Grade

The grade of a magnet corresponds to the so-called energy product. The energy product is the product of the magnetic flux density B and the magnetic field strength H and is a physical measure of the magnetic energy stored per unit volume. Magnets have a force effect proportional to their energy product when magnets of the same size are compared. If two magnets are the same size and shape, a magnet with twice the energy product will also have twice the force effect compared to another magnet.

The gauss oersted unit of measure for the energy product

The energy product can be measured in gauss oersteds. 1 gauss oersted is the product of the unit gauss for the magnetic flux density (you could also use the unit tesla) and the unit oersted for the magnetic field strength. Due to the unusual conversion factor 1 oersted = 79,577 A/m, the energy product in the standard units of energy density J/m³ has a fundamentally different numerical value than in the units gauss oersteds. The following applies: 1 000 gauss oersteds = 7,9577 J/m³.

Typically, good permanent magnets store several hundred kJ/m³ of magnetic energy. That’s a few tens of mega gauss oersteds (MGOe).

Expressing the grade in numerical values and letters

The grade of a magnet is conveyed as a number. The numerical value corresponds to the energy product in the unit MGOe.

In addition to the numerical value for the energy product, the specified grade of a magnet often includes a letter to indicate the maximum operating temperature. The maximum operating temperature of permanent magnets is limited, since a permanent magnet is created due to remanence after the magnetisation of a ferromagnetic material. Remanence completely disappears at the latest when the material-specific Curie temperature is exceeded.

However, heating to lower temperatures can also lead to partial demagnetisation. Therefore, the maximum operating temperature should not be exceeded. The convention is that magnets with the letter N can be used up to 80 °C, M up to 100 °C, H up to 120 °C, SH up to 150 °C, UH up to 180 °C and EH up to 200 °C.

Examples of grade specifications

The grade designation N45 describes a magnet with an energy product of 45 MGOe (358 kJ/m³) that can be used at temperatures up to 80 °C. With a thickness of a few cm and a pole area of the north pole or south pole of around 20 cm²², such a magnet would have an adhesive force of over 1 000 N on soft iron. With such a magnet, you could lift a block of iron weighing 100 kg. However, the letter N indicates that the magnetic forces can be lost above 80 °C. After exposure, the magnet would not adhere even if it cooled down again. It would have to be magnetised again.
A magnet of the same size but with grade M50 instead of N45 could be used up to 100 °C and would have around 10 % more magnetic energy.

Since magnetic forces depend on magnetic energy, magnets with a higher grade also have stronger force effects than magnets with a lower grade. In contrast to the N45 magnet, the M50 magnet is therefore not only more resistant to heat but would also have a greater adhesive force of approx. 10 % at the same size (it has 50/45 = 1,11 times the energy product and therefore also 1,11 times the magnetic force effect).

However, the specified adhesive force depends heavily on the shape of the magnet, surface conditions and purity of the attracted iron. These are idealised, calculated values. If the surface of the iron block is rough or the material is coated, or if the magnet does not have a flat pole surface, the adhesive force will be reduced accordingly.