Conducted noise suppression up to GHz range by spin-sprayed Ni _0.2Zn_xFe_2.8-xO₄ (x = 0.3, 0.6) films having different natural resonance frequencies

In order to apply to a novel, flexible type of GHz noise suppressors, we prepared Ni_0.2Zn_xFe_2.8-xO ₄ films with x=0.3 and 0.6 and thicknesses of 2 and 5 μm, by spin spray ferrite plating from an aqueous solution on polyimide sheets at 90°C. Placing the films onto a microstrip line, we measured transmission loss ΔP_loss and reflection parameter S₁₁ at 10 MHz-10 GHz. As x increased from 0.3 to 0.6, f_r (natural resonance frequency) decreased from 350 to 50 MHz, which resulted in decreasing f_c (a frequency from which ΔP_loss begins rising) from 400 to 100 MHz. This means we can tune f_c of the films by changing the Zn concentration x. At 8 GHz, ΔP_loss obtained by the ferrite films increased from 40% to 70% when their thickness increased from 2 to 5 μm. We obtained S11<10%, irrespective of Zn concentration, in the whole measurement frequency range. By the films with x=0.3 and 2 μm thickness we obtained ΔPloss=40%, which was as strong as that obtained by a commercially available composite sheet type noise suppressor of 25 μm thickness that are made of ferromagnetic metal flakes embedded in a flexible polymer matrix. Moreover, ΔP_loss by the ferrite film increased to 70% when the thickness was increased to 5 μm. Therefore, our NiZn ferrite films are promising to be actually used as GHz noise suppressors with tunable working frequencies that exhibit stronger noise suppression than the commercialized composite type of noise suppressors.