This talk will highlight the importance of field-matter interactions in the synthesis of nanomaterials to control their microstructure, defects and to achieve phase selective synthesis. Application of external magnetic fields during chemical vapor deposition (CVD) offers an independent and additional processing parameter to manipulate the evolution of microstructure and phase composition by influencing atomistic processes involved in the thin film growth. We report here on the role of applied magnetic fields (0.5 T) in altering the mass transport of the iron precursor ([FeIII(OtBu)₃]₂) during a cold-wall CVD process that affected the crystal growth and phase composition. Thin films grown under similar set of CVD parameters in the presence or absence of external magnetic fields showed striking differences in the elemental compositions and preferred growth directions of crystallites constituting the CVD deposits. Whereas selective formation of homogeneous magnetite (Fe₃O₄) films was observed under field-assisted chemical vapor deposition, co-existence of hematite (α-Fe₂O₃) and amorphous iron(III) oxide was confirmed under zero-field conditions. Moreover, the application of external magnetic field (0.5 T) produced larger particulates with enhanced crystallinity and out-of-plane magnetic anisotropy, as confirmed by microstructural studies (SEM, AFM and XRD) and magnetization (VSM) data. Comparison of the coercive field (Hc) in films obtained in magnetic fields (Hc=11mT) and without external field (Hc=60mT) indicated lower defect concentration in the former case. X ray photoemission electron microscopy (X-PEEM) in absorption mode at the O-K and Fe-L3,2 edges for both the field-assisted and zero-field samples confirmed the selective formation of magnetite.