Tohru Oikawa

Room-Temperature Electron Spin Transport in a Highly Doped Si Channel

We report on the first demonstration of generating a spin current and spin transport in a highly doped Si channel at room temperature (RT) using a four-terminal lateral device with a spin injector and a detector consisting of an Fe/MgO tunnel barrier. Spin current was generated using a nonlocal technique, and spin injection signals and Hanle-type spin precession were successfully detected at 300 K, thus proving spin injection with the elimination of spurious signals. The spin diffusion length and its lifetime at RT were estimated to be 0.6 µm and 1.3 ns by the Hanle-type spin precession, respectively.

Electrical Spin Injection into Silicon Using MgO Tunnel Barrier

We demonstrate the evidence of electrical spin injection into silicon (Si) using the Fe/MgO tunnel barrier by the following two methods: 1) non-local (NL) four-terminal magnetoresistance (MR) scheme with their respective corresponding ferromagnetic (FM) electrode and 2) Hanle-type spin precession scheme. These results are compatible and in agreement with each other. The spin injection signals in the non-local scheme were observed up to 150 K, and the spin diffusion length (¿N) was estimated to be 2.8 ¿m at 8 K. The experimental data completely matches every detail of the Hanle measurements equation, and spin lifetime (¿) was estimated to be 9.44 ns at 8 K. We will be able to discuss the physical properties of a pure spin current in silicon by this compelling data.

Comparison of spin signals in silicon between nonlocal four-terminal and three-terminal methods

The three-terminal (3T) measurement is a method of detecting spin accumulation at a ferromagnetic/semiconductor interface. Spin polarization (P) at the injector with an electric field (Pinjector) and that at the detector without an electric field (Pdetector) were measured separately by using the nonlocal (NL)-Hanle and 3T measurements, and Pinjector and Pdetector exhibited the same behavior with increasing temperature. We also found that the spin lifetime (τsp) in highly doped silicon measured by using the 3T method coincides with that estimated by the NL-Hanle measurement, which shows that the localized state does not exist at the interface.

Temperature dependence of spin diffusion length in silicon by Hanle-type spin precession

The Hanle-type spin precession method was carried out in association with nonlocal (NL) magnetoresistance measurement using a highly doped (5×1019 cm−3) silicon (Si) channel. The spin diffusion length obtained by the Hanle-method is in good agreement with that of the gap dependence of NL signals. We have evaluated the interface and bulk channel effects separately, and it was demonstrated that the major factor of temperature dependence of NL signals originates from the spin polarization reduction at interface between the tunnel barrier and silicon.

Evidence of Electrical Spin Injection Into Silicon Using MgO Tunnel Barrier

We demonstrate the evidence of electrical spin injection into silicon (Si) using the Fe/MgO tunnel barrier by the following two methods: 1) non-local (NL) four-terminal magnetoresistance (MR) scheme with their respective corresponding ferromagnetic (FM) electrode and 2) Hanle-type spin precession scheme. These results are compatible and in agreement with each other. The spin injection signals in the non-local scheme were observed up to 150 K, and the spin diffusion length (?N) was estimated to be 2.8 ?m at 8 K. The experimental data completely matches every detail of the Hanle measurements equation, and spin lifetime (?) was estimated to be 9.44 ns at 8 K. We will be able to discuss the physical properties of a pure spin current in silicon by this compelling data.

Investigation of the inverted Hanle effect in highly doped Si

The underlying physics of the inverted Hanle effect appearing in Si was experimentally investigated using a Si spin valve, where spin transport was realized up to room temperature. No inverted-Hanle-related signal was observed in a non-local 4-terminal scheme even the same ferromagnetic electrode was used, whereas the signal was detected in a non-local 3-terminal scheme. Although the origin of the inverted Hanle effect has been thought to be ascribed to interfacial roughness beneath a ferromagnetic electrode, our finding is inconsistent with the conventional interpretation. More importantly, we report that there were two different Hanle signals in a non-local 3-terminal scheme, one of which corresponds to the inverted Hanle signal but the other is ascribed to spin transport. These results strongly suggest that (1) there is room for discussion concerning the origin of the inverted Hanle effect, and (2) achievement of spin transport in a non-local 4-terminal scheme is indispensable for further understanding of spin injection, spin transport and spin coherence in Si. Our new findings provide a new and strong platform for arising discussion of the physical essence of Hanle-related spin phenomena.

Effect of spin drift on spin accumulation voltages in highly doped silicon

We investigated the effect of spin drift on spin accumulation in highly doped silicon (Si) by using a non-local three-terminal (NL-3T) and four-terminal (NL-4T) methods, and have clarified that the spin accumulation voltages in a NL-3T device were modulated due to spin drift and that spin lifetime can be accurately extracted by employing a modified spin drift-diffusion equation. The extracted spin lifetime is 4-7 ns, which is slightly shorter than the intrinsic spin lifetime (8 ns) measured in the NL-4T method in the same Si device because of spin drift. It is elucidated that the spin drift effect should be considered for the precise estimation of spin lifetime in Si by NL-3T method.

Local magnetoresistance in Fe/MgO/Si lateral spin valve at room temperature

Room temperature local magnetoresistance in two-terminal scheme is reported. By employing 1.6 nm-thick MgO tunnel barrier, spin injection efficiency is increased, resulting in large non-local magnetoresistance. The magnitude of the non-local magnetoresistance is estimated to be 0.0057 Ω at room temperature. As a result, a clear rectangle signal is observed in local magnetoresistance measurement even at room temperature. We also investigate the origin of local magnetoresistance by measuring the spin accumulation voltage of each contact separately.

Local and non-local magnetoresistance with spin precession in highly doped Si

We have demonstrated comparison of spin accumulation voltages in local and non-local (NL) magnetoresistance measurements without any spurious megnetoresistive signals. The spin transport length in a local measurement was larger than that in a NL measurement. An intensity of the local signals was explained by a conductivity mismatch theory. Depending on the relative magnetic configurations, Hanle-type spin precession signals in the local and the NL schemes showed a clear difference in signal shapes. The local transporting spins are accelerated in the direction of the electric field, and precess around the direction of the magnetic field under the detector electrode.

Observation of large spin accumulation voltages in nondegenerate Si spin devices due to spin drift effect: Experiments and theory

A large spin-accumulation voltage of more than 1.5 mV at 1 mA, i.e., a magnetoresistance of 1.5 {\Omega}, was measured by means of the local three-terminal magnetoresistance in nondegenerate Si-based lateral spin valves (LSVs) at room temperature. This is the largest spin-accumulation voltage measured in semiconductor-based LSVs. The modified spin drift-diffusion model, which successfully accounts for the spin drift effect, explains the large spin-accumulation voltage and significant bias-current-polarity dependence. The model also shows that the spin drift effect enhances the spin-dependent magnetoresistance in the electric two terminal scheme. This finding provides a useful guiding principle for spin metal-oxide semiconductor field-effect transistor (MOSFET) operations.