This article will simplify how magnets are designated and define the differences between magnet grades. We will focus specifically on the strongest rare earth magnets available: neodymium magnets (NdFeB) and samarium cobalt (SmCo) magnets.

Before You Begin — Your Application

When we assist customers with magnet selection we will undoubtedly ask questions about the application. This helps us understand the environment and requirements of the magnet. But in about 50% of situations, our customers cannot share the application due to confidentiality. If that is the case, we will ask:

  1. What is the maximum operating temperature the magnet will be exposed to?
  2. What other materials (steel, aluminum, plastic, etc.) will the magnet be next to?
  3. What is the size of the area available for the magnet?
  4. What tolerances are required?
  5. Is the magnet in an assembly or operating independently?
  6. Are there specific coating requirements?

While these are only a starting point, they typically lead to other questions about operating environment and constraints.

Neodymium and samarium cobalt magnets are available in many grades. Several factors and considerations go into design, but for this article we will focus on magnet strength, magnet coercivity, and the considerations for selection.

Magnet Strength

The strength of a magnet is described by the specification known as BHmax, the maximum energy density of the magnet, defined in mega-gauss-oersteds (MGOe). On a magnetic demagnetization curve, this is the highest point of the magnet’s strength — its maximum energy product.

Neodymium (NdFeB) Magnets

For neodymium magnets, the BHmax generally ranges from 30 MGOe to 55 MGOe. The higher the number, the stronger the magnet. Neodymium magnets produce the highest MGOe of any permanent magnet material. Common grades include N35, N38, N40, N42, N45, N48, N50, N52, and N55.

Samarium Cobalt (SmCo) Magnets

For samarium cobalt magnets, the BHmax ranges from 16 MGOe to 32 MGOe. As with neodymium, the higher the number, the stronger the magnet. Common grades include 16, 18, 20, 22, 24, 26, 28, 30, and 32. The MGOe range for neodymium is higher than for SmCo, indicating that neodymium magnets are stronger than samarium cobalt magnets.

Magnet Coercivity

Coercivity (“Hci”) is a magnet’s ability to withstand demagnetization forces — temperature or other magnetic forces working against the magnet. Manufacturers commonly use a lettering system after the grade to communicate this: M, H, SH, UH, EH, and TH.

When a letter follows a magnet grade, that material has a greater ability to resist demagnetizing forces. Using heat as a typical demagnetizing force, general guidelines are:

No letter (e.g., N38, N45, N52): Up to ~80 °C
M (e.g., N35M, N42M): Up to ~100 °C
H: Up to ~120 °C
SH: Up to ~150 °C
UH: Up to ~180 °C
EH: Up to ~200 °C
TH: Up to ~220 °C

These are general specifications; geometry and operating conditions also play a role.

Considerations

Strength

Why not just specify the highest strength every time? Because the highest strength is not always the right choice. A sensor magnet, for example, may be specified to produce a particular field at a defined distance — overshooting that field can cause the sensor to read incorrectly. In motors, an N48 may match the windings well while an N55 may overheat them. In a holding application, a stronger magnet may be over-spec while a weaker one may not hold enough.

Defining pull or holding strength, magnetic field requirements, and the saturation point of surrounding materials helps narrow magnet choice.

Cost

As a general rule, the higher the grade, the higher the price — and the higher the coercivity letter after the grade, the higher the price. An N48H costs more than an N48; an N48SH costs more than an N48H. A lower grade with a higher coercivity letter can also cost more than a higher-grade base material. For example, an N35SH will likely cost more than an N38 or even an N40.

Application

We may not always know the final application. There are situations where a magnet can be combined with other materials, or even other magnets, to better influence the field — creating a situation where a lesser grade magnet may work fine. Focused or shaped magnetic fields are a separate topic, but if you need a tightly focused field, contact us and we can work through the details.

Size and Shape of Magnet

Size and shape also influence material selection. A very thin or very small magnet may benefit from a higher coercivity letter than the data table alone would suggest. For example, a thin N45H listed as suitable for 120 °C may behave better as N45SH at the same temperature when geometry creates an unfavorable demagnetization environment.

Not All Grades Are Created Equal

Permanent magnet selection often means determining trade-offs. You may need to decide whether strength or coercivity is more important, or whether neodymium is a better fit than samarium cobalt. As you push for higher strength, available coercivity options narrow. If you need a magnet to handle 180 °C, a UH grade is required.

For prototyping, many standard grades and coatings are available through our eCommerce division SuperMagnetMan. For application-specific selection and validation, our engineering team can help you weigh the trade-offs before committing to a production grade.