Zero-field spin-lattice relaxation of the Cr3+ ion in ruby

The spin-lattice relaxation time T 1 of the Cr 3+ ion in the Al 2 O 3 lattice has been measured in zero magnetic field. The theoretical value of T 1 in this particularly simple case is shown to agree well with the measured value for all types of crystal growth at low Cr 3+ concentration. The anomalous concentration dependence observed in the range 0·02-0·1% Cr 3+ for all samples except those grown by the vapour phase modification of the Verneuil process is ascribed to the inhomogeneous Cr 3+ ion distribution in most crystals.