FNA Annual Report 2010 - Technische Universiteit Eindhoven

FNA Annual Report 2010 6.Results 43
additional broadening of the curves. Thereby, this approach could allow for a more quantitative analysis of
changes in the OMAR curves resulting from changes in the operating conditions or the material properties.
In the second topic, we investigated spin-polarized
transport through an organic layer, for instance in an
organic spin valve.
The main mechanism for loss of
polarization in most inorganic semiconductors, which is
related to spin–orbit coupling, is negligible in organic
materials. However, there might still be other
mechanisms that cause a smaller but non-zero loss of spin
polarization. We conjectured that the hyperfine fields are
the main source of polarization loss in organic materials,
which results from mixing between the spin-up and spindown
electrons by precession of spins about these
random fields.
We theoretically investigated this effect of the hyperfine
fields on spin polarization. We explicitly included the
hopping transport characteristic for organic
semiconductors. Due to spatial and energetic disorder,
the charges hop from one localized site to another. The
longer the time they spend on a site, the larger the loss of
spin polarization. We showed that an external magnetic
field larger than the typical hyperfine-field strength
reduces the loss of spin polarization. Hence, such an
external field causes the polarization to persist over a larger distance, leading to a magnetic-field dependent
increase of the spin-diffusion length. Using these findings, we could very accurately fit experimental data on the
magnetoresistance of organic spin valves reported in literature. Moreover, we made predictions about the effect
of changing the orientation of the magnetic field, thereby manipulating the spins during transport.
Output
Plastic Spintronics; spin transport and intrinsic magnetoresistance in organic semiconductors
Wiebe Wagemans
PhD thesis, June 2010