Microwave absorbing properties of composites containing ultra-low loading of optimized microwires

A set of composite absorbers with ultra-low loading of down to 0.017 wt.% short-cut microwires were prepared and investigated in terms of their magnetic and microwave properties in the X-band (8-12 GHz). The key parameters that could factor into the performance of the present absorbers have been systematically studied from micro to macro scale, i.e., the magnetic domain structure modulated by different internal stress scenarios, microwire aspect and composite filling ratio, and relative importance of impedance matching and material loss. Longer wires (9 mm) and wires after suitable treatment lead to the enhancement of microwave absorbing property mainly due to the larger magnetic loss resulted from the natural ferromagnetic resonance, which is induced in the domain rotation and reversal process. The internal stress modification by removal of glass and joule annealing has profound effects in formulating the microwave absorption frequency and intensity of microwire composites by tailoring the domain structure and local anisotropy field. It is found that the composites with microwires annealed at 132 mA for 15 min obtain the minimum reflection loss and broadest absorption bandwidth with reference to −10 dB at 1.5 mm thickness, which is −25.7 dB (0.81 GHz) at 11.39 GHz caused by the relative high attenuation constant and best matching condition where Zreal is 1.026 and Zimag is −0.102. This indicates that impedance matching plays a dominant role in tailoring the microwave absorbing property. Remarkably, the unconventional absorption frequency - matching thickness relation based on the quarter-wavelength model offers possibility to address the limitation of thickness for low frequency absorbing application.