Since the magnetic measurements are reported per gram of fiber rather than per quantity of SCO particles aloneand given that all [Fe(Htrz)₂(trz)]BF₄ phase mixtures display similar abrupt and almost complete spin transitions the increase in magnetic signal likely reflects a higher number of SCO particles formed as the FRR increases in Class I fibers. As discussed in the morphological characterization, at a FRR of 0.5, the limited amount of Fe(II) injected into the main microfluidic channel, dispersed across a relatively broad RD zone, produces larger particles. However, the limited diffusion of Fe(II) becomes a bottleneck in the formation of more [Fe(Htrz)₂(trz)]BF₄ particles. In contrast, at FRRs of 4 and 9, the increased Fe(II) input and its complete diffusion into Htrz (and vice versa) result in the formation of more SCO particles, correlating with the rise in magnetic signal (Table and Figure ). The slight decrease in the magnetic signal observed between FRR 4 to 9 is likely attributed to the narrowing of the RD zone. This reduction in width constrains the space available for the growth and formation of [Fe(Htrz)₂(trz)]BF₄ particles, resulting in a lower number of particles being formed (Figure ). Furthermore, the magnetic signals observed for Class II fibers differ significantly from those of Class I. The HS fraction remains stable across varying FRRs, which can be explained with prior arguments. Specifically, in Class II fibers, phase mixtures across different FRRs are similar, with no crystallographic evolution detected, resulting in a consistent HS fraction (Figure ). This finding suggests that the crystallographic evolution across FRRs in Class I fibers is key to the HS state stabilization. Additionally, it supports our hypothesis that a shortage of Htrz in Class I fibers drives their crystallographic evolution toward a phase mixture with a higher proportion of polymorph II, whereas the excess ligand in Class II fibers prevents this evolution, maintaining a constant HS fraction. As for the magnetic signal, at FRR 0.5, the Fe(II) quantity injected in the microfluidic device during the formation of Class II fibers is near the minimum threshold necessary for the formation and growth of [Fe(Htrz)₂(trz)]BF₄ particles. This leads to a greater number of particles than at FRRs of 4 and 9, where a substantial shortage of Fe(II) results in fewer particles and, consequently, a lower magnetic signal (Table and Figure ). We also investigated the effects of increasing or reducing the TFR during the production of Class I fibers. However, these adjustments either inhibited the formation of SCO particles within the fibers or resulted in the clogging of the device (Table ). Given the lack of success in these trials, we chose not to pursue the synthesis of Class II fibers under similar conditions, as meaningful comparisons between the two classes would not be feasible.