An Unconventional Ferromagnet Plays Host to a Conventional Superconductor

Under most conditions, conventional superconductivity and ferromagnetism are mutually exclusive. In conventional superconductivity, the current is carried by pairs of electrons in states with opposite magnetic moment (spin), termed a spin singlet state. Ferromagnets, however, prefer to have electrons in states with the same magnetic moment. The superconducting current (supercurrent) thus decays very rapidly when in contact with a ferromagnet, which normally prohibits the existence of singlet pairs. Researchers in the NSF MRSEC–supported Center for Materials for Information Technology (MINT) at the University of Alabama, working in collaboration with scientists at Delft University of Technology in the Netherlands and Brown University, have observed for the first time a supercurrent that prevails over a substantial distance through a ferromagnet – the unique ferromagnet chromium dioxide (CrO2) – by creating a “spin triplet” instead of singlet pairs. CrO2 is a truly remarkable ferromagnetic material that has been demonstrated to be simultaneously an excellent metal for majority spin electrons and an insulator for minority spin electrons. For this reason, CrO2 belongs to a small class of exotic materials called half-metals. The material is also known for its large scale use in the past for the fabrication of superior quality audiotapes.

The research team connected two conventional niobium titanium nitride (NbTiN) superconducting contacts with each other through a single crystal thin film layer of the ferromagnetic half-metal CrO2, as shown in the figure, while varying the distance between the contacts. At temperatures below -263ºC (or 10 Kelvin), they observed a supercurrent through the structure proving the occurrence of superconductivity. The supercurrent persisted even when the contacts were separated by a distance of ~ 1 μm. Furthermore, the supercurrent was dependent on the orientation of the magnetization in the ferromagnet, establishing the subtle interplay of these normally disparate phenomena. These results provide strong evidence for the conversion from spin singlet pairs to spin triplets – that is, to triplets of electrons with two of the spins in parallel – at the superconductor-ferromagnet interface. While such a “spin triplet” conversion in a ferromagnet was predicted in theory, the present work offers the first experimental evidence for the phenomenon. Since the supercurrent can be switched with the direction of the magnetization, it opens up new possibilities for promising applications such as magnetically controllable superconducting devices capable of operating at extremely high frequencies.

R. S. Keizer, S. T. B. Goennenwein, T. M. Klapwijk, G. Miao, G. Xiao, and A. Gupta, Nature 439, 825 (2006).