The antiferromagnetic crystal Mn${\mathrm{F}}_2$ presents a fluorescence band which peaks at ∼ 6300 Å and is ∼ 600-Å wide at 77°K and peaks at ∼ 5820 Å and is ∼ 525-Å wide at 4.2°K. Going down in temperature, the fluorescence shows a sharp increase at $\sim \frac{T_N}{2}$ ($T_N=67\mathrm{}$°K Néel temperature). This fluorescence is due to localized traps which reside below the Mn absorption edge; in contrast, the absorption spectrum is due to exciton-type processes in which the energy is delocalized and moves freely throughout the crystal. When Er impurities are present three additional groups of fluorescence lines located at ∼ 6710, ∼ 9760, and ∼ 15 300 Å are observed and were assigned by us to transitions from the ${}_{}{}^4{}_{}{}^{}F_{\frac{9}{2}}^{}{}_{}{}^{}$, ${}_{}{}^4{}_{}{}^{}I_{\frac{11}{2}}^{}{}_{}{}^{}$, and ${}_{}{}^4{}_{}{}^{}I_{\frac{13}{2}}^{}{}_{}{}^{}$ levels to the ${}_{}{}^4{}_{}{}^{}I_{\frac{15}{2}}^{}{}_{}{}^{}$ level of ${\mathrm{Er}}^{3+}$, respectively. The excitation spectrum of each transition reveals that most of the Er excitation energy is due to Mn → Er energy transfer; this process was found to be of the nonradiative type and to be more efficient at ∼ 77 than at 4.2°K. Both the thermal dependence of the fluorescence and that of the energy transfer are explained as due to the emptying of the higher Mn trap into the lower Mn excitonic band.

• Received 14 May 1973

DOI:https://doi.org/10.1103/PhysRevB.8.5232