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We report the highest two-terminal magnetoresistance (MR) ratio at room temperature in semiconductor (SC)-based vertical spin-valve (VSV) devices on a silicon (Si) platform. Here, we demonstrate all-epitaxial ferromagnet (FM)–germanium (Ge)–FM VSV devices with Co2FeSi as one of two FM electrodes. In addition to the high spin polarization of Co2FeSi, the relatively low resistance in the parallel magnetization state due to the strong Fermi-level-pinning effect at the p-type Ge/Co2FeSi interface causes the enhancement in the MR ratios. As a result, we achieve two-terminal MR ratios of over 1% even at room temperature through a 20-nm-thick Ge intermediate layer in VSV devices on Si. We emphasize that the use of Co-based Heusler alloys is effective even for SC-based VSV structures on Si to obtain high MR ratios at room temperature. [ABSTRACT FROM AUTHOR]

Compensated ferrimagnetic Heusler compounds with high spin polarization and a low net magnetic moment are strategically important materials for spin-logic and further energy-efficient spintronic applications. However, the element-resolved magnetic ordering of these compensated ferrimagnets remains an open issue. Here, we report a direct observation of the spin and orbital moments of the B2 phase Mn2CoAl thin film using the synchrotron-based x-ray magnetic circular dichroism technique. An ferrimagnetic ordering between Mn and Co elements and a compensated-ferrimagnet-like small net magnetic moment of only 0.34 μB/f.u. were observed unambiguously in B2 Mn2CoAl. Antiparallel coupling between Mn and Co is attributed to the mixture of the Mn(B) and Al occupation in the B2 phase Mn2CoAl lattice. This work demonstrates great potential of the compensated ferrimagnetic half-metallic inverse Heusler compounds Mn2CoAl for spintronic applications. [ABSTRACT FROM AUTHOR]

We study spin torque oscillators comprised of a perpendicular spin injection layer (SIL) and a planar field generating layer to reveal the influence of the spin polarization of SIL material on the critical current density, JC, to induce microwave oscillation. Two systems with different SIL are compared: one with a highly spin-polarized Heusler alloy, Co2Fe(Ga0.5Ge0.5) (CFGG), and the other a prototypical Fe2Co alloy. Cross sectional scanning transmission electron microscopy observations show the B2-ordered structure in a 3-nm-thick CFGG SIL, a prerequisite for obtaining half-metallic transport properties. Current induced microwave oscillations are found at frequencies of ~15 GHz for both systems. However, the current needed to cause the oscillations is ~50% smaller for films with the CFGG SIL compared to those of the Fe2Co SIL. These results are in accordance with micromagnetic simulations that include spin accumulation at the SIL. [ABSTRACT FROM AUTHOR]

Fully epitaxial pseudo spin-valves (PSVs) using 10-nm-thick Co2Fe(Ga0.5Ge0.5) (CFGG) ferromagnetic layers and a 5-nm-thick AgZn space layer annealed at 630°C show a large current-perpendicular-to-plane giant magnetoresistance (CPP-GMR) output with resistance-change area product, ΔRA, of 21.5 mΩ μm² and MR ratio of 59.6% at room temperature. These values are substantially enhanced to ΔRA of 59.8 mΩ μm² and MR ratio of 200.0% at 10K. The large MR is attributed to the high spin polarization of the CFGG electrodes with the enhanced L21 ordering induced by the atomic diffusion of Zn through the CFGG layers. The CPP-PSV shows relatively large ΔRA of 10.9mΩ μm² with the MR ratio of 25.6% for the low annealing temperature of 350°C, which is a practically useful feature for read sensor applications. [ABSTRACT FROM AUTHOR]

We investigate spin transport and accumulation in n+-Si using Heusler compound Co2FeSi/MgO/Si on insulator (SOI) devices. The magnitudes of the non-local four- and three-terminal Hanle effect signals when using Heusler compound Co2FeSi/MgO/SOI devices are larger than when using CoFe/MgO/SOI devices, whereas the preparation methods of MgO layers on SOI are exactly same in both devices. Different bias voltage dependencies on the magnitude of spin accumulation signals are also observed between these devices. Especially, Co2FeSi/MgO/SOI devices show large spin accumulation signals compared with CoFe/MgO/SOI devices in the low bias voltage region less than 1000mV in which the increase of the spin polarization is expected from the estimation of the density of states in Heusler compound Co2FeSi and CoFe under spin extraction conditions. These results indicate that the species of ferromagnetic material definitely affects the magnitude and behavior of the spin signals. The use of highly polarized ferromagnets such as Heusler compounds would be important for improving the spin polarization and the magnitude of spin signals through Si channels. [ABSTRACT FROM AUTHOR]

We show spin torque oscillation at zero external magnetic field using a point-contact giant magnetoresistance device with a highly spin-polarized Co2Fe0.4Mn0.6Si (CFMS) Heusler alloy. The device emitted a high rf power (Prf) signal with a narrow spectral linewidth of 3 MHz without external magnetic field. Depending on the applied current, mode changes were clearly observed, and a large Prf of 25.7 nW was achieved. The large Prf and the high coherency of oscillation are attributable to the combination of the high spin-polarization of CFMS and the advantages of pointcontact geometry. [ABSTRACT FROM AUTHOR]

Anisotropic magnetoresistance (AMR) effect has been systematically investigated in various Heusler compounds Co2MnZ and Co2FeZ (Z=Al, Si, Ge, and Ga) epitaxial films and quantitatively summarized against the total valence electron number NV. It was found that the sign of AMR ratio is negative when NV is between 28.2 and 30.3, and turns positive when NV becomes below 28.2 and above 30.3, indicating that the Fermi level (EF) overlaps with the valence or conduction band edges of half-metallic gap at NV~28.2 or 30.3, respectively. We also find out that the magnitude of negative AMR ratio gradually increases with shifting of EF away from the gap edges, and there is a clear positive correlation between the magnitude of negative AMR ratio and magnetoresistive output of the giant magnetoresistive devices using the Heusler compounds. This indicates that AMR can be used as a facile way to optimize a composition of half-metallic Heusler compounds having a high spin-polarization at room temperature. [ABSTRACT FROM AUTHOR]