Shift and broadening of electronic transitions in a dilute antiferromagnet:Fe1−xZnxF2

The effects of dilution on the crystalline field, the exchange field, and the Néel temperature have been studied for the dilute antiferromagnet ${\mathrm{Fe}}_{1-x}{\mathrm{Zn}}_x{\mathrm{F}}_2$ in the range of $0<~x<~0.75$. Excitations within the ${}_{}{}^5{}_{}{}^{}T_{2g}^{}{}_{}{}^{}$ manifold of ${\mathrm{Fe}}^{2+}$ ions have been measured by Raman scattering, and the one-magnon sideband transition in the 21 500-${\mathrm{cm}}^{-1\mathrm{}}$ region was studied by optical absorption. Both transitions shift and broaden as the zinc concentration is increased. The behavior of the main features of the spectra are interpreted in terms of a molecular-field model. From the Raman measurements a linear change of the magnetic anisotropy with zinc dilution was deduced, while the shift of the one-magnon sideband indicates a linear variation of the exchange field and the Néel temperature with $x$. The low-temperature linewidths are interpreted as originating from the random distribution of ${\mathrm{Fe}}^{2+}$ environments and strains induced by the zinc ions. The results confirm many theoretical assumptions of previous papers.