Reference: Daniel V. Schroeder, An Introduction to Thermal Physics, (Addison-Wesley, 2000) – Problem 3.21.
As another example of the formulas we obtained for the two-state paramagnet we can look at the 1951 experiment by Purcell and Pound described in Schroeder on page 102. The dipoles here are provided by lithium nuclei, which is a real-life paramagnet with four spin states, although for the purposes of this problem, we’ll pretend it has only two states. The magnetization is given by
The values in the experiment are
The magnetization per particle is
The energy difference between the parallel and antiparallel dipole alignments is , so in this experiment, the energy of a photon required to perform a flip is
This corresponds to a wavelength which can be calculated from Planck’s formula
With a wavelength of around 20 metres, the photon is in the radio wave region of the spectrum.