Pure Spin Current Transport Research Articles

Half‐Metallic Heusler Alloy/GaN Heterostructure for Semiconductor Spintronics Devices

AbstractBecause spin‐orbit coupling in wurtzite semiconductors is relatively weak compared with that in zincblende ones, the III‐nitride semiconductor GaN is a promising material for high‐performance optical semiconductor spintronic devices such as spin lasers. For the purpose of reducing the operating power of spin lasers, it is necessary to demonstrate highly efficient electrical spin injection from a ferromagnetic material into GaN with a low‐resistance contact. Here, an epitaxial half‐metallic Heusler alloy Co2FeAlxSi1−x(CFAS)/GaN heterostructure is developed by inserting an ultrathin Co layer between the CFAS and GaN. The CFAS/n+‐GaN heterojunctions clearly show tunnel conduction with very small rectification and a low resistance‐area product of ≈3.8 kΩµm2, which is several orders of magnitude smaller than those reported in previous work, at room temperature. Nonlocal spin signals and a Hanle effect curve are observed at low temperatures using lateral spin‐valve devices with the CFAS/n+‐GaN contacts, suggesting pure spin current transport in bulk GaN. The spin transport is observed at temperatures as high as room temperature, with a high spin polarization of 0.2 at a low bias voltage less than 2.0 V. This study is expected to open a path to GaN‐based spintronic devices with highly spin‐polarized and low‐resistance contacts.

Experimental extraction of donor-driven spin relaxation in n -type nondegenerate germanium

Using pure spin current transport measurements in lateral spin-valve devices, we study the spin relaxation in an $n$-type nondegenerate Ge layer, which is moderately doped Ge (P: $\ensuremath{\sim}{10}^{18}$ ${\mathrm{cm}}^{\ensuremath{-}3}$). The obtained spin diffusion length $({\ensuremath{\lambda}}_{\mathrm{Ge}})$ of the nondegenerate Ge is two to three times greater than that of heavily doped degenerate Ge (P: $\ensuremath{\sim}{10}^{19}$ ${\mathrm{cm}}^{\ensuremath{-}3}$) in the temperature range from 8 to 100 K. We find that the electron spin lifetime ($\ensuremath{\tau}$) for the nondegenerate Ge is monotonically increased with decreasing temperature $(T)$. The increase in $\ensuremath{\tau}$ at temperatures less than 50 K can be interpreted in terms of the donor-driven spin relaxation mechanism including the 1/$\sqrt{T}$ behavior in multivalley semiconductors, proposed by Song et al. [Y. Song, O. Chalaev, and H. Dery, Phys. Rev. Lett. 113, 167201 (2014)]. We note that it is important for the $\ensuremath{\tau}$ of the moderately doped nondegenerate Ge to partly consider the $T$-independent component of spin relaxation in addition to the 1/$\sqrt{T}$ component.

Revealing the importance of interfaces for pure spin current transport

The authors measure spin transport and spin absorption across interfaces in lateral spin-valve devices and correlate this electrical characterization to direct imaging of the defect structure of the buried interfaces.

Hopping-Dominated Spin Transport in Unintentionally Doped Organic Semiconductors.

We report a spin diffusion theory to predict unusual pure spin current transport in unintentionally doped organic semiconductors. We demonstrate that the feasibility of pure spin current transport via polaron hopping at a low carrier density. Our theoretical prediction, 40 nm, for spin diffusion length (SDL) in dinaphtho[2,3-b:2,3-f]thieno[3,2-b]thiophene (DNTT) is in very good agreement with experimental data. Interestingly, SDL can be prolonged by restraining molecular geometry structure disorder and reducing the reorganization energy. In comparison with anisotropic organic materials, the SDL in isotropic ones increases up to 60%. Our results open up a new avenue to design organic spintronics devices with long SDL and low carrier density.

Strong spin orientation-dependent spin current diffusion and inverse spin Hall effect in a ferromagnetic metal

