Figure 8 - Air gap influence.
Let us look back at the magnetized magnet in the electromagnet.
If after returning the magnetizing field to zero
(the induction in the sample is at Br), instead of reversing
the field we introduce an air gap between the magnet and
the electromagnetic pole, the magnet will still become
somewhat demagnetized and the flux density in the sample
will decrease below Br to some Bd value as shown in Figure 9.
Figure 9 - Bd (flux density) versus air gap size.