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In this paper, we introduced a comparative study of Sm-Tm-substituted Sr-Ba nanohexaferrites (NHFs), Sr_0.5Ba_0.5Tm<i>_x</i>Sm<i>_x</i>Fe_{12&#8722;2<i>x</i>}O₁₉ with <i>x</i> = 0.00&#8722;0.05, manufactured via both citrate sol-gel auto-combustion and ultrasonication approaches. The phase formation of M-type hexaferrite (HF) for both compositions was confirmed by X-ray diffraction (XRD) powder pattern, Fourier-transform infrared (FT-IR) spectra, scanning and transmission electron microscopy (SEM and TEM) micrographs, energy dispersive X-ray (EDX) spectra, and elemental mappings. The magnetic properties at room temperature (RT) and low temperature (<i>T</i> = 10 K) were also investigated. M-H loops revealed ferrimagnetic nature for various prepared nanohexaferrites via sol-gel and ultrasonication routes. The <i>M_s</i> (saturation magnetization) and <i>M_r</i> (remanence) values increased with increasing Tm-Sm substituting contents. <i>M_s</i> and <i>M_r</i> reached their maximum values at <i>x</i> = 0.04 in the case of samples prepared using the sol-gel technique and at <i>x</i> = 0.03 for those prepared via ultrasonication route. <i>M-H</i> loops were very broad in samples prepared via ultrasonication route in comparison to those produced by means of the sol-gel approach, implying that the products synthesized via ultrasonication route have greater values of coercivity (<i>H_c</i>). The variations in <i>H_c</i> values with respect to Tm-Sm substitutions were governed by the evolutions in the magneto-crystalline anisotropy. Diffuse reflectance spectra (DRS) were employed to estimate the direct band gap of pristine and co-substituted Sr_0.5Ba_0.5Fe₁₂O₁₉ hexaferrites. Optical energy band gaps (<i>E</i>_g) of pristine samples were significantly tuned by co-substitution of Tm³⁺ and Sm³⁺ ions. <i>E</i>_g values of the Sr_0.5Ba_0.5Fe₁₂O₁₉ sample, which was synthesized using the sol-gel method, decreased almost linearly from 1.75 to 1.45 eV by increasing co-doped ion content. However, we observed a sharp drop from 1.85 eV to an average of 1.50 eV for the samples, which were synthesized using the ultrasonication approach.