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The high frequency magnetic spectra of Sr 2+ or Sr 2+ -Gd 3+ doped Co 2 Z hexaferrites were systematically investigated. Of particular interest is the high impedance modulus measured in such dual-ion modified Z-type polycrystalline hexaferrites that have considerable potential for applications as anti -electromagnetic interference (EMI) over a frequency range of 0.5–5 GHz. The experiments indicate that the combination of Sr 3+ and Gd 3+ cations effectively tailors the anisotropy field while concomitantly increasing magnetization, yielding an increase in both real and imaginary components of the complex permeability. Consequently, this results in an enhanced impedance modulus as high as 384 Ω at 2 GHz. The structure and magnetic properties of the hexaferrite materials were characterized by X-ray diffractometry (XRD), scanning electron microscopy (SEM), vibrating sample magnetometry (VSM), and S-parameter measurements using a vector network analyzer (VNA). Furthermore, the physical origins of the improved high frequency performance are discussed. The present results provide a pathway to the development of anti -EMI hexaferrites operating in the GHz range. [ABSTRACT FROM AUTHOR]