  We investigate transport through 6 to 10 nm thin epitaxial GaAs(001) barriers
sandwiched between polycrystalline iron films. Apart from a pronounced
tunneling magnetoresistance effect (TMR) at low magnetic fields we observe a
distinct negative magnetoresistance (MR) at low and a positive MR at higher
temperatures. We show that the negative MR contribution is only observed for
the ferromagnetic iron contacts but is absent if iron is replaced by copper or
gold electrodes. Possible explanations of the negative MR involve suppression
of spin-flip scattering or Zeeman splitting of the tunneling barrier.
