Linear and nonlinear 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 ${\mathrm{Sm}}_{0.55}{\mathrm{Tb}}_{0.45}{\mathrm{FeO}}_3$ 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 ${\mathrm{\Gamma }}_4\left(G_a{F}_c\right)\stackrel{T_1}{\leftrightarrows }{\mathrm{\Gamma }}_{24}\left({\mathrm{G}}_{\mathrm{ac}}{\mathrm{F}}_{\mathrm{ac}}\right)\stackrel{T_2}{\leftrightarrows }{\mathrm{\Gamma }}_2\left({\mathrm{G}}_{\mathrm{c}}{\mathrm{F}}_{\mathrm{a}}\right)$ at $T_1\simeq 220\mathrm{K}$ and $T_2\simeq 130\mathrm{K}$ and carefully studied the following evolution of Raman scattering on magnetic excitations at these transitions. Notably, the intermediate magnetic structure ${\mathrm{\Gamma }}_{24}$ displays an exceptionally broad temperature range $\mathrm{\Delta }T=T_1-T_2\simeq 90\mathrm{K}$ in mixed ${\mathrm{Sm}}_{0.55}{\mathrm{Tb}}_{0.45}{\mathrm{FeO}}_3$ 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.

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• Magnons
• Optical phonons
• Spin reorientation transition
• Spin-phonon coupling
• Antiferromagnets
• Magnetic insulators
• Density functional calculations
• Fourier transform infrared spectroscopy
• Raman spectroscopy