Superconductivity Spintronics

miércoles, 23 de junio de 2010

Superconductivity Spintronics

Superconductivity SpintronicsSuperconductivity and ferromagnetism are generally considered competing phenomena - the former associated with electron attraction in an anti-parallel spin orientation to form Cooper pairs, while the latter encourages the parallel alignment of spins. The combination, therefore, of ferromagnetic and superconducting materials in close proximity has led to a range of novel and interesting phenomena, including "π-shift" Josephson Junctions in which the phase of the supercurrent undergoes a sign change due to the oscillatory order parameter in the ferromagnetic barrier leads to re-entrant superconducting critical currents with temperature and ferromagnetic layer thickness and spin-polarisation dependent proximity effects. More generally, the combination of superconducting and ferromagnetic materials can potentially create artificial analogues of the rather rare materials that exhibit both superconductivity and ferromagnetism simultaneously, such as RuSr2GdCu2O8 or heavy fermion materials such as UGe2.

Key to understanding the interactions between superconductors and non-superconductors, including ferromagnets, is the process of Andreev reflection. In conventional Andreev reflection, an electron incident on a normal metal/superconductor interface at energy close to the Fermi energy and below the superconducting gap energy is reflected as a hole with opposite momentum and spin. Furthermore the reflected hole carries information both on the phase of the electron state and the macroscopic phase of the superconductor. In this fashion, the effect of the superconducting condensate can 'leak' into the non-superconducting material over a length scale that can differ from the superconductor's coherence length. In a spin polarised material, the spin dependency of the density of states results in suppression of the Andreev reflection. Thus, information on the local properties of ferromagnetic materials, such as the spin-polarisation and spin-flip scattering, are obtainable through measurements of Andreev reflection at superconductor-ferromagnet interfaces.

We are investigating interaction of superconductivity with artificial magnetic structures, developing a new breed of hybrid superconducting-magnetic devices in which the devices are actively controllable through the magnetic or electron-spin configuration. This represents a convergence of superconductivity and spintronics and so depends upon, and contribute to current spintronics research.