Halliday, Resnick & Walker — Maxwell's Equations; Magnetism of Matter: Problem 39

39 The exchange coupling mentioned in Module 32-8 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 \(1.5 \times 10^{-23}\text{ J/T}\) (cobalt), and (b) the minimum energy required to turn a second identical dipole end for end in this field. (c) By comparing the latter with the mean translational kinetic energy of 0.040 eV, what can you conclude?