Spin-Spin Relaxation at Very Low Temperatures in Chromic Methylammonium Alum

We have measured the absorption due to spin-spin relaxation in chromic methylammonium alum at frequencies from 0.1 to 10 Mc/sec and at temperatures from 0.06° to 1°K. At 1°K the spin-spin relaxation time, ${\tau }_s$, is found to be 4.2×${10}^{-10\mathrm{}}$ sec and to be decreasing slowly with increasing temperature. Below 1°K., ${\tau }_s$ increases rapidly. The absorption of energy is found to be proportional to the square of the frequency, $\nu$, at the higher temperatures, but varies less rapidly at lower temperatures, being proportional to ${\nu }^{1.83}$ at 0.06°K. The results are analyzed in terms of Broer's theory of spin-spin relaxation, and, not surprisingly, the theory is found to need refinement in order to be applicable at temperatures where Curie's law is not obeyed. It is concluded that measurements of this type, which give essentially the same kind of information as is obtained from resonance line shapes, should be able to throw light upon the behavior of substances near their antiferromagnetic transition points.