What is the demagnetizing field and how does it form?

[sokrates]

Can anybody enlighten me on what demagnetizing field is and how it comes to existence?

 

[Meir Achuz]

At the end of a bar magnet, H points in the opposite direction to M and B. This is due to the boundary condition on H at the end of a magnet. In many books, this is called a
"demagnetizing field". That is a misnomer, because it is B and not H that acts to polarize the atoms in the iron. Since B near the end of the magnet is 1/2 its value at the middle, there is some tendency for the magnet to lose a bit of its magnetization (M) over time.

 

[sokrates]

I'll try to absorb what you said, and then come back.
Thanks for your post