Fig. 7. RL curves of Fe₃O₄ (a), CuS (b), CSF-0.025 (c), CSF-0.075 (d), CSF-140 (e) and CSF-180 (f) with a filler loading of 50 wt% and those of CSF-160 heterostructure with filler loadings of 30 wt% (g), 40 wt% (h) and 50 wt% (i); the testing frequency range is 2–18 GHz.