  We report the results of magnetoresistance measurements in vertical organic
spin valves with the magnetic field oriented perpendicular to the layer stack.
The magnetoresistance measurements were performed after carefully preparing
either parallel or antiparallel in-plane magnetization states of the magnetic
electrodes in order to observe traces of Hanle precession. Due to the low
mobility in organic semiconductors the transit time of spin polarized carriers
should allow for precession of the spins in perpendicular fields which in
statistical average would quench the magnetoresistance. However, in none of the
experiments we do observe any change in resistance while sweeping the
perpendicular field, up to the point where the electrode's magnetization starts
to reorient. This absence of Hanle type effects indicates that the
magnetoresistance is not based on the injection of spin polarized electrons
into the organic semiconductor but rather on tunneling through pinholes
superimposed with tunneling anisotropic magnetoresistance.
