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D.J. Hykel, C. Paulsen, D. Aoki, J.R. Kirtley and K. Hasselbach

In the conventional BCS theory, Cooper pairs are in a spin singlet state, and the application of a magnetic field will break the Cooper pairs and destroy the superconducting state. Therefore, ferromagnetism and supercon- ductivity are usually two antagonist and competing electronic states due to the ferromagnetic interactions and the large internal fields they can create. Both states, however can coexist if the ferromagnetic domains are small compared to the superconducting coherence length, or in the case of exotic superconductivity with spin triplet pairing of electrons. In the last fifteen years the ferromagnetic superconductors UGe2, URhGe and UCoGe were discovered. Upon cooling at ambient pressure URhGe and UCoGe undergo first a transition in a ferromagnetic state before superconductivity appears at very low temperature. These type II superconductors exhibit a huge upper critical field up to several tens of tesla, in spite of the low critical temperature of less than 0.5 K. Our highly sensitive microscopic and macroscopic magnetization measurements at very low temperatures and low external magnetic fields have allowed to explain how both states can coexist.

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article on open access archive Hal

Physical Review Letters article