Exchange-coupled pairs ofYb3+ions inSrF2

Detailed investigations of the spin-lattice relaxation of single ${\mathrm{Yb}}^{3+}$ ions at trigonal sites in ${\mathrm{SrF}}_2$ are reported. The measurements were performed in the 2–12 K temperature range, in the X band, by using a pulse-sequence saturation method and compared with earlier Q-band measurements. At T≥7.5 K the relaxation is described at both frequencies by an Orbach process. At lower temperatures (2≤T≤6 K), $T_1$ exhibits a departure from a linear temperature dependence, unexpected shorter values, and lack of anisotropy. This is interpreted by considering cross relaxation with fast relaxing pairs of exchange-coupled ${\mathrm{Yb}}^{3+}$ ions with the same trigonal symmetry and Zeeman splittings. The spin-lattice relaxation of pairs is governed by an Orbach process which involves transitions between the S’=1 ground and S’=0 excited states, separated by J=-12.5 K. Consequently, the low-temperature (2≤T≤12 K) relaxation of single ${\mathrm{Yb}}^{3+}$ ions is described by $T_1^{\mathrm{-}1}$ = 142.2 / exp(12.5/T)-1 +7.731$B^5$coth( hν / 2kT )+0.197×${10}^9$exp(-111.67 / T ) . .sp The analysis of ESR spectra confirms the presence of such weakly exchange-coupled ${\mathrm{Yb}}^{3+}$ pairs with J<0, representing about 1% of the total content of ${\mathrm{Yb}}^{3+}$ ions.