Tunable twin matching frequency (f<inf>m1</inf> /f<inf>m2</inf>) behavior of Ni<inf>1-x</inf> Zn<inf>x</inf> Fe<inf>2</inf> O <inf>4</inf> /NBR composites over 2-12.4 GHz: A strategic material system for stealth applications

The gel to carbonate precipitate route has been used for the synthesis of Ni1-x Znx Fe2 O 4 (x = 0, 0.25, 0.5 and 0.75) bulk inverse spinel ferrite powder samples. The optimal zinc (50%) substitution has shown the maximum saturation magnetic moment and resulted into the maximum magnetic loss tangent (tanδm) > -1.2 over the entire 2-10 GHz frequency range with an optimum value ∼-1.75 at 6 GHz. Ni 0.5 Zn 0.5 Fe2 O 4 - Acrylo-Nitrile Butadiene Rubber (NBR) composite samples are prepared at different weight percentage (wt%) of ferrite loading fractions in rubber for microwave absorption evaluation. The 80 wt% loaded Ni 0.5 Zn 0.5 Fe2 O 4 /NBR composite (FMAR80) sample has shown two reflection loss (RL) peaks at 5 and 10 GHz. Interestingly, a single peak at 10 GHz for 3.25 mm thickness, can be scaled down to 5 GHz by increasing the thickness up to 4.6 mm. The onset of such twin matching frequencies in FMAR80 composite sample is attributed to the spin resonance relaxation at ∼5 GHz (f m1) and destructive interference at λm /4 matched thickness near ∼10 GHz (f m2) in these composite systems. These studies suggest the potential of tuning the twin frequencies in Ni 0.5 Zn 0.5 Fe2 O 4 /NBR composite samples for possible microwave absorption applications.