Although flux growth is a simple technique, it does have one tricky part, which is figuring out what flux to use. The flux can be one of the elements in the system, a mixture of the elements in the system, or an additional element. As the borocarbides are cooled, they can undergo two basic types of phase transitions. The first is the phase transition of the conduction electrons, the electrons that have enough energy to zip freely through the material, at the superconducting transition temperature. The second phase transition is the transition of the rare-earth element's magnetic moments at the magnetic transition temperature.
Magnetism usually kills superconductivity.
Pure lanthanum, for example, conducts electricity without resistance at temperatures below 6 Kelvin. But if just a fraction of a percent of the magnetic rare-earth element gadolinium is added to the lanthanum, it destroys the superconducting state. In a magnetic superconductor, however, there is only weak coupling between the moments of the rare-earth elements that give rise to the magnetism and the conduction electrons that give rise to the superconductivity. Because the couple is so weak, the material can be simultaneously superconducting and magnetic. A nickel borocarbide that includes the rare-earth element holmium, for example, becomes superconducting at 8.5 Kelvin.