Fachgruppe Nanostrukturierte Materialien    

We report the results of magnetoresistance measurements in vertical organic spin valves with themagnetic 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 do we 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. These measurements suggest that even for clear spin-valve signatures, control experiments should be established which can confirm or disprove the electrical detection of injected spin-polarized carriers, similar to the Hanle precession in inorganic spin valves.