Pure spin current transport has become the central point of the state-of-the-art spintronics. While most spin current phenomena have been extensively explored, aspects of the pure spin current injected into ferromagnetic metals are far from completely understood. The reports on a fundamental problem, i.e. the spin relaxation asymmetry with spin current polarization collinear or transverse to the magnetization of ferromagnetic metals, are quite controversial. By employing a Y3Fe5O12 (YIG)/Cu/Ni80Fe20 (Py)/Ir25Mn75 (IrMn) spin valve heterostructure with the thermal inverse spin Hall effect (ISHE) of a Py well separated from other thermoelectric transport and thermal Hall effects, we find that the ISHE signal amplitude in 10 nm Py increases by 80% when changing the relative orientation of the YIG and Py magnetization from orthogonal (⊥) to collinear (||). Moreover, the spin-diffusion length λsf and effective spin Hall angle theta _{{mathrm{SH}}}^{{mathrm{eff}}} of Py are also spin orientation dependent and vary from lambda _{{mathrm{sf}}}^ bot = 1.0 ± 0.1 nm to lambda _{{mathrm{sf}}}^parallel = 2.8 ± 0.5 nm with theta _{{mathrm{SH}}}^{{mathrm{eff}}}left( bot right)/theta _{{mathrm{SH}}}^{{mathrm{eff}}}left( parallel right) = 1.5, respectively. Our results demonstrate magnetization orientation-dependent spin relaxation and spin injection efficiency of a pure spin current, revealing that exchange interactions in ferromagnetic metals strongly affect the transport of the pure spin current.

Role of interface quality for the spin Hall magnetoresistance in nickel ferrite thin films with bulk-like magnetic properties

We utilized spin Hall magnetoresistance (SMR) measurements to experimentally investigate pure spin current transport in thin film heterostructures of nickel ferrite (NiFe2O4,NFO) and normal metals (NM) Ta and Pt. We grew (001)-oriented NFO thin films by pulsed laser deposition on lattice-matched magnesium gallate (MgGa2O4) substrates, thereby significantly improving their magnetic and structural properties. We performed SMR measurements at room temperature in patterned Hall bar structures for charge currents applied in the [100]- and [110]-directions of NFO. We found that the extracted SMR magnitude for NFO/Pt heterostructures depends crucially on the Pt resistivity of the investigated Hall bar structure. We further study this resistivity scaling of the SMR effect at different temperatures for NFO/Pt. Our results suggest that the spin mixing conductance of the NFO/Pt interface and the Pt resistivity depend on the interface quality and thus a correlation between these two quantities exists.

Nonvolatile Electric‐Field Control of Ferromagnetic Resonance and Spin Pumping in Pt/YIG at Room Temperature

AbstractElectric‐field‐controlled magnetism is of importance in realizing energy efficient, dense and fast information storage and processing. Strain‐mediated converse magneto‐electric (ME) coupling between ferromagnetic and ferroelectric heterostructure shows promise for realizing electric‐controlled magnetism at room temperature and is attracting a number of recent investigations. However, such ME‐effect studies have mainly focus on magnetic metals. In this work, high quality yttrium iron garnet (Y3Fe5O12 (YIG)) films are deposited directly onto (100)‐oriented single‐crystal Pb (Mg1/3Nb2/3)0.7Ti0.3O3 (PMN‐PT) substrates by means of magnetron sputtering. The electric‐field‐induced polarization switching and lattice strain in the PMN‐PT substrate results in two distinct magnetization states in the YIG film that are nonvolatile and electrically reversible. Because of the direct contact between the YIG and the PMN‐PT substrate, an efficient ME coupling and an almost 90° rotation of the easy axis of the YIG film can be realized. Furthermore, the electric‐field‐controlled hysteresis loop‐like ferromagnetic resonance field shifts and spin pumping signals are observed in Pt/YIG/PMN‐PT heterostructures. Thus, the obstacle is overcome via growing high‐quality YIG thin films directly onto PMN‐PT substrates and an efficient manipulation of magnetism and pure spin current transport by electric field is thereby realized. These findings are instructive for future low‐power magnetic insulator‐based spintronic devices.

Crystal orientation effect on spin injection/detection efficiency in Si lateral spin-valve devices

We show evident crystal orientation effect on pure spin current transport in Si-based lateral spin-valve (LSV) devices with epitaxially grown CoFe/MgO tunnel contacts. When we compare nonlocal spin signals between LSV devices along 1 0 0 (Si1 0 0) and 1 1 0 (Si1 1 0), we find that the magnitude of the spin signals for Si1 0 0 LSV devices is always larger than that for Si1 1 0 LSV devices. The analyses based on the one-dimensional spin diffusion model reveal that the spin diffusion length and spin lifetime between Si1 0 0 and Si1 1 0 LSV devices are comparable, while the spin injection/detection efficiency in Si1 0 0 LSV devices is evidently larger than that in Si1 1 0 ones. Possible origins of the difference in the spin injection/detection efficiency between Si1 0 0 and Si1 1 0 LSV devices are discussed.

