  In this thesis we study electron transport through magnetic nanocontacts and
nanowires with ab initio quantum transport calculations. The aim is to gain a
thorough understanding of the interplay between electrical conduction and
magnetism in atomic-size conductors and how it is affected by different aspects
as e.g. the atomic structure and the chemical composition of the conductor. To
this end our ab initio quantum transport program ALACANT which combines the
non-equilibrium Green's function formalism (NEGF) with density functional
theory (DFT) calculations has been extended to describe spin-polarized systems.
We present calculations on nanocontacts made of Ni as a prototypical magnetic
material. We find that atomic disorder in the contact region strongly reduces
the a priori high spin-polarization of the conductance leading to rather
moderate values of the so-called ballistic magnetoresistance (BMR). On the
other hand, we show that the adsorption of oxygen in the contact region could
strongly enhance the spin-polarization of the conduction electrons and thus BMR
by eliminating the spin-unpolarized s-channel. Finally, we show that short
atomic Pt chains suspended between the tips of a nanocontact are magnetic in
contrast to bulk Pt. However, this emergent nanoscale magnetism barely affects
the overall conductance of the nanocontact making it thus difficult to
demonstrate by simple conductance measurements. In conclusion, we find that
spin-transport through atomic-scale conductors is quite sensitive to the actual
atomic structure as well as to the chemical composition of the conductor. This
presents both, opportunities and challenges for the realization of future
nanoscale spintronics devices.
