Exchange interactions in mixed ${\mathrm{Yb}}^{3+}$-${\mathrm{Cr}}^{3+}$ and ${\mathrm{Yb}}^{3+}$-${\mathrm{Ho}}^{3+}$ dimers: An inelastic-neutron-scattering investigation of ${\mathrm{Cs}}_3$${\mathrm{Yb}}_{1.8}$${\mathrm{Cr}}_{0.2}$${\mathrm{Br}}_9$ and ${\mathrm{Cs}}_3$${\mathrm{Yb}}_{1.8}$${\mathrm{Ho}}_{0.2}$${\mathrm{Br}}_9$

The two title compounds were synthesized and investigated with the inelastic-neutron-scattering (INS) technique. They contain mixed YbM${\mathrm{Br}}_9^{3\mathrm{-}}$ (M=${\mathrm{Cr}}^{3+}$, ${\mathrm{Ho}}^{3+}$) dimers as discrete units, and the magnetic excitations of mixed ${\mathrm{Yb}}^{3+\mathrm{-}}$ ${\mathrm{Cr}}^{3+}$ and ${\mathrm{Yb}}^{3+\mathrm{-}}$ ${\mathrm{Ho}}^{3+}$ dimers could thus be observed. The ${\mathrm{Yb}}^{3+\mathrm{-}}$ ${\mathrm{Cr}}^{3+}$ dimer has three INS transitions, for which anisotropic exchange, as well as zero-field splitting of ${\mathrm{Cr}}^{3+}$, has to be included in the exchange Hamiltonian. For the ${\mathrm{Yb}}^{3+\mathrm{-}}$ ${\mathrm{Ho}}^{3+}$ dimer the effect of the exchange interaction manifests itself as a broadening and a splitting of the crystal-electric-field levels of the isolated ${\mathrm{Ho}}^{3+}$ ion. Taking into account the full (2J+1) ground-state multiplet of ${\mathrm{Ho}}^{3+}$, as well as anisotropic exchange, gives a satisfactory description of this dimer.