Spin injection into silicon in three-terminal vertical and four-terminal lateral devices with Fe/Mg/MgO/Si tunnel junctions having an ultrathin Mg insertion layer

Abstract

We demonstrate that the spin injection/extraction efficiency is enhanced by an ultrathin Mg insertion layer (\ensuremath{\leqslant}2 nm) in $\mathrm{Fe}\text{/}\mathrm{Mg}\text{/}\mathrm{MgO}\text{/}{n}^{+}\text{\ensuremath{-}}\mathrm{Si}$ tunnel junctions. In diode-type vertical three-terminal devices fabricated on a Si substrate, we observe the narrower three-terminal Hanle (N-3TH) signals indicating true spin injection into Si and estimate the spin polarization in Si to be 16% when the thickness of the Mg insertion layer is 1 nm, whereas no N-3TH signal is observed without the 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 also observe clear spin transport signals, such as nonlocal Hanle signals and spin-valve signals, in a lateral four-terminal device with the same $\mathrm{Fe}\text{/}\mathrm{Mg}\text{/}\mathrm{MgO}\text{/}{n}^{+}\text{\ensuremath{-}}\mathrm{Si}$ tunnel junctions fabricated on a Si-on-insulator substrate. It is 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 polarization. Our analytical functions explain the experimental results very well. 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.