It is possible to demagnetise a Neodymium magnet by applying an external magnetic field in a direction opposite to the direction of magnetisation (DoM) of the NdFeB magnet. The extent of demagnetisation is a function of the total magnetic circuit (and hence the Intrinsic working point), the temperature (and hence the actual BH curve shape for the NdFeB in use) and the magnitude of the externally applied demagnetising field. If the Intrinsic working point if known, a line drawn on the BH curve from the origin (B=0, H=0) to the Intrinsic working point is drawn (this is the Intrinsic Load Line). If the magnitude of the externally applied demagnetising field is known (Ha), the Intrinsic Load Line is translated along so the start of the load line has moved from H=0 to H=Ha. The slope of the load line is the same but the working point in the Intrinsic curve has moved. If the new Intrinsic working point has not entered the region of the ‘knee’ of the Intrinsic curve, the demagnetisation of the magnet is minimal, possibly so little it cannot be measured.
When the external field is removed, the Intrinsic working point ‘recoils’ back but at a slope equal to the Intrinsic curve slope where H=0 and B=Br. If the Intrinsic working point had entered the region of the ‘knee’, the recoil does not follow the original BH curve shape – a new BH curve shape is produced which has a lower new Br (this is the effect of demagnetisation). Applying an external field again up to the same magnitude of the original Ha has no further demagnetising effect – only if Ha exceeds the original Ha will further demagnetisation be seen. If the Ha equals Hc (the coercive force), the Neodymium magnet will appear to have no magnetic output whilst the field is applied but removing the field will have caused potentially very high demagnetisation.
But if Ha equals Hci (the Intrinsic coercive force, when Ha is removed, the Neodymium magnet will be totally demagnetised (i.e. it will show no magnetic performance). To minimize the effects of externally applied fields, higher Hci rated Neodymium magnets may be used and the design can be improved to give a better Intrinsic Load Line (also known as a higher Pci or Intrinsic Permeance Coefficient, where Pci=Pc+1 and Pc is the Permeance Coefficient based on using the Normal Curve). Where possible a lower operating temperature for the application may also assist (this is unlikely to be designed in though).