Single phase (Mn_0.2Fe_0.2Zn_1.2)_x substituted Ba₂Co₂Fe₁₂O₂₂ (Co₂Y) hexaferrites with a self-aligned sheet stacked, highly c-axis oriented and multi-domain structure were fabricated to widen their electromagnetic wave (EMW) absorption band via a solid state reaction (SSR) method without an applied magnetic field. The microstructure and magnetic properties of the prepared Ba₂Co_2−1.6x(Mn_0.2Fe_0.2Zn_1.2)_xFe₁₂O₂₂ (x = 0.5, 1.0, 1.25) hexaferrites were hereby confirmed, and their EMW absorption performance was significantly superior to that of traditional ferrite-based absorbers. The minimum reflection loss (RL_min) of sample x = 0.5 was as strong as −48.4 dB at 7.19 GHz with a thickness of 2.5 mm, and the maximum effective absorption bandwidth (EAB_max) reached as wide as 10.24 GHz (7.76–18.0 GHz) with a thickness of only 1.86 mm. Notably, for samples x = 1.0 and x = 1.25, when the thickness was 3.0 mm and 3.2 mm, the EAB_max was 3.61 (4.06–7.67 GHz) and 3.65 GHz (3.81–7.46 GHz), respectively, almost covering the C-band (4.0–8.0 GHz), indicating their excellent low-frequency EMW absorption performance. The excellent EMW absorption properties of the prepared Y-type hexaferrites were attributed to the multiple regulation of ion substitution, self-aligned sheet stacking, c-axis orientation and the multi-domain structure, which optimized their impedance matching and broadened their ferromagnetic resonance (FMR) frequency range. The EMW dissipation mechanism of the prepared Y-type hexaferrites included multiple reflections, natural resonance, exchange resonance, conductive loss, dipole polarization and interfacial polarization. The findings shed new light on the synthesis of hexaferrite-based EMW absorbers with a wider absorption bandwidth.