The generation, manipulation and detection of a pure spin current (i.e., the flow of spin angular momentum without a charge current) are prospective approaches for realizing next-generation spintronic devices with ultra-low electric power consumption. Conventional ferromagnetic electrodes such as Co and NiFe have been utilized as spin injectors to generate pure spin currents in nonmagnetic channels. However, the generation efficiency of pure spin currents is extremely low at room temperature, giving rise to a serious obstacle for device applications. Here we demonstrate the generation of giant pure spin currents at room temperature in lateral spin valve devices with a highly ordered Heusler-compound Co 2 FeSi (CFS) spin injector. The generation efficiency of pure spin currents from the CFS spin injectors is 10 times greater than that of the NiFe injectors, indicating that Heusler compound spin injectors with high spin polarization enable us to materialize a high-performance lateral spin device. The present study is a technological jump in spintronics, and indicates the great potential of ferromagnetic Heusler compounds with half metallicity for generating pure spin currents.

We study ferromagnetic properties of Heusler-alloy Co2FeSi epilayers grown on silicon (Si). The magnetic moment and in-plane magnetic anisotropy of the Co2FeSi/Si(111) epilayers vary significantly with the growth temperature (TG) even in the low-temperature region (TG≤200 °C). These features are induced by reaction phases formed at the interface between Co2FeSi and Si. At TG=100 °C, however, we can obtain both highly ordered L21 structures on Si and high-quality Co2FeSi/Si heterointerfaces at the same time. This fact will open a road to realize a Co-based half-metallic spin injector and detector for Si-based spintronic devices.

We study room-temperature generation and detection of pure spin currents using lateral spin-valve devices with electrodes formed from the Heusler compounds Co 2 FeSi (CFS) or Fe 3 Si (FS). The magnitude of the nonlocal spin-valve signals is strongly affected by resistivity variations observed particularly in low-temperature-grown Heusler compounds containing ordered structures. From an analysis based on a one-dimensional spin diffusion model, we find that the spin polarization monotonically increases with decreasing resistivity, which depends on the structural ordering, for both the CFS and FS electrodes, and show that CFS has a larger spin polarization than FS.In the field of spintronics, evaluation of the spin polarization of ferromagnetic materials is essential to understand the general physics of materials and spin-related transport properties in device structures. To realize high-performance devices, the use of highly spin-polarized ferromagnets is required. The Co-based Heusler compounds, which are halfmetallic ferromagnets, 1-5 are promising for obtaining huge magnetoresistance effects in vertical device structures 2,6-8 and for electrical spin injection into semiconductors. 9 The spin polarizations of these Co-based Heusler compounds have been directly measured by using the point-contact Andreev reflection (PCAR) technique, 10 the Valet-Fert model with the current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) effect, 11,12 and the Julliere model with the tunneling magnetoresistance (TMR) effect. 13,14 Although relatively high values have been reported, it is technically difficult to obtain the precise spin polarization at room temperature for these Heusler compounds. First of all, although the PCAR method allows a relatively precise estimation of the spin polarization, it is limited to low temperatures. Using the Valet-Fert model with CPP-GMR, only limited information can be obtained about the overall series resistance of the device, which is a combination of that of the ferromagnetic electrodes, nonmagnetic spacer, and interfaces. 11,12 In the case of the Julliere model with TMR, the tunnel barrier used in most magnetic tunnel junctions is composed of crystalline MgO in order to allow the subsequent epitaxial growth of the top Heusler-compound electrode. 13,14 In this case, the spin polarization of the Heusler-compound electrodes may be overestimated if the spin-filter effect of the MgO barrier predominantly enhances the TMR effect. 15,16 Furthermore, for the above two vertical structures, since the bottom and top electrodes are fabricated under different conditions, this can lead to different degrees of structural ordering, which can have a strong influence on the spin polarization of the Heusler compounds. 2,11,14 On the other hand, multiterminal lateral structures can provide a significant amount of information on spin-related phenomena because of the flexibility of the probe configuration. In particular, nonlocal spin-valve (NLSV) measurements enable the detection of the pure spin current ...

This paper reviews the recent progress in germanium (Ge) spintronics on the basis of the electrical spin injection from ferromagnets (FM), where Ge is a next generation semiconductor for applications such as CMOS and optical communication on the silicon platform. In general, four-terminal nonlocal voltage measurements in FM–Ge lateral spin-valve devices are important to discuss the spin transport and spin relaxation in n-Ge. First, to obtain relatively low contact resistance compared to the FM/MgO/Ge contacts, we introduce the formation of high-quality FM/Ge heterointerfaces with a phosphorus δ-doped Ge layer, where the atomic arrangement matching at the interface between the (1 1 1) surface of body-centered cubic FMs and Ge(1 1 1) is important. Next, we explain electrical detections of the spin transport in degenerate n-Ge. Owing to the Heusler alloy/Ge Schottky-tunnel barrier contacts, we obtain relatively large spin signals compared to those detected by using conventional CoFe contacts. Furthermore, we can experimentally determine the spin diffusion length and the spin lifetime in degenerate n-Ge by quantitatively analyzing the contact-distance dependence of the spin signals and the Hanle-effect curves. Since we can clarify the temperature dependence of the spin lifetime from 8 to 296 K, the spin relaxation mechanism in n-Ge can be understood as a consequence of the intervalley spin-flip scattering in the conduction band. We propose an advantage over GaAs systems by comparing the spin lifetimes between Ge and GaAs at around room temperature. Finally, we describe future prospects of Ge spintronics, including vertically fabricated device structures such as vertical spin MOSFETs and spin LEDs.

Using high-quality Fe3Si/n + -Ge Schottky-tunnel-barrier contacts, we study spin accumulation in an n-type germanium (n-Ge) channel. In the three-or two-terminal voltage measurements with low bias current conditions at 50 K, Hanle-effect signals are clearly detected only at a forward-biased contact. These are reliable evidence for electrical detection of the spin accumulation created in the n-Ge channel. The estimated spin lifetime in n-Ge at 50 K is one order of magnitude shorter than those in n-Si reported recently. The magnitude of the spin signals cannot be explained by the commonly used spin diffusion model. We discuss a possible origin of the difference between experimental data and theoretical values.