The coupling responsible for ferromagnetism is not the mutual magnetic interaction energy between two magnetic dipoles. To show this we will calculate (a) the magnetic field a distance of 10 nm away along the dipole axis from an atom with magnetic dipole moment (cobalt), and (b) the minimum energy required to turn a second identical dipole end for an end in this field.

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The magnetic field along the axis of a magnetic dipole of moment at a distance x from it is given by

.

It is given that a cobalt atom has magnetic dipole moment . The magnetic field at a distance of 10 nm from it along the dipole axis will therefore be

With this magnetic field the minimum energy required for turning a second identical dipole end to end will be

Substituting the values of and B, we find

We will calculate the thermal temperature at which the mean thermal kinetic energy of a cobalt atom will be comparable to

As the sample of the ferromagnetic material has to have a temperature of less than for the spin-spin alignment, we conclude that the spin-spin coupling is not the mutual magnetic interaction energy of elementary dipoles for ferromagnetism.