The χT vs T curves (χ being the mass paramagnetic susceptibility) for Class I and II fibers exhibit an abrupt hysteretic transition, similar to the behavior displayed by Class I and II particles (Figure , 2p, S17 and Table ). The hysteresis loop is significantly reduced in comparison to bulk [Fe(Htrz)2(trz)]BF4 polymorph I (Figure ) for both fiber classes synthesized at three different FRRs. This reduction in cooperativity can likely be attributed again to the mixture of [Fe(Htrz)2(trz)]BF4 polymorphs I and II present in our SCO composite fibers, as has been shown before in mixed phase [Fe(Htrz)2(trz)]BF4 nanoparticles prepared using micellar approaches. Notably, for Class I fibers, a gradual increase in the Fe(II) HS fraction at low temperatures is observed as the FRR rises from 0.5 to 4 and 9, accompanied by an overall increase in the magnetic signal during thermal cycling. Although HS stabilization could, in principle, be attributed to nanoparticle size reduction where surface Fe atoms with incomplete coordination spheres tend toward the HS statesuch effects typically occur only in extremely small spin-crossover particles, specifically with sizes ranging from 6 to 10 nm. Therefore, the HS stabilization in Class I fibers likely stems from an alternative mechanism.