We're often asked about what surface field for a certain size magnet is. In terms of surface fields, it can’t be calculated by some formula.

What is surface field of magnet?
In general, the magnetic state of a magnetic material is not only related with the history of the magnetization, but also the magnetic field which this material located. Permanent magnet will produce a demagnetizing field as soon as magnetized. However this demagnetizing field is affected by performance, shape and size of magnetic material. When the magnetizing field decreases and disappears, the magnetic state of the material will return to the point of remanence from saturation point along the hysteresis loop, enter the portion of the second quadrant demagnetization curve, and then walk back along the demagnetization curve to the reversing field which is equal to the point of demagnetizing field. B value at this point is the surface magnetic field of magnet. Therefore, the surface force is only B value of a point at magnetic material demagnetization curve that is less than the total value of the remanence Br.

To put it simply, the surface magnetic force refers to the strength of the B-field in Gauss of specific shape magnets. It is related to the residual magnetism and the shape of magnet.

Formula on the internet?
However, if you have a search on the internet, it will return certain results with surface field formula. This formula applies to these magnets with regular shape and regular magnetized direction, such as block and disc magnet. The difference between the theoretical and the practical results is within 5%. But for an irregular shape magnet, such as segment magnet, the difference is bigger, about 10%.

How is surface field measured in practice?
There is a tool which called gauss meter. It is the professional measurement tool for magnet industry. You can buy one from here. And we also have viewing film which helps us to view magnetic fields of permanent magnet.

Why does a smaller magnet have a higher surface field than a bigger one?
That’s because, for same grade magnets, the bigger draw ratio (long and thin shape magnets), the higher the surface field is. Otherwise, the smaller draw ratio (flat shape magnets), the lower the surface field is.

Can we know the grade of magnet by gauss meter results?
No, different gauss meters, test place and distance, magnet environment etc. all these affect the result of surface field. To figure out what the grade of magnet, we need test by demagnetization curve measurement and aging test.

How to test the accurate surface filed of magnet by gauss meter?
First, the tested permanent magnet is placed separately;
Second, the hall probe must be located in the middle of geometric center.
Magnetic induction line is a kind of contour distribution, even with a slight movement of hall probe or uneven hand power, the data of surface field will be fluctuated greatly.

What is the quality control standard of magnet before shipment?
Generally, we use flux meter or magnetic moment instrument as below picture. They can simulate the real assembly environment of rare earth magnets with copper coils surrounded; calculate the whole data of permanent magnet accurately. The control standard is ＜6%.