Interfacial spin modulation in MWCNT/Co-Zn ferrite nanocomposites and coupled magnetic and microwave spin resonance phenomena

Interfacial spin modulation plays a pivotal role in tailoring the magnetic anisotropy and spin resonance dynamics of nanocomposites by disrupting superexchange pathways and altering local spin environments. In this study, we present the oxidative functionalization followed by solvothermal synthesis of multi-walled carbon nanotube (MWCNT)/Co₀.₇Zn₀.₃Fe₂O₄ (CZFO) magnetic nanocomposites (MNCs) and systematic investigation, focusing on the correlation between interfacial structure and magnetic relaxation behaviour. X-band ferromagnetic resonance (FMR) analysis reveals a gradual shift in resonance field from 3493.74 to 3500.75 G and a reduction in Landé g-factor from 2.26 to 2.16 with increasing MWCNT concentration (0 to 16 wt.%), indicating increased spin disorder and reduced spin–orbit coupling at the interface. The spin concentration also increases from 6.60 × 10²² to 6.91 × 10²² spins/g, suggesting enhanced paramagnetic site density due to defect-induced and chemically active surfaces. The spin–spin relaxation time shows a slight increase from 8.29 to 8.68 ps, confirming stable spin coherence despite interfacial modulation. Vibrating sample magnetometry (VSM) data demonstrate a controlled reduction in saturation magnetization from 60.77 to 51.79 emu/g and a nonlinear variation in coercivity, which decreases from 205 to 179 Oe, attributed to surface spin canting, dipolar interactions, and local anisotropy fields. The magnetocrystalline anisotropy constant exhibits a substantial drop from 6.84 × 10⁵ to 1.50 × 10⁵ erg/cm³, consistent with Co²⁺ orbital suppression and increased structural distortion at the CZFO–MWCNT interface. Structural characterization confirms a cubic spinel phase with progressive lattice expansion (from 8.358 to 8.416 Å), reduced crystallite size (from 10.64 to 9.20 nm), and increased microstrain, supporting the emergence of lattice-induced spin frustration. Raman spectroscopy shows an increasing Iᴅ/Iɢ ratio from 1.17 to 1.23, highlights defect engineering in carboxyl-functionalized MWCNTs, facilitating covalent ferrite attachment, while FTIR analysis also confirms strong chemical bonding and functional integration of CZFO onto oxidized MWCNT surfaces. Collectively, these results demonstrate that interfacial engineering via MWCNT integration enables quantitative modulation of both static and dynamic magnetic properties. The work provides a coherent link between spin structural disorder, magnetic relaxation, and anisotropy tailoring, advancing the design of nanocomposites for high-frequency spintronic and microwave applications.