Why can’t the grade be determined by the magnet’s surface magnetism?

When it comes to surface magnetic flux density, I’ve noticed there are so many questions. For example, customers often ask: ‘Which is greater—the surface magnetic flux density at the centre of a magnet or at its edge? Why?’, ‘What is the relationship between a magnet’s surface magnetic flux density and its remanence?’, ‘Is the surface magnetic flux density the same at the N-pole and S-pole of a magnet?’, and so on. Today, I’d like to discuss why one cannot judge a magnet’s magnetic performance based on its surface magnetic flux density.

Firstly, surface magnetic flux density is influenced by the magnet’s dimensions and shape. Even for neodymium-iron-boron magnets of the same grade—for example, N52—if the dimensions, thickness or structure differ, the measured surface magnetic field values will vary considerably. Generally, thicker magnets can generate a stronger surface magnetic field, whilst thinner magnets will have a relatively lower surface magnetic field, even when made from high-performance materials. Therefore, it is impossible to accurately determine the material grade based solely on surface magnetic field values.

Secondly, the method of magnetisation and measurement conditions also affect the surface magnetic field results. Axial, radial and multipole magnetised magnets exhibit completely different magnetic field distributions. In particular, multipole magnetic rings, due to the small spacing between their poles, often yield surface magnetic field values that differ markedly from those of standard bipolar magnets. Furthermore, factors such as the distance between the gaussmeter probe and the magnet’s surface, the measurement location, and the instrument’s accuracy can all lead to variations in the measurement results.

Measurement errors may also exist between gaussmeters of different brands; for instance, the author has encountered discrepancies of around 150–200 gauss between Chinese-made and Japanese gaussmeters.

Measuring the surface magnetisation of a magnet using a Gauss meter

The indicators truly used to determine magnetic performance grades are material parameters such as remanence (Br), coercivity (Hcj) and maximum energy product (BHmax); these data must be obtained using specialised magnetic performance testing equipment and standard testing methods. Therefore, surface magnetic flux density can only serve as a reference indicator for the effectiveness of a magnet’s application, and cannot be used as a basis for determining whether it belongs to magnetic performance grades such as N35, N42 or N52.

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