We hypothesize that the HS stabilization is driven by variations in the polymorph I and II ratios of [Fe(Htrz)₂(trz)]BF₄, as it has been observed previously. According to our crystallographic analysis, all systems crystallize as mixtures of polymorphs I and II. However, in Class I fibers, a progressive shift from a phase mixture containing more polymorph I to a different phase mixture containing more polymorph II can be seen as the FRR increases from 0.5 to 9 (Figure ). Interestingly, this control over the mixture of polymorphs and stabilization of the HS fraction was previously achieved by tuning the concentration of Fe(II) in the reaction media while keeping a stoichiometric ratio with Htrz. In our case, this compositional and magnetic tuning is achieved on-the-fly by precisely controlling the RD zone, specifically the Fe(II):Htrz ratio, during the reaction process.