  We demonstrated that the spin injection/extraction efficiency is enhanced by
an ultrathin Mg insertion layer (<= 2 nm) in Fe/Mg/MgO/n+-Si tunnel junctions.
In diode-type vertical three-terminal devices fabricated on a Si substrate, we
observed the narrower three-terminal Hanle (N-3TH) signals indicating true spin
injection into Si, and estimated the spin polarization in Si to be 16% when the
thickness of the Mg insertion layer is 1 nm, whereas no N-3TH signal was
observed without Mg insertion. This means that the spin injection/extraction
efficiency is enhanced by suppressing the formation of a magnetically-dead
layer at the Fe/MgO interface. We have also observed clear spin transport
signals, such as non-local Hanle signals and spin-valve signals, in a lateral
four-terminal device with the same Fe/Mg/MgO/n+-Si tunnel junctions fabricated
on a Si-on-insulator substrate. It was found that both the intensity and
linewidth of the spin signals are affected by the geometrical effects (device
geometry and size). We have derived analytical functions taking into account
the device structures, including channel thickness and electrode size, and
estimated important parameters; spin lifetime and spin polarizations. Our
analytical functions well explain the experimental results. Our study shows the
importance of suppressing a magnetically-dead layer, and provides a unified
understanding of spin injection/detection signals in different device
geometries.
