  We investigate the origin of purported "spin accumulation" signals observed
in local "three-terminal" (3T) measurements of ferromagnet/insulator/n-Si
tunnel junctions using inelastic electron tunneling spectroscopy (IETS).
Voltage bias and magnetic field dependences of the IET spectra were found to
account for the dominant contribution to 3T magnetoresistance signals, thus
indicating that it arises from inelastic tunneling through impurities and
defects at junction interfaces and within the barrier, rather than from spin
accumulation due to pure elastic tunneling into bulk Si as has been previously
assumed.
