Ni-Substitution-Induced Microstructural Evolution and Electromagnetic Loss Regulation in U-Type Hexaferrite Ceramics for Enhanced Microwave Absorption

U-type hexaferrites Ba4Co2Fe36-xNixO60 (x = 0.0–0.4) with nominal Ni substitution were prepared by a conventional solid-state reaction. X-ray diffraction analysis indicates that all samples are predominantly composed of the U-type phase. Scanning electron microscopy observations reveal that the samples exhibit faceted and polyhedral grains with polygonal and triangular outlines. With increasing Ni content, the magnetic properties are effectively regulated, with the saturation magnetization increasing from 12.8 emu·g−1 to 14.5 emu·g−1 and the coercivity exhibiting a non-monotonic variation. Meanwhile, the complex permittivity and permeability are systematically tuned, leading to improved electromagnetic attenuation capability and impedance matching. At a thickness of 6.0 mm, the x = 0.2 sample exhibits the broadest absorption bandwidth, defined as a reflection loss of 9.3 GHz (RL ≤ −5 dB), whereas the x = 0.4 sample achieves a minimum reflection loss (RLmin) of −36.0 dB and an effective absorption bandwidth of 6.8 GHz (RL ≤ −5 dB). These results reveal that Ba4Co2Fe36-xNixO60 is a great potential absorber in electromagnetic (EM) wave absorption.