Effect of magnetic order on the fluorescence dynamics ofTbF3

The dynamical properties of ${\mathrm{Tb}}^{3+}$ fluorescence in ${\mathrm{TbF}}_3$ have been investigated above and below its ferromagnetic phase transition ($T_C$=3.95 K). Near and below $T_C$ the magnetic interactions lead to splittings and shifts of the optical lines reflecting the interionic interactions and the onset of the ordered state. A dramatic (factor of 10) lengthening of the exciton decay occurs both when lowering the temperature below $T_C$ and when applying an appropriate external magnetic field along the easy axis above $T_C$. Throughout the range from 300 to 1.6 K, the experimental fluorescence decays are in agreement with a rate-equation model involving fast-diffusion energy transfer among intrinsic ions. The observed decrease in diffusion coefficient can be understood either as the result of a large reduction of the resonant energy transfer between intrinsic ions or as the result of a decrease in the trapping probability in the ferromagnetic or aligned paramagnetic phases. Either process may be understood in terms of the short-range ${\mathrm{Tb}}^{3+}$-${\mathrm{Tb}}^{3+}$ interactions and the properties of the ordered state.