|MadSci Network: Physics|
To answer your question, first we should clerify what are magnets and how they interact with each other. The magnetic field of a permanent magnet comes from numerous unpaired electrons in the molecules of the magnet. Due to its spin, each unpaired electron generates a magnetic field which is very similar to that of a small magnet. Such a magnetic source is called a magnetic dipole. We can regard a permanent magnet as numerous small magnets (magnetic dipoles) binded together through springs. The interaction between two magnets is a collective effect of all their magnetic dipoles. Therefore, now it is clear why we can not block one polarity of a permanent magnet, that is because we can not break a magnetic dipole into two monopoles. A permanent magnet is always a collection of magnetic dipoles, no matter how we split it. However, we can design the shape and way of magnetization (distribution of the orientations of magnetic dipoles) of permanent magnets to let them interact in specific ways.
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