In the Ising model for ferromagnetism a lower energy is assumed when two spin magnetic dipoles are aligned parallel to each other and the energy is higher when they are antiparallel.

If I take two simple bar magnets I see precisely the opposite behavior: if I force their axes to be parallel they prefer to be in the antiparallel configuration (so that the N of one magnet is near to the S of the other).

What happens in ferromagnets that makes the dipole to prefer to be parallel to one another?


The reason that the systems energy is lowered when spins are aligned comes from the coulomb (electrostatic) potential, not magnetism. The details are non-trivial, but basically if you combine the Pauli Exclusion Principle with the Coulomb Potential you find that the ground state occurs when spins are aligned. The repulsion with two macroscopic magnets is a magnetic repulsion, completely separate from the coulomb interaction that aligns spins. The same magnetic repulsion also occurs to counteract the spin aligning, but it is much weaker than the coulomb interaction that aligns the spins


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