Neodymium magnets

Neodymium magnets come in different grades such as N35, N42 or N52.  What do these numbers mean?  How does the grade relate to the strength of a magnet?  Does a magnet have a Gauss number?

What are Magnet Grades?

A magnet grade is a good measure of the strength of a magnet.  In general, higher numbers indicate a stronger magnet.

The number comes from an actual material property, the Maximum Energy Product of the magnet material, expressed in MGOe (Megagauss Oersteds).  It represents the strongest point on the magnet’s Demagnetization Curve, or BH Curve.

The pull force from a magnet varies with the grade or N number.  Double the N number and you’ll find roughly double the pull force.

How do you measure the strength of a magnet?

It depends on what is meant by strength.  Two common measures of a magnet’s strength are the pull force and the strength of the magnetic field.

Pull force is how much force you have to pull on a magnet to move it away from something, such as a steel surface or another magnet. The specific way the magnet is tested can have a huge influence on the measured strength.

What is the maximum allowable temperature of a neodymium magnet?  What happens if a magnet goes above this temperature?  Temperature issues and neodymium magnets can be a complex subject.  In this article, we’ll try and simplify some of these concepts, and answer a few common questions.

Two key temperatures are the Maximum Operating Temperature and the Curie Temperature.

Curie Temperature is the temperature at which all magnetization of the magnet is lost.

Maximum Operating Temperature is a general number given for each different grade of magnet material.  While not exact, it’s a good guideline which applies to many situations.  In between the Max Operating temperature and the Curie Temperature, some percentage of the magnetization is irreversibly lost.

Some Definitions:

Magnetic Strength: Throughout this article, magnetic strength is the key measure.  The product of B times H is often used to described the strength of neodymium magnets.  For example, grade N42 magnets have a maximum BH (called BHMAX) of 42MGOe.  This number directly relates to Pull Force Case 1, the attractive force from a single magnet stuck to a large steel plate.  You could also express this as magnetic field, measured in Gauss, at some specific location.  We specify Surface Field in Gauss, measured at the surface of the magnet.  Magnetic Strength is not power, or work, and cannot be expressed in terms of power.

Reversible losses: Up to the Max Operating temperature, you will see some loss of magnetic strength at those elevated temperatures.  When you bring the magnet back down to room temperature, it returns to the original strength.  These losses are small, typically within 5 to 10 percent.

Irreversible losses: Above the Max Operating temperature, some magnetization is lost.  When you bring the magnet back down to room temperature, it will be weaker than it was before the heating process.  A magnet that has had irreversible losses could theoretically be re-magnetized to the original strength, or very close to it.

Permanent losses: Above temperatures where the magnet material was originally sintered, structural changes will occur to permanently demagnetize a magnet.  No amount of externally applied magnetic field will bring the magnet’s strength back.  For neodymium magnets, this temperature is very high, typically above 900°C to 1000°C.

What about temperature cycling?  This is a popular question.  As long as you’re staying below the Maximum Operating Temperature, it doesn’t matter how many times you cycle the magnet’s temp.  No irreversible losses occur until you get above the Max Operating temperature.  Even then, if you lose some strength because the magnet strayed above that temperature, you wouldn’t incur any successive losses by repeatedly going up to that same temperature.

What about N52?  If you compare the N52 curves to the more common N42, you’ll see that there are trade-offs when you choose the most powerful magnet grade available.  In some cases, especially with thinner magnets, N52 magnets can have lower Max Operating temperature than N42.  Be sure to consider this in your selection process.

What about neodymium magnet performance at very low temperatures?  How about cryogenic temperatures?

If you start at room temperature (20°C) and get colder, the strength of the magnet increases by a small amount.  Below about -125°C, the strength then drops much more quickly.

Still, even at temperatures near absolute zero, there is still a good deal of magnetic strength left.  Even at the boiling point of liquid nitrogen, -196°C, there’s still roughly 87% of the field strength you would find at room temperature.

Neodymium magnets are certainly usable at these low temperatures, but the loss in strength below -150°C should be planned for.  In most cases, the strength should return to normal once the magnet is brought back up to room temperature.

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