Enhanced super-hyperfine structure of EPR spectra of $\mathrm U^{3+}$ ion in the Van Vleck paramagnet $\mathrm{LiTmF}_4$

The observation of the super-hyperfine structure (SHFS) of EPR spectra due to enhanced nuclear magnetism is reported. The X-band spectrum of $\mathrm U^{3+}$ ion introduced into the Van Vleck paramagnet $\mathrm{LiTmF}_4$ was measured in the temperature range of 5–20 K and compared with the spectra of $\mathrm{LiLuF}_4$:$\mathrm U^{3+}$ and $\mathrm{LiYF}_4$:$\mathrm U^{3+}$ single crystals. The spectra reveal well resolved and strikingly different SHFS. SHFS of $\mathrm{LiLuF}_4$:U$^{3+}$ is due to fluorine ions forming the nearest surroundings of the $\mathrm U^{3+}$ ion. The main contribution to SHFS of the $\mathrm U^{3+}$ spectrum in $\mathrm{LiTmF}_4$ is coming from $\mathrm{Tm}^{3+}$ ions with highly enhanced nuclear gyromagnetic tensor.