 Theory Group
London (City University/Imperial) The London theory group is well known for their work on oscillatory exchange coupling and transport in magnetic multilayers, in particular spin-dependent tunnelling in magnetic tunnel junctions. The group has developed over the last ten years very versatile computer codes for evaluating the charge current from the real-space Kubo formula. The codes can be applied to magnetic multilayers of an arbitrary composition. The magnetic multilayers are "grown on a computer" by "depositing" individual atomic planes one by one, using the method of adlayers. We can handle by this method multilayers of up to several hundred atomic planes thick. The input into these codes is a fully realistic band structure of each constituent layer forming the multilayer. We use a multi-orbital tight-binding (TB) method with TB parameters determined from preliminary fully self-consistent local density functional LMTO-TB calculations performed in house. These codes are now so accurate that the conductance of a perfect epitaxial tunnelling junction can be determined quite rigorously for junctions with insulating barriers as thick as 20 atomic planes. Most recently, we have developed a unified self-consistent theory of transport of spin and charge in magnetic multilayers. The theory, which is based on the nonequilibrium Keldysh formalism, allows us to calculate in a unified way spin currents and local spin densities at zero bias, corresponding to equilibrium exchange coupling, as well as transport spin and particle currents which determine spin-current torques acting on the magnetic layers in a multilayer under external bias. Using the same formalism, we can also determine non-equilibrium (transport) spin densities in any atomic plane of the structure under bias. This can again all be done for a fully realistic ab-initio band structure of the multilayer, and codes for calculating spin currents in Co/Cu multilayers have already been successfully implemented and tested. To our knowledge, we are the only theory group world-wide which is currently capable of calculating local spin currents and transport spin densities from first principles, taking into account rigorously the contributions from all the layers of the multilayer. We also have the capability to introduce imperfections into magnetic multilayers using the lateral supercell method in our codes. Role in Network- Calculations of the magnetoresistance of epitaxial tunnelling junctions for new combinations of barrier and electrode materials and for different crystal orientations
- Design and calculations of the magnetoresistance and other properties of novel composite tunnelling junctions exploiting resonant tunnelling
- Fully realistic calculations of the spin-transfer torques in magnetic multilayers
- Microscopic calculations of the current-induced switching paths in junctions with uniaxial and shape anisotropies
- Discussion of the dynamical stability of current-induced switching paths using the Landau-Lifshitz-Gilbert equation with microscopically calculated spin-current torques
- Calculations of the accumulated spin densities in multilayers under bias and of both the injected spin currents and spin densities in paramagnets in contact with a ferromagnetic electrode
- Calculation of domain wall motion due to a current-induced torque
Personnel Prof. J. Mathon (City) Prof. D. M. Edwards (Imperial) Dr. A. Umerski (OU Visiting Fellow)
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