Spin-Lattice Relaxation ofTm2+in CaF2, SrF2, and BaF2

Abstract

Measurements were made of the spin-lattice relaxation time of the ground state of ${\mathrm{Tm}}^{2+}$ in Ca${\mathrm{F}}_{2}$, Sr${\mathrm{F}}_{2}$, and Ba${\mathrm{F}}_{2}$ over the magnetic field range of 1 to 12 kG and temperature range of 1.3 to 20\ifmmode^\circ\else\textdegree\fi{}K. The one-phonon relaxation process was unambiguously identified by its characteristic fourth-power dependence on frequency and its linear temperature dependence. The Raman process with its ${T}^{9}$ dependence begins to dominate the relaxation above 5\ifmmode^\circ\else\textdegree\fi{}K. The angular dependence of the relaxation permitted the separation of the effect of the two local modes, ${E}_{g}$ and ${T}_{2g}$, on the one-phonon process. Calculations were in good agreement with theory, and they showed that the second-order terms of the orbit-lattice interaction dominate the relaxation process. Continuous measurement of the relaxation over this magnetic field range was made possible by a technique which is independent of any microwave sources. The detection was done by monitoring the magnetic circular dichroism of the broad optical absorption bands.