Spin-Lattice Interaction in Ruby Measured by Electron Spin Resonance in Uniaxially Stressed Crystals

The spin-lattice Hamiltonian first proposed by Van Vleck to explain electronic spin-lattice relaxation has been experimentally determined for ${\mathrm{Cr}}^{3+}$ ions in ruby single crystals. Its magnitude was obtained through measurements on the effect of applied uniaxial stress on the ESR spectrum of the ${\mathrm{Cr}}^{3+}$ ions. The spin-lattice Hamiltonian was found to be a quadratic spin operator, in agreement with Van Vleck's prediction, with no evidence for any other spin dependence. It was also determined through these measurements that the ${\mathrm{Cr}}^{3+}$ ions can be considered, for interactions with lattice modes, to occupy two inequivalent types of site in the ${\mathrm{Al}}_2$ ${\mathrm{O}}_3$ lattice.