Suppression of spin rectification effects in spin pumping experiments

Spin pumping (SP) is a well-established method to generate pure spin currents allowing efficient spin injection into metals and semiconductors avoiding the problem of impedance mismatch. However, to disentangle pure spin currents from parasitic effects due to spin rectification effects (SRE) is a difficult task that is seriously hampering further developments. Here we propose a simple method that allows suppressing SRE contribution to inverse spin Hall effect (ISHE) voltage signal avoiding long and tedious angle-dependent measurements. We show an experimental study in the well-known Py/Pt system by using a coplanar waveguide (CPW). Results obtained demonstrate that the sign and size of the measured transverse voltage signal depends on the width of the sample along the CPW active line. A progressive reduction of this width evidences that SRE contribution to the measured transverse voltage signal becomes negligibly small for sample width below 200 μm. A numerical solution of the Maxwell equations in the CPW-sample setup, by using the Landau-Lifshitz equation with the Gilbert damping term (LLG) as the constitutive equation of the media, and with the proper set of boundary conditions, confirms the obtained experimental results.

Enhanced spin accumulation in nano-pillar-based lateral spin valve using spin reservoir effect

Lateral spin valves are ideal nanostructures for investigating spin-transport physics phenomena and promoting the development of future spintronic devices owing to dissipation-less pure spin current. The magnitude of the spin accumulation signal is well understood as a barometer for characterizing spin current devices. Here, we develop a novel fabrication method for lateral spin valves based on ferromagnetic nanopillar structures using a multi-angle deposition technique. We demonstrate that the spin-accumulation signal is effectively enhanced by reducing the lateral dimension of the nonmagnetic spin channel. The obtained results can be quantitatively explained by the confinement of the spin reservoir by considering spin diffusion into the leads. The temperature dependence of the spin accumulation signal and the influence of the thermal spin injection under a high bias current are also discussed.

Ultrahigh Spin Filter Efficiency, Giant Magnetoresistance and Large Spin Seebeck Coefficient in Monolayer and Bilayer Co-/Fe-/Cu-Phthalocyanine Molecular Devices

The spin related electrical and thermoelectric properties of monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices in a parallel spin configuration (PC) and an anti-parallel spin configuration (APC) between the V-shaped zigzag-edged graphene nanoribbon electrodes and the center bilayer MPc molecules are investigated by combining the density functional theory and non-equilibrium Green’s function approaches. The results show that there is an ultrahigh spin filter efficiency exceeding 99.99995% and an ultra-large total conductance of 0.49996G0 for FePc-CoPc molecular devices in PC and a nearly pure charge current at high temperature in an APC and a giant MR ratio exceeding 9.87 × 106% at a zero bias. In addition, there are pure spin currents for CuPc and FePc molecular devices in PC, and an almost pure spin current for FePc molecular devices in an APC at some temperature. Meanwhile, there is a high SFE of about 99.99585% in a PC and a reserved SFE of about −19.533% in an APC and a maximum MR ratio of about 3.69 × 108% for the FePc molecular device. Our results predict that the monolayer and bilayer MPc (M = Co, Fe, Cu) molecular devices possess large advantages in designing high-performance electrical and spintronic molecular devices.

A Physically Meaningful Relativistic Description of the Spin State of an Electron

We introduced a new model to present the states of a two-state quantum system. The space is the complexified Minkowski space. The Lorentz group acts by the linear extension of its action on the four-vectors. We applied this model to represent the spin state of an electron or any relativistic spin 1/2 particle. The spin state of such particle is of the form U+iS, where U is the four-velocity of the particle in the lab frame, and S is the 4D spin in this frame. Under this description, the transition probability between two pure spin states ϱ1 and ϱ2 of particles moving with the same velocity are defined by use of Minkowski dot product as 12<ϱ2|ϱ1>. This transition probability is Lorentz invariant, coincide with the quantum mechanics prediction and thus agree with the experimental results testing quantum mechanics predictions based on Bell’s inequality. For a a particle of mass m and charge q with the spin state ϱ, the total momentum is mcϱ and the electromagnetic momentum is qϱ. This imply that the Landé g factor for such particles must be g=2. We obtain an evolution equation of the spin state in an electromagnetic field which defines correctly the anomalous Zeeman effect and the fine structure splitting.

Pure spin photocurrent in non-centrosymmetric crystals: bulk spin photovoltaic effect

Spin current generators are critical components for spintronics-based information processing. In this work, we theoretically and computationally investigate the bulk spin photovoltaic (BSPV) effect for creating DC spin current under light illumination. The only requirement for BSPV is inversion symmetry breaking, thus it applies to a broad range of materials and can be readily integrated with existing semiconductor technologies. The BSPV effect is a cousin of the bulk photovoltaic (BPV) effect, whereby a DC charge current is generated under light. Thanks to the different selection rules on spin and charge currents, a pure spin current can be realized if the system possesses mirror symmetry or inversion-mirror symmetry. The mechanism of BSPV and the role of the electronic relaxation time $$\tau$$ τ are also elucidated. We apply our theory to several distinct materials, including monolayer transition metal dichalcogenides, anti-ferromagnetic bilayer MnBi2Te4, and the surface of topological crystalline insulator cubic SnTe.

Simultaneous pure T2 and varying T2′-weighted BOLD fMRI using Echo Planar Time-resolved Imaging (EPTI) for mapping laminar fMRI responses

Spin-echo (SE) BOLD fMRI has high microvascular specificity, but its most common acquisition method, SE-EPI, suffers from T2' contrast contamination with undesirable draining vein bias. To address this, in this study, we extended a recently developed multi-shot EPI technique, Echo-Planar Time-resolved Imaging (EPTI), to laminar SE-fMRI at 7T to obtain pure spin-echo BOLD contrast with minimal T2' contamination for improved specificity. We also developed a framework to simultaneously obtain a series of asymmetric SE (ASE) images with varying T2' weightings, and extracted from the same data equivalent conventional SE multi-shot EPI images with different ETLs, to investigate the T2'-induced macrovascular contribution across the spin-echo readout. A low-rank spatiotemporal subspace reconstruction was implemented for the SE-EPTI acquisition, which incorporates corrections for both shot-to-shot phase variations and dynamic B0 drifts. SE-EPTI was used in a visual task fMRI experiment to demonstrate that i) the pure SE image provided by EPTI results in the highest microvascular specificity; ii) the ASE EPTI image series, with a graded introduction of T2' weightings at time points farther away from the pure SE, show a gradual sensitivity increase accompanied by a larger and larger draining vein bias; iii) a longer ETL in the conventional SE EPI acquisition will induce more draining vein bias. Consistent results were observed across multiple subjects, demonstrating the robustness of the proposed technique for SE-BOLD fMRI with high specificity.