated line.