The influence of microstructural changes from nano to micron grain size regime towards their structural, magnetic, electrical and microwave properties of gadolinium iron garnet (Gd3Fe5O12) has been investigated systematically in this research work. Raw materials were milled via high-energy ball milling (HEBM) followed by subsequent sintering (600–1400 °C) process. X-ray diffraction (XRD) analysis had shown that single phase with garnet structure and highest crystallinity of Gd3Fe5O12 was formed at 1000 °C. The B–H hysteresis loop unveil the evolution development of magnetic behaviour (paramagnetism–ferrimagnetism) in samples thru variation of different sintering temperature. The values of linewidth (∆H) can be grouped into two groups which are; sample increased from 600 to 1000 °C due to shape and strain induced anisotropy while decreasing from 1100 to 1400 °C influenced by magnetocrystalline anisotropy with the increment of the sintering temperature.

An efficient microwave absorber is essential for reducing electromagnetic interference in modern technologies. Here we highlight a magnetic-catalyst-assisted route to carbon nanotubes (CNTs): CNTs were grown by CVD using Ni–Zn ferrite/carbonyl iron (NZF/CI) as magnetic catalysts, embedding magnetic nanophases within the CNT network. This magnetic–dielectric coupling increases magnetic loss (μ″) through natural-resonance and eddy-current pathways. When combined with a graded double-layer structure which CNT/epoxy (matching/ front) and carbon black (CB)/epoxy (absorbing/back), synergistically optimizing impedance matching and inlayer attenuation across the X- and Ku-bands. Single-layer CNT/epoxy and double-layer CNT/epoxy–CB/epoxy nanocomposites were fabricated and characterized by Raman spectroscopy, field emission scanning electron microscopy (FESEM), and a PNA network analyzer over the X- and Ku-band frequency ranges. Results revealed that the optimized double-layer structure, comprising a 1 mm CNT as a matching layer and a 1 mm carbon black as an absorbing layer, achieved a − 10 dB absorption bandwidth of 2.58 GHz and over 99.99 % absorption at 11.14 GHz. Microstructural analysis shows nanosized CB (~41 nm) uniformly distributed, providing abundant interfacial sites and micro-capacitive contacts that elevate ε″, while entangled CNTs (outer diameter ~61 nm; spiral/twisted/net-like) promote multiple internal reflections and efficient dissipation. The close contact between CNTs and NZF/CI further strengthens magnetic loss and stabilizes impedance matching. These results demonstrate a cost-effective, magnetic–dielectric double-layer strategy for high-performance and thin microwave absorbers.