The ability to manipulate electron spin in organic molecular materials offers a new and extremely tantalizing route towards spin electronics, both from fundamental and technological points of view. This is mainly due to the unquestionable advantage of weak spin-orbit and hyperfine interactions in organic molecules, which leads to the possibility of preserving spin-coherence over times and distances much longer than in conventional metals or semiconductors. Here we demonstrate theoretically that organic spin valves, obtained by sandwiching an organic molecule between magnetic contacts, can show a large bias-dependent magnetoresistance and that this can be engineered by an appropriate choice of molecules and anchoring groups. Our results, obtained through a combination of state-of-the-art non-equilibrium transport methods and density functional theory, show that although the magnitude of the effect varies with the details of the molecule, large magnetoresistance can be found both in the tunnelling and the metallic limit.

Ab initio computational methods for electronic transport in nanoscaled systems are an invaluable tool for the design of quantum devices. We have developed a flexible and efficient algorithm for evaluating I-V characteristics of atomic junctions, which integrates the nonequilibrium Green's function method with density functional theory. This is currently implemented in the package SMEAGOL. The heart of SMEAGOL is our scheme for constructing the surface Green's functions describing the current-voltage probes. It consists of a direct summation of both open and closed scattering channels together with a regularization procedure of the Hamiltonian and provides great improvements over standard recursive methods. In particular it allows us to tackle material systems with complicated electronic structures, such as magnetic transition metals. Here we present a detailed description of SMEAGOL together with an extensive range of applications relevant for the two burgeoning fields of spin and molecular electronics.

We have developed an efficient simulation tool 'GOLLUM' for the computation of electrical, spin and thermal transport characteristics of complex nanostructures. The new multi-scale, multi-terminal tool addresses a number of new challenges and functionalities that have emerged in nanoscale-scale transport over the past few years. To illustrate the flexibility and functionality of GOL-LUM, we present a range of demonstrator calculations encompassing charge, spin and thermal transport, corrections to density functional theory such as local density approximation +U (LDA+U) and spectral adjustments, transport in the presence of non-collinear magnetism, the quantum Hall effect, Kondo and Coulomb blockade effects, finite-voltage transport, multi-terminal transport, quantum pumps, superconducting nanostructures, environmental effects, and pulling curves and conductance histograms for mechanically-controlled breakjunction experiments.New J. Phys. 16 (2014) 093029 J Ferrer et al that non-equilibrium transport codes are quite difficult to handle, in part because of their complex input data structures, which can create a steep learning curve, and also because they carry very heavy computational demands. As a consequence, we have devised the new code GOLLUM to be more user friendly, with simple and easy to understand input and output structures, and with no accuracy parameters to tune. We present now a short summary of the features and functionalities of the two programs to better appreciate their differences. SMEAGOL is a non-equilibrium Green's function (NEGF) program that computes the charge and spin transport properties of two-terminal junctions subject to a finite voltage bias. SMEAGOL cannot read a user-defined tight-binding Hamiltonian. Instead, it reads the meanfield Hamiltonian from the program SIESTA [27] and is tightly bound to the old versions of it. SMEAGOL can read from SIESTA Hamiltonians carrying non-collinear spin arrangements as well as the spin-orbit interaction. SIESTA and SMEAGOL have indeed been used successfully to simulate the magnetic anisotropies of atomic clusters [28][29][30] as well as the spin transport functionalities of several atomic chains and molecular junctions subjected to strong spin-orbit interaction [31,32]. However, SMEAGOL does not profit from other recent density functionals. Examples are the van der Waals family of functionals or those based on the local density approximation + U (LDA+U) approach.GOLLUM is a program that computes the charge, spin and electronic contribution to the thermal transport properties of multi-terminal junctions. In contrast to NEGF codes, GOLLUM is based on equilibrium transport theory, which means that it has a simpler structure, is faster, and consumes less memory. The program has been designed for user-friendliness and takes a considerable leap towards the realization of ab initio multi-scale simulations of conventional and more sophisticated transport functionalities.The simpler interface of GOLLUM allows it to read model tight-binding Hamiltonians. Fur...