▎ 摘　　要

Based on a tight-binding model and a recursive Green's function technique, spin-dependent ballistic transport through tiny graphene sheets (flakes) is studied. The main interest is focused on electrical conductivity, giant magnetoresistance (GMR), and shot noise. It is shown that when graphene flakes are sandwiched between two ferromagnetic electrodes, the resulting GMR coefficient may be quite significant. This statement holds true both for zigzag and armchair chiralities, as well as for different aspect (width/length) ratios. Remarkably, in absolute values the GMR of the armchair-edge graphene flakes is systematically greater than that corresponding to the zigzag-edge graphene flakes. This finding is attributed to the different degree of conduction channel mixing for the two chiralities in question. It is also shown that for big aspect ratio flakes, three-dimensional end-contacted leads, very much like invasive contacts, result in nonuniversal behavior of both conductivity and Fano factor.