Random metamaterials with double negative electromagnetic parameters in dense CuCr₂Se₄–CoCr₂Se₄ binary spinels

Metamaterials exhibiting simultaneous negative permittivity (<i>ε</i> < 0) and negative permeability (<i>μ</i> < 0) offer exceptional electromagnetic properties, enabling advanced applications such as subwavelength imaging, wireless power transfer, and advanced antenna systems. Chromium-based selenium spinel composites have emerged as promising candidates for random double-negative metamaterials (DNMs), although achieving tunable and desired double-negative characteristics remains challenging. Here, CoCr₂Se₄, distinguished by high magnetic spin resonance in high-frequency fields and low conductivity, was selected to modulate the electromagnetic properties of CuCr₂Se₄-based composites. A series of (1 - <i>x</i>)CuCr₂Se₄·<i>x</i>CoCr₂Se₄ samples were synthesized <i>via</i> a solid-state reaction. Double-negative properties were achieved in the MHz frequency ranges, specifically within 841-1000 MHz for <i>x</i> = 0.2, 638-1000 MHz for <i>x</i> = 0.4, and 484-1000 MHz for <i>x</i> = 0.6. The negative permittivity arises from the plasma oscillations of delocalized carriers, well described by the Drude model. Meanwhile, negative permeability is explained through the suppression of eddy currents-due to the low conductivity of CoCr₂Se₄-which mitigates skin-depth limitations and enhances magnetic resonance. This composite design not only reduces electromagnetic attenuation but also improves magnetic responsiveness under alternating fields. By clarifying the selection rationale for metallic ferromagnetic spinel composites, this work enhances the theoretical understanding and tuning mechanisms of negative <i>ε</i> and <i>μ</i>. It thereby broadens the scope of metamaterial research and facilitates the development of single-phase double-negative materials, especially for low-frequency near-field applications.