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Interference of Magnetism and Superconductivity
Workshop | Mon, Jul 18, 2016 | Fri, Sep 02, 2016
Directors:Superconductivity and ferromagnetism are the two antagonistic phenomena: a superconductor ex-pels a magnetic field, which in turn destroys the superconductivity. The microscopic theory of super-conductivity has been created by J. Bardeen, L. Cooper and J. Schriffer (BCS) in 1957. Following this BCS theory the electrons due to the interaction with the crystalline lattice may form pairs, called ‘Cooper pairs’. The spins and the moment of electrons in these ‘Cooper pairs’ are opposite. The mag-netic field destroys this singlet superconductivity via the so-called orbital effect and the paramagnet-ic effect. The orbital effect is just the manifestation of the Lorentz force which acts in the opposite directions on the electrons of the Cooper pairs having the opposite momentum. The paramagnetic effect is related with the Zeeman energy of electron spins—the magnetic field favours the orienta-tion of the electrons spins along the field direction. In a ferromagnet a huge internal field acts on the electron spins, thus preventing the coexistence of singlet superconductivity and ferromagnetism.
On the other hand, the ferromagnetism may be transformed into a special type of domain structure, while appearing in the superconducting phase. In the last decade the spectacular discovery of super-conductivity in the ferromagnetic superconductors UGe2 and URhGe provided a strong evidence in favor of the existence of unconventional types of superconductivity – triplet superconductivity, which is insensitive to the paramagnetic effect.
The coexistence of superconductivity and ferromagnetism may be easily achieved in artificially fabri-cated superconductor/ferromagnet heterostructures. Due to the proximity effect, the Cooper pairs can penetrate into the F layer and induce superconductivity there. In such a case we have a unique opportunity to study the properties of superconducting electrons under the influence of a large ex-change field. In addition, it is possible to study the interplay between superconductivity and mag-netism in a controlled manner, since by varying the layer thicknesses we change the relative strengths of two competing orderings. The behavior of the superconducting condensate under these conditions is quite peculiar. The Cooper pair wave function extends from superconductor to ferro-magnetic with damped oscillatory behavior. This results in many new effects: spatial oscillations of the electron density of states, a nonmonotonic dependence of the critical temperature of S/F multi-layers and bilayers on the ferromagnet layer thickness, and the realization of “Pi” Josephson junc-tions in S/F/S systems. Moreover the singlet superconducting pairs (or strictly speaking the super-conducting correlations) can be transformed by a suitable magnetic interface or interlayer (known as a “spin mixer”) into triplet pairs in which the spins are parallel.
The great importance of superconductivity and magnetism for applications is evident and we may expect that their combination in the same system will be at the origin of the new fascinating func-tionalities. The recent progress of the works on the superconducting spintronics is extremely promis-ing and opens the new horizons for superconductivity and magnetism applications.Spin-valve behav-ior in complex S/F structures gives another example of the interesting interplay between magnetism and superconductivity, an effect that is promising for potential applications.
