  Spin-dependent transport through two coupled single-level quantum dots weakly
connected to ferromagnetic leads with collinear magnetizations is considered
theoretically. Transport characteristics, including the current, linear and
nonlinear conductance, and tunnel magnetoresistance are calculated using the
real-time diagrammatic technique in the parallel, serial, and intermediate
geometries. The effects due to virtual tunneling processes between the two dots
via the leads, associated with off-diagonal coupling matrix elements, are also
considered. Negative differential conductance and negative tunnel
magnetoresistance have been found in the case of serial and intermediate
geometries, while no such behavior has been observed for double quantum dots
coupled in parallel. It is also shown that transport characteristics strongly
depend on the magnitude of the off-diagonal coupling matrix elements.
