Hiroshi Idzuchi

Giant enhancement of spin accumulation and long-distance spin precession in metallic lateral spin valves

The non-local spin injection in lateral spin valves is strongly expected to be an effective method to generate a pure spin current for potential spintronic application. However, the spin-valve voltage, which determines the magnitude of the spin current flowing into an additional ferromagnetic wire, is typically of the order of 1 μV. Here we show that lateral spin valves with low-resistivity NiFe/MgO/Ag junctions enable efficient spin injection with high applied current density, which leads to the spin-valve voltage increasing 100-fold. Hanle effect measurements demonstrate a long-distance collective 2π spin precession along a 6-μm-long Ag wire. These results suggest a route to faster and manipulable spin transport for the development of pure spin-current-based memory, logic and sensing devices.

Enhanced spin accumulation obtained by inserting low-resistance MgO interface in metallic lateral spin valves

We have systematically investigated the interface contributions to the spin injection characteristics in permalloy/MgO/Ag lateral spin valves. The spin valve signal remarkably increases with MgO thickness and reaches a maximum when the interface resistance is about 100 fΩ m2 for 1 nm thick MgO, which is two orders of magnitude lower than that of the typical tunnel junction. Our quantitative analysis based on the spin-dependent diffusion equation considering variable spin polarization in the MgO layer well describes the observed trend in the spin valve signals.

5d iridium oxide as a material for spin-current detection

Devices based on pure spin currents have been attracting increasing attention as key ingredients for low-dissipation electronics. To integrate such spintronics devices into charge-based technologies, electric detection of spin currents is essential. The inverse spin Hall effect converts a spin current into an electric voltage through spin-orbit coupling. Noble metals such as Pt and Pd, and also Cu-based alloys, have been regarded as potential materials for a spin-current injector, owing to the large direct spin Hall effect. Their spin Hall resistivity ρSH, representing the performance as a detector, is not large enough, however, due mainly because of their low charge resistivity. Here we report that a binary 5d transition metal oxide, iridium oxide, overcomes the limitations encountered in noble metals and Cu-based alloys and shows a very large ρSH~38 μΩ cm at room temperature.

Spin relaxation mechanism in silver nanowires covered with MgO protection layer

Spin-flip mechanism in Ag nanowires with MgO surface protection layers has been investigated by nonlocal spin injection using permalloy/Ag lateral spin valves. The spin flip events mediated by surface scattering are effectively suppressed by the MgO capping layer. The spin relaxation process was found to be well described in the framework of Elliott-Yafet mechanism (R. J. Elliott, Phys. Rev. 96, 266 (1954); Y. Yafet, in Solid State Physics, edited by F. Seitz and D. Turnbull (Academic, New York, 1963), pp. 1–98) and then the probabilities of spin-filp scattering for phonon or impurity mediated momentum scattering is precisely determined in the nanowires. The temperature dependent spin-lattice relaxation follows the Bloch-Gruneisen theory (V. F. Bloch, Z. Phys. 59, 208 (1930); V. E. Gruneisen, Ann. Phys. 5, 530 (1933)) and falls on to a universal curve of Ag as in the conduction-electron-spin resonance data for bulk.

Revisiting the measurement of the spin relaxation time in graphene-based devices

A long spin relaxation time (tausf) is the key for the applications of graphene to spintronics but the experimental values of tausf have been generally much shorter than expected. We show that the usual determination by the Hanle method underestimates tausf if proper account of the spin absorption by contacts is lacking. By revisiting series of experimental results, we find that the corrected tausf are longer and less dispersed, which leads to a more unified picture of tausf derived from experiments. We also discuss how the correction depends on the parameters of the graphene and contacts.

Effect of anisotropic spin absorption on the Hanle effect in lateral spin valves

We have succeeded in fully describing dynamic properties of spin current including the different spin absorption mechanism for longitudinal and transverse spins in lateral spin valves, which enables to elucidate intrinsic spin transport and relaxation mechanism in the nonmagnet. The deduced spin lifetimes are found independent of the contact type. From the transit-time distribution of spin current extracted from the Fourier transform in Hanle measurement data, the velocity of the spin current in Ag with Py/Ag Ohmic contact turns out much faster than that expected from the widely used model.

Spin injection properties in trilayer graphene lateral spin valves

We report on the electrical injection and detection of spin accumulation in trilayer-graphene/MgO/Permalloy lateral spin-valve (LSV) structure. Non-local spin valve signal is clearly observed in the LSV, indicating that spin coherence extends underneath all ferromagnetic contacts. We also show that low-resistivity graphene/MgO/Py junctions enable efficient spin injection and detection in LSV with high applied current density, which leads to large spin accumulation of 120 μV at room temperature. A spin diffusion length of 1.5 μm was obtained for the injector-detector separation dependence of spin valve signal measurements carried out at room temperature, while at T = 10 K, the diffusion length increases to 2.3 μm.

Effect of Annealing on Interfacial Spin Polarization and Resistance in Permalloy/MgO/Ag Lateral Spin Valves

We have investigated the effect of annealing on the nonlocal spin valve signal in lateral spin valves with permalloy/MgO/Ag junctions. The results show that annealing up to 400 °C pushes up the spin polarization of the MgO interface to 38%, resulting in an increase in the nonlocal spin valve signal of up to 77.2 mΩ. Further increase in the annealing temperature up to 500 °C causes a decrease in the spin valve signal of 48.0 mΩ. However, the high spin polarization of 55% could be advantageous for the enhancement of the spin accumulation in lateral spin valves.

Spin Diffusion Characteristics in Magnesium Nanowires

The spin diffusion characteristics of magnesium have been investigated by using lateral spin-valve structures consisting of permalloy spin injector and detector electrodes bridged by a magnesium-nanowire. Large spin valve signals of 3.6 and 1.1 mΩ were observed at 10 K and room temperature (RT). From the spin injector–detector distance dependence, the spin diffusion length in magnesium was determined to be 720 nm at 10 K and 230 nm at RT, indicating magnesium matches common materials such as Al, Cu, and Ag in terms of the spin diffusion length.

Impact of interface properties on spin accumulation in dual-injection lateral spin valves

We study spin accumulation in dual-injection lateral spin valves (DLSVs) with Ni80Fe20(/MgO)/Ag or Co50Fe50/MgO/Ag junctions. In Ohmic NiFe/Ag junctions, there is negligible enhancement in the spin accumulation for the dual scheme compared with the conventional single scheme. In contrast, large spin valve signals of 233 and 480 m Ω are observed for DLSVs with NiFe/MgO/Ag and CoFe/MgO/Ag junctions, respectively. The experimental results are analyzed with a one-dimensional spin diffusion model, taking into account the junctions and their structures. The efficient generation of a pure spin current IS/IC up to 0.55 is realized.