PXRD patterns of Class II fibers obtained at higher FRRs of 4 and 9 indicate the presence of residual Htrz, whereas at a lower FRR of 0.5, the diffractogram shows pure [Fe(Htrz)₂(trz)]BF₄ (Figure and). This observation aligns with our simulation results and previous discussion (Figures and), though with an inverse rationale to that of Class I fibers. At low FRR of 0.5, the Fe(II) ions supplied to the microfluidic device diffuse completely across the sheath flow, occupying the entire cross-section of the device and allowing full reaction with Htrz. Conversely, at FRRs of 4 and 9, the insufficient availability of Fe(II) ions results in unreacted Htrz within the fibers. Following the basic physicochemical characterization, we examined the magnetic properties of the SCO composite fibers by means of temperature-dependent magnetic susceptibility measurements (Figure and). Before beginning the discussion, it is important to clarify that the magnetic properties presented here correspond to the second heating-cooling cycle, as Class I and II fibers suffer from a shrinking of the magnetic hysteresis after the first cycle. This shrinkage is not new in this family of [Fe(Htrz)2(trz)]BF4 compounds and has been observed in the polymorph II of bulk [Fe(Htrz)2(trz)]BF4 and [Fe(Htrz)2(trz)]BF4 nanoparticles obtained using micelles as confined reactors. In general, such changes have been associated with an irreversible transition from a metastable to a stable crystalline phase upon heating the sample. For the first to third magnetic thermal cycles, we refer the reader to the SI (Figure ).