Reduced interfacial magnetic moment of Y3Fe5O12 by capping Pt

The proximity effects of the Pt layers on the magnetic properties of the ultrathin Y3Fe5O12 (YIG) films have been systematically investigated. A significant decrease in the saturation magnetization is observed after capping the Pt films on the YIG films, indicating that a strong interaction occurs between Pt and YIG. In contrast, the magnetic properties remain essentially unchanged for YIG capping with the Cu films. The X-ray magnetic circular dichroism measurements show that a selective charge transfer occurs from Pt to the tetrahedral site Fe of YIG, leading to a reduction of the magnetic moment of the interfacial YIG. Our findings provide important clues to further understanding the pure spin current transport at the Pt/YIG interface.

Pure spin current transport in a SiGe alloy

Using four-terminal nonlocal magnetoresistance measurements in lateral spin-valve devices with Si0.1Ge0.9, we study pure spin current transport in a degenerate SiGe alloy (n ∼ 5.0 × 1018 cm−3). Clear nonlocal spin-valve signals and Hanle effect curves, indicating generation, transport, and detection of pure spin currents, are observed. The spin diffusion length and spin lifetime of the Si0.1Ge0.9 layer at low temperatures are reliably estimated to be ∼0.5 µm and ∼0.2 ns, respectively. This study demonstrates the possibility of exploring physics and developing spintronic applications using SiGe alloys.

Observation of the spin Nernst effect.

The observation of the spin Hall effect triggered intense research on pure spin current transport. With the spin Hall effect, the spin Seebeck effect and the spin Peltier effect already observed, our picture of pure spin current transport is almost complete. The only missing piece is the spin Nernst (-Ettingshausen) effect, which so far has been discussed only on theoretical grounds. Here, we report the observation of the spin Nernst effect. By applying a longitudinal temperature gradient, we generate a pure transverse spin current in a Pt thin film. For readout, we exploit the magnetization-orientation-dependent spin transfer to an adjacent yttrium iron garnet layer, converting the spin Nernst current in Pt into a controlled change of the longitudinal and transverse thermopower voltage. Our experiments show that the spin Nernst and the spin Hall effect in Pt are of comparable magnitude, but differ in sign, as corroborated by first-principles calculations.

Pure spin current transport in gallium doped zinc oxide

We study the flow of a pure spin current through zinc oxide by measuring the spin Hall magnetoresistance (SMR) in thin film trilayer samples consisting of bismuth-substituted yttrium iron garnet (Bi:YIG), gallium-doped zinc oxide (Ga:ZnO), and platinum. We investigate the dependence of the SMR magnitude on the thickness of the Ga:ZnO interlayer and compare to a Bi:YIG/Pt bilayer. We find that the SMR magnitude is reduced by almost one order of magnitude upon inserting a Ga:ZnO interlayer and continuously decreases with increasing interlayer thickness. Nevertheless, the SMR stays finite even for a 12 nm thick Ga:ZnO interlayer. These results show that a pure spin current indeed can propagate through a several nm-thick degenerately doped zinc oxide layer. We also observe differences in both the temperature and the field dependence of the SMR when comparing tri- and bilayers. Finally, we compare our data to the predictions of a model based on spin diffusion. This shows that interface resistances play a crucial role for the SMR magnitude in these trilayer structures.

Temperature-independent spin relaxation in heavily doped n -type germanium

We experimentally study the spin relaxation mechanism in heavily doped $n$-type germanium (Ge) layers by electrically detecting pure spin current transport. The spin diffusion length (${\ensuremath{\lambda}}_{\mathrm{Ge}}$) in heavily doped $n$-type Ge layers at 125 K is less than $0.7\phantom{\rule{4pt}{0ex}}\ensuremath{\mu}\mathrm{m}$, much shorter than that expected in the recent study by Dushenko et al. We find that the spin relaxation time ${\ensuremath{\tau}}_{\mathrm{s}}$ is independent of temperature in the range of 8 to 125 K, which can be interpreted by the recent theory by Song et al. This study clarifies that the spin-relaxation mechanism at low temperatures in degenerate Ge is dominated by extrinsic scattering with impurities.

Exchange-Dominated Pure Spin Current Transport in Alq3 Molecules.

We address the controversy over the spin transport mechanism in Alq3 utilizing spin pumping in the Y3Fe5O12/Alq3/Pd system. An unusual angular dependence of the inverse spin Hall effect is found. It, however, disappears when the microwave magnetic field is fully in the sample plane, excluding the presence of the Hanle effect. Together with the quantitative temperature-dependent measurements, these results provide compelling evidence that the pure spin current transport in Alq3 is dominated by the exchange-mediated mechanism.