Linear and non-linear couplings of magnetic and lattice excitations are at the heart of many fascinating magnetophononic phenomena observed in rare-earth orthoferrites, the distinctive feature of which is the tendency to spin-reorientation transitions. Here we report the results of the experimental study of the spin and lattice dynamics in the Brillouin zone center of the rare-earth orthoferrite Sm$_{ 0.55}$Tb$_{ 0.45}$FeO$_{ 3}$ by using polarized infrared reflectivity and Raman scattering spectroscopic techniques. The obtained results were supported by the first-principles calculations, which allowed us to reliably identify the parameters of most infrared- and Raman-active phonons. We reveal the spin-reorientation transitions $\Gamma_{ 4}(G_{ a}F_{ c}) \overset{ T_{ 1}}\longleftrightarrow \Gamma_{ 24}(G_{ ac}F_{ ac}) \overset{ T_{ 2}}\longleftrightarrow \Gamma_{ 2}(G_{ c}F_{ a})$ at $T_{ 1} \simeq 220$ K and $T_{ 2} \simeq 130$ K and carefully studied the following evolution of Raman scattering on magnetic excitations at these transitions. Notably, the intermediate magnetic structure $\Gamma_{ 24}$ displays an exceptionally broad temperature range $\Delta{ T} = T_{ 1} - T_{ 2} \simeq 90$ K in mixed Sm$_{ 0.55}$Tb$_{ 0.45}$FeO$_{ 3}$ compared to pure rare-earth orthoferrites. We attribute this broadening of the intermediate phase to the modification of the magnetocrystalline anisotropy as a result of the inhomogeneous magnetic structure caused by the random distribution of rare-earth $\mathrm{ Sm}^{ 3+}$ and $\mathrm{ Tb}^{ 3+}$ ions. We found neither change in the parameters of Raman-active $B_{ 1g}$ phonons nor the appearance of new phonons induced by spin-reorientation transitions, which have been reported in $\mathrm{ SmFeO}_{ 3}$. We assume that our results provide a solid basis for more deeper understanding of magnetophononic phenomena in rare-earth orthoferrites.
Categories: cond-mat.mtrl-sci