The exchange coupling mentioned in Section 32-11 as being responsible for ferromagnetism is not the mutual magnetic interaction between two elementary magnetic dipoles. To show this, calculate

(a) The magnitude of the magnetic field a distance of 10 nm away, along the dipole axis, from an atom with magnetic dipole moment 1.5 x 10-23 J/T (cobalt), and

(b) The minimum energy required to turn a second identical dipole end for end in this field.