To carry out this thought experiment, Si and Yamamoto created a model system that idealizes what exists in nature. Their jumping off point was a well-studied phenomenon known as the Kondo effect -- which also has its roots in quantum magnetic effects. Based on what they knew of this effect, they created a model of a "Kondo lattice," a fine-grained mesh of electrons that behaved like those that had been observed in Kondo studies of real-world materials.
Si and Yamamoto were able to use the model to provide a rigorous answer about the fine-grained origins of metallic ferromagnetism. Furthermore, the ferromagnetic state that was predicted by the model turned out to have quantum properties that closely resemble those observed experimentally in heavy fermion ferromagnets.