What Causes Neodymium Magnets to Demagnetize: Heat, Shock and Opposing Fields

A customer once sent us a steel drum of neodymium magnets back as defective, marked simply "gone weak in six months". The parts were fine to look at, no chips, no rust, but they had lost close to a third of their pull. The story of almost every magnet that fades comes down to three things, usually heat, sometimes a knock, occasionally a field pushing the other way, and it is worth knowing which one you are dealing with before you blame the supplier.

How Heat Causes Neodymium Magnets to Lose Strength

Heat is the one we see most. Within its rated range a Neodymium (NdFeB) magnet gives back about 0.11 percent of its flux for every degree Celsius it warms, and that much comes straight back when it cools. The real damage starts above the maximum operating temperature, roughly 80 degrees Celsius for a plain N-grade part, 150 for SH and 200 for EH. Past that line the tiny grains that carry the field begin to flip, and cooling does not flip them back. A designer who reads the Curie point, the 310 to 400 degrees where the alloy stops being magnetic at all, as a working limit has usually ruined the magnet long before the thermometer gets near it.

Can Physical Shock Demagnetize Neodymium Magnets?

A hard knock is easier to underestimate. A sintered neodymium magnet is a ceramic at heart, hard and brittle at once, so a drop onto a bench or two big magnets allowed to slam together can jar enough grains loose to measurably weaken the part even when the only visible harm is a chipped corner. Cold makes the same blow count for more: below about minus 40 degrees the magnet grows more brittle and its coercivity dips, so a shock that shrugs off in a warm shop does real magnetic damage in a frozen yard or a cryogenic fixture.

How Opposing Magnetic Fields Cause Permanent Demagnetization

The least obvious cause is another field. Every grade has a demagnetization curve, and the magnet stays safe while it works above the knee of that curve. Park it near a stronger magnet with like poles facing, run it past a degaussing coil, or load a motor hard enough that the armature field pushes back, and the local field can shove the working point below the knee. What you get is a partial, permanent loss that no amount of cooling repairs; the part has to be re-magnetized, and if it was hot when the damage happened it may never fully come back.

So before a grade is fixed we ask what the magnet actually feels: how hot it gets inside the assembly rather than in the room, whether any moving steel or nearby magnet can push against it, and what it will be dropped, bolted or welded near. Send us the duty cycle and the environment and we will pick the grade, the coating and the magnetizing spec that keep the part well clear of its knee, so it demagnetizes on your schedule and not in your customer's hands.