Syzygium polyanthum Extract‐Assisted Biogenic Synthesis of Co‐Doped Fe 3 O 4 Nanoparticles: Structural, Magnetic, and Antimicrobial Insights

Co‐doped Fe 3 O 4 nanoparticles from iron sand with Syzygium polyanthum extract were successfully synthesized. The nanoparticle phase was exclusively Co‐doped Fe 3 O 4 with a cubic spinel crystal structure, where the replacement of Fe 3+ (ionic radius = 69 pm) by Co 2+ (ionic radius = 74 pm) enhanced the lattice parameters from 8.360 to 8.378 Å. The oxygen position parameter increased after cobalt substitution , with values ranging from 0.3890 to 0.3893. The overlapping FeO and CoO bonds were detected in the range of approximately 442–534 cm –1 as a result of cobalt substitution . The success of S. polyanthum extract integration was confirmed by the presence of CH (698–956 cm –1 ), CO (1365 cm –1 ), and CC (1636 cm –1 ) bonds, originating from phenolic compounds (flavonoids, saponins, and tannins) and aromatic compounds . The saturation magnetization decreased from 35.49 to 24.956 emu/g as the Co composition increased, which can be theoretically explained by Neel's sublattice model. Remarkably, the excellent broad‐spectrum antimicrobial performance of the nanoparticles against different microbial strains ( B. subtilis , E. coli , and C. albicans ) can be attributed to the synergistic role of Co substitution and S. polyanthum extract. These findings demonstrate the utilization of natural resources to fabricate new broad‐spectrum antimicrobial agents.