Ferromagnetic nuclear resonance characterization of novel magnetic nanocomposite

Permanent magnets are essential for sustainable technologies. Rare-earth-based magnets (NdFeB) dominate high-performance applications due to their excellent properties but suffer from supply and sustainability issues. Ferrites are abundant and cheap but offer lower performance. Gap magnets emerge as intermediate materials balancing efficiency and sustainability. This thesis studies nanocomposites of magnetoplumbite-type hexaferrite (SrFe$_{12}$O$_{19}$, SFO) and spinel-type ferrite (CoFe$_{2}$O$_{4}$, CFO), aiming to exploit superexchange interactions between hard and soft phases to optimize their properties. SFO is the hard phase at room temperature, while CFO hardens at low temperature due to its higher coercivity. The experimental characterization was performed using ferromagnetic nuclear resonance (FNR), which exploits internal hyperfine fields as sensitive local probes. SFO was studied in the 4--320 K range, assigning spectral peaks to the crystallographic Fe sites and confirming the structural model. The evolution of hyperfine fields with SFO/CFO composition was then analyzed, revealing how the local magnetic environment changes with phase ratio. Our zero-field NMR results provide unique insight into nanoscale interfacial exchange interactions in SFO--CFO nanocomposites with varying phase ratios, where harder and softer magnetic components coexist in the same nanoparticle. The comparison with bulk SFO allowed reliable peak assignment and revealed site-specific frequency shifts from the inclusion of CFO, evidencing interfacial exchange fields across the nanoparticle. Measurements of the enhancement factor established a direct link between magnetic hardening and structural parameters. Overall, this work demonstrates the capability of zero-field NMR to probe the nanoscopic interplay between magnetic domains with different anisotropy fields, offering new perspectives on magnetic hardening in exchange-coupled nanocomposites for future applications.