Efficient Spin Transport Research Articles

Superconducting magnetic Wollaston prism for neutron spin encoding.

A magnetic Wollaston prism can spatially split a polarized neutron beam into two beams with different neutron spin states, in a manner analogous to an optical Wollaston prism. Such a Wollaston prism can be used to encode the trajectory of neutrons into the Larmor phase associated with their spin degree of freedom. This encoding can be used for neutron phase-contrast radiography and in spin echo scattering angle measurement (SESAME). In this paper, we show that magnetic Wollaston prisms with highly uniform magnetic fields and low Larmor phase aberration can be constructed to preserve neutron polarization using high temperature superconducting (HTS) materials. The Meissner effect of HTS films is used to confine magnetic fields produced electromagnetically by current-carrying HTS tape wound on suitably shaped soft iron pole pieces. The device is cooled to ~30 K by a closed cycle refrigerator, eliminating the need to replenish liquid cryogens and greatly simplifying operation and maintenance. A HTS film ensures that the magnetic field transition within the prism is sharp, well-defined, and planar due to the Meissner effect. The spin transport efficiency across the device was measured to be ~98.5% independent of neutron wavelength and energizing current. The position-dependent Larmor phase of neutron spins was measured at the NIST Center for Neutron Research facility and found to agree well with detailed simulations. The phase varies linearly with horizontal position, as required, and the neutron beam shows little depolarization. Consequently, the device has advantages over existing devices with similar functionality and provides the capability for a large neutron beam (20 mm × 30 mm) and an increase in length scales accessible to SESAME to beyond 10 μm. With further improvements of the external coupling guide field in the prototype device, a larger neutron beam could be employed.

Current-based detection of nonlocal spin transport in graphene for spin-based logic applications

Graphene has been proposed for novel spintronic devices due to its robust and efficient spin transport properties at room temperature. Some of the most promising proposals require current-based readout for integration purposes, but the current-based detection of spin accumulation has not yet been developed. In this work, we demonstrate current-based detection of spin transport in graphene using a modified nonlocal geometry. By adding a variable shunt resistor in parallel to the nonlocal voltmeter, we are able to systematically cross over from the conventional voltage-based detection to current-based detection. As the shunt resistor is reduced, the output current from the spin accumulation increases as the shunt resistance drops below a characteristic value R*. We analyze this behavior using a one-dimensional drift-diffusion model, which accounts well for the observed behavior. These results provide the experimental and theoretical foundation for current-based detection of nonlocal spin transport.

Unveiling Self‐Assembled Monolayers' Potential for Molecular Spintronics: Spin Transport at High Voltage

Molecular magnetic tunnel junctions using self-assembled monolayers (SAMs) as tunnel barriers show stable and efficient spin transport properties. Large tunnel magnetoresistance with a flat bias voltage dependence of the magnetoresistance is observed in La(2/3) Sr(1/3) MnO(3) /dodecylphosphonic acid SAM/Co nanocontacts. This opens the door to spintronic tailoring though SAM engineering and could also lead to new venues for spin injection in organic devices.

Highly efficient spin transport in epitaxial graphene on SiC

Spin information processing is a possible new paradigm for post-CMOS (complementary metal-oxide semiconductor) electronics and efficient spin propagation over long distances is fundamental to this vision. However, despite several decades of intense research, a suitable platform is still wanting. We report here on highly efficient spin transport in two-terminal polarizer/analyser devices based on high-mobility epitaxial graphene grown on silicon carbide. Taking advantage of high-impedance injecting/detecting tunnel junctions, we show spin transport efficiencies up to 75%, spin signals in the mega-ohm range and spin diffusion lengths exceeding 100 {\mu}m. This enables spintronics in complex structures: devices and network architectures relying on spin information processing, well beyond present spintronics applications, can now be foreseen.

Cu2Oas a nonmagnetic semiconductor for spin transport in crystalline oxide electronics

We probe spin transport in Cu_{2}O by measuring spin valve effect in La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/Co and La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/La_{0.7}Sr_{0.3}MnO_{3} epitaxial heterostructures. In La_{0.7}Sr_{0.3}MnO_{3}/Cu_{2}O/Co systems we find that a fraction of out-of-equilibrium spin polarized carrier actually travel across the Cu_{2}O layer up to distances of almost 100 nm at low temperature. The corresponding spin diffusion length dspin is estimated around 40 nm. Furthermore, we find that the insertion of a SrTiO_{3} tunneling barrier does not improve spin injection, likely due to the matching of resistances at the interfaces. Our result on dspin may be likely improved, both in terms of Cu_{2}O crystalline quality and sub-micrometric morphology and in terms of device geometry, indicating that Cu_{2}O is a potential material for efficient spin transport in devices based on crystalline oxides.

Comparison of the structural, electronic and magnetic properