Tomoaki Inokuchi

Effect of the interface resistance of CoFe/MgO contacts on spin accumulation in silicon

We show direct evidence for importance of the interface resistance to electrically create large spin accumulation in silicon (Si). With increasing the thickness of the tunnel barrier in CoFe/MgO/n+-Si devices, a marked enhancement of spin accumulation signals can be observed in the electrical Hanle-effect measurements. To demonstrate room-temperature detection of the spin signals in three-terminal methods, the influence of the spin absorption from Si into CoFe through a tunnel barrier should be taken into account.

Electrical Spin Injection into n-GaAs Channels and Detection through MgO/CoFeB Electrodes

Spin injection and detection between MgO tunnel barrier/CoFeB electrodes and n-GaAs channels were investigated by means of all electrical transport measurements. The magnetoresistance of the GaAs/MgO/CoFeB junctions and non-local signals through the lateral GaAs channel were detected in the same sample. The non-local signals were detected when the magnetization directions of two CoFeB electrodes differed. This correspondence between the magnetization directions and the non-local signals clearly shows that the injection and detection of the spin-polarized current are realized in the GaAs/MgO/CoFeB systems.

Spin-Based MOSFETs for Logic and Memory Applications and Spin Accumulation Signals in CoFe/Tunnel Barrier/SOI Devices

New innovative ferromagnetic source/drain technologies on Si for next-generation-transistor applications are researched and developed using CoFe/AlO_x<i>n</i>⁺-Si and CoFe/MgO <i>n</i>⁺-Si junctions. As evidence of the spin accumulation in the <i>n</i>⁺-Si conduction channels, nonlocal spin signals and four-terminal nonlocal-Hanle signals are presented for CoFe/MgO/SOI devices. The spin diffusion times determined by four-terminal nonlocal-Hanle signals are consistent with those observed in three-terminal Hanle signals. The relatively long spin diffusion time of τ<i>s</i>=1.4 nsec and relatively large spin polarization <i>P</i>=0.43 at room temperature for CoFe/MgO/SOI devices were observed, when fitting to the existing diffusion model for spin injection and accumulation. We have observed the marked enhancement of the absolute value of three-terminal voltage changes via Hanle-type spin precessions (|Δ<i>V</i>|) as a function of interface resistance in the temperature range between 20 K and 300 K. We also have observed the asymmetric bias voltage dependence on Δ<i>V</i>. In terms of the reason of marked enhancement of |Δ<i>V</i>| as a function of interface resistance, the spin absorption into ferromagnet would be most effective. For the explanation of the asymmetric bias voltage dependence, we should take into account two additional possible origins. Moreover, we succeed in decreasing the interface resistance for CoFe/MgO/ <i>n</i>⁺-Si junctions down to 36 Ωμm² by using evaporation method for MgO deposition.

Current-induced magnetization switching under magnetic field applied along the hard axis in MgO-based magnetic tunnel junctions

The authors have investigated the dependence of the critical current density (Jc) for the current-induced magnetization switching (CIMS) on external magnetic fields applied along the hard axis of a free layer (Hhard) and on the duration of pulse current in MgO-based magnetic tunnel junctions. The Jc and the intrinsic current density (Jc0), derived from the dependence of the Jc on the pulse duration, decreased as ∣Hhard∣ increased. These reductions of Jc and Jc0 would be attributed to the decrease of energy barrier for CIMS and the increase of the spin transfer efficiency.

Maximum magnitude in bias-dependent spin accumulation signals of CoFe/MgO/Si on insulator devices

We study in detail how the bias voltage (Vbias) and interface resistance (RA) depend on the magnitude of spin accumulation signals (|ΔV| or |ΔV|/I, where I is current) as detected by three-terminal Hanle measurements in CoFe/MgO/Si on insulator (SOI) devices with various MgO layer thicknesses and SOI carrier densities. We find the apparent maximum magnitude of spin polarization as a function of Vbias and the correlation between the magnitude of spin accumulation signals and the shape of differential conductance (dI/dV) curves within the framework of the standard spin diffusion model. All of the experimental results can be explained by taking into account the density of states (DOS) in CoFe under the influence of the applied Vbias and the quality of MgO tunnel barrier. These results indicate that it is important to consider the DOS of the ferromagnetic materials under the influence of an applied Vbias and the quality of tunnel barrier when observing large spin accumulation signals in Si.

Local magnetoresistance through Si and its bias voltage dependence in ferromagnet/MgO/silicon-on-insulator lateral spin valves

Local magnetoresistance (MR) through silicon (Si) and its bias voltage (Vbias) (bias current (Ibias)) dependence in ferromagnet (FM)/MgO/silicon-on-insulator lateral spin valves are investigated. From the experimental measurements, we find that the local-MR through Si increases with increasing Vbias. This anomalous increase of local-MR as a function of Vbias can be understood by considering the standard drift-diffusion theory improved by taking into account the difference in the interface resistances and first order quantum effect between FM/MgO/Si (source) and Si/MgO/FM (drain) interfaces. The interface resistance dependence on experimentally obtained local-MR ratios also agrees with the improved standard spin diffusion theory. These results indicate that experimentally observed local-MR is certainly related to the spin signal through the Si bulk band.

Spin injection and detection between CoFe/AlOx junctions and SOI investigated by Hanle effect measurements

Spin injection and detection properties in Co50Fe50/AlOx/SOI (Si on insulator) junctions were investigated by using Hanle effect measurements up to room temperature. Cross-sectional transmission electron microscope images and Fourier transform images of the Co50Fe50/AlOx/SOI junction, fabricated by using appropriate oxidation condition, indicate the AlOx layer became single-crystal-like and spin injection and detection were realized at room temperature. In contrast, in junction fabricated by using excess oxidation condition, AlOx layer became poly-crystal-like and spin injection was not realized though spin injection was realized below 150 K. These results indicate that appropriate oxidation is essential to fabricate AlOx tunnel barrier with good crystallinity and realize spin injection and detection at room temperature.

Read/write operation of spin-based MOSFET using highly spin-polarized ferromagnet/MgO tunnel barrier for reconfigurable logic devices

For future high-performance reconfigurable logic devices, we developed a novel spin-based MOSFET; “Spin-Transfer-Torque-Switching MOSFET (STS-MOSFET)” that enables the read/write performance and memorization of the configuration with nonvolatility by using the ferromagnetic electrodes and the spin-polarized current through Si channel and spin-transfer torque switching in magnetic tunnel junctions (MTJs) on the source/drain. The read/write operation of the STS-MOSFET was first demonstrated in this work. The highly spin-polarized ferromagnet/MgO tunnel barrier electrodes and the MTJs using their structure for the source/drain showed great possibility to realize our proposed STS-MOSFET and to enhance their performance.

Scalability of spin field programmable gate array: A reconfigurable architecture based on spin metal-oxide-semiconductor field effect transistor

The scalability of a field programmable gate array (FPGA) using a spin metal-oxide-semiconductor field effect transistor (MOSFET) (spin FPGA) with a magnetocurrent (MC) ratio in the range of 100–1000% is discussed for the first time. The area and speed of million-gate spin FPGAs are numerically benchmarked with CMOS FPGA for 22, 32, and 45 nm technologies including a 20% transistor size variation. We show that the area is reduced and the speed is increased in spin FPGA due to the nonvolatile memory function of spin MOSFET.Scalability of Field Programmable Gate Array (FPGA) using spin MOSFET (spin FPGA) with magnetocurrent (MC) ratio in the range of 100% to 1000% is discussed for the first time. Area and speed of million-gate spin FPGA are numerically benchmarked with CMOS FPGA for 22nm, 32nm and 45nm technologies including 20% transistor size variation. We show that area is reduced and speed is increased in spin FPGA owing to the nonvolatile memory function of spin MOSFET.

Voltage-control spintronics memory (VoCSM) having potentials of ultra-low energy-consumption and high-density

We propose a new spintronics-based memory employing the voltage-control-magnetic-anisotropy effect as a bit selecting principle and the spin-orbit-torque effect as a writing principle. We have fabricated the prototype structure, and successfully demonstrated the writing scheme specific to this memory architecture.