What Is the Difference Between Maximum Operating Temperature and Curie Temperature?

What Is Maximum Operating Temperature?

The maximum operating temperature, also called the maximum working temperature, is the highest temperature a magnet can typically withstand in normal use without significant permanent loss of magnetic strength. Below it, a neodymium magnet gets a little weaker as it warms and recovers fully when it cools.

Each neodymium grade class has its own rating, set mainly by the letters after the number (N42, N42SH and so on). The rating is a guideline, not a guarantee: the real limit for your magnet also depends on its shape, what it’s attached to and the conditions it works in.

What Is Curie Temperature?

The Curie temperature is the temperature at which a magnetic material loses its ferromagnetic properties. Above it, the material can no longer hold magnetism, and the magnet is fully demagnetized.

The Curie temperature is a property of the alloy itself, so it changes little from grade to grade. In our grade data it’s about 310 °C (590 °F) for standard N grades and about 340 to 350 °C (644 to 662 °F) for the higher-temperature grades. It’s a physical ceiling, not a design limit: a magnet loses much of its strength long before it gets there.

Maximum Operating Temperature vs. Curie Temperature

Why Does a Magnet Lose Strength Before Its Curie Temperature?

Heat affects a neodymium magnet in two ways.

• Reversible loss. A warm magnet is a little weaker, roughly 0.1% for each degree Celsius above room temperature, and full strength returns when it cools. A standard magnet at 80 °C is several percent weaker while it’s hot.
• Irreversible loss. Heat also lowers the magnet’s coercivity, its resistance to being demagnetized. Once the magnet’s own field, or an outside field, can overcome that resistance, part of the material flips direction. That part stays flipped when the magnet cools, so some strength is lost for good. This is a form of demagnetization, and it’s what the maximum operating temperature guards against.

Strength lost this way can only be restored by remagnetizing the magnet in an industrial magnetizer, and only if the heat hasn’t damaged the material. For most buyers, replacing the magnet is simpler.

What Happens If a Neodymium Magnet Gets Too Hot?

It depends on how hot, and for how long:

1. Below its maximum operating temperature: it’s slightly weaker while warm, then back to normal once it cools.
2. A little above it: it loses a small part of its strength for good. You might notice it holds a bit less than before.
3. Well above it: the permanent loss grows quickly. The magnet may hold only a fraction of its rating, or hold unevenly across its face.
4. At or above the Curie temperature: it’s fully demagnetized and no longer acts as a permanent magnet.

Heat can also damage what’s around the magnet. Nickel plating handles these temperatures, but adhesive backings, rubber and plastic coatings and epoxy usually have lower limits than the magnet itself, so check them separately. Soldering and welding heat a magnet far past its limits; see can I solder or weld to neodymium magnets?

How Grade, Shape, Magnetic Circuit and Conditions Affect Heat Resistance

Magnet Grade

The letters after the number set the temperature class. Higher classes, such as H, SH, UH and EH, use alloys with higher coercivity, so they resist demagnetization at higher temperatures. The number still sets the strength: an N42SH magnet is about as strong as an N42 at room temperature.

Shape

Thin magnets, with little thickness for their diameter, are more easily demagnetized by heat than thicker magnets of the same grade. Engineers describe this with the permeance coefficient (Pc), or load line: the lower it is, the sooner a magnet starts losing strength as it warms. A very thin disc on its own may need a higher grade, or more temperature margin, than a thick one.

Magnetic Circuit

A magnet attached to steel, or set in a steel cup like a pot magnet, works in a more closed magnetic circuit and holds up better to heat. A magnet standing alone in air, or working across a large air gap, is more vulnerable.

Operating Conditions

Opposing magnetic fields, such as two magnets repelling each other or the fields inside a motor, push a magnet toward demagnetization and lower its real temperature limit. Long exposure, repeated heating and cooling, and hot spots near motors, heaters or lighting add up too.

Is N52 More Heat Resistant than N42?

No. N52 has the same temperature rating as N42: both are standard N-class grades rated about 80 °C (176 °F). N52 is stronger, but its minimum coercivity in our grade data is slightly lower (Hcj 11 kOe or more, versus 12 kOe or more for N42), so in thin shapes or demanding circuits it can begin losing strength a little sooner. Strength and heat resistance are separate specifications. See N42 vs. N52.

What Does the SH Magnet Grade Mean?

SH is a high-temperature class with a typical maximum operating temperature of about 150 °C (302 °F). It doesn’t add strength: our 1 × 3/8 Inch Disc is rated 37.8 lb in both N42 and N42SH. The difference is that the N42SH version keeps that strength at temperatures that would permanently weaken a standard N42.

Which Magnet Grade Should I Choose for High-Temperature Applications?

1. Find the peak temperature, not the average. Include sunlight, enclosed spaces and nearby motors, heaters or lights. A car dashboard in summer sun, for example, can approach or exceed the 80 °C rating of standard grades.
2. Choose a grade class rated above that peak, with some margin.
3. Add more margin for thin magnets, magnets without steel behind them, magnets that repel each other or sit near other strong fields, and long exposure.
4. Check everything else that gets hot, such as adhesive, coating, glue and the parts around the magnet.
5. Test in real conditions before committing to a large order.

See our stock high-temperature magnets in the High Temperature Neodymium Magnets collection. Other grades, including H, UH and EH, are available as custom magnets. For temperatures beyond what neodymium can handle, samarium cobalt magnets are the usual choice; see neodymium vs. samarium cobalt.

Practical Takeaway

Design to the maximum operating temperature, not the Curie temperature. Pick a grade class rated above your peak temperature, leave extra margin for thin magnets, open circuits and opposing fields, and remember that N52 is about strength, not heat. If a project runs hot, an SH grade or a custom high-temperature grade will keep its strength where a standard magnet would fade.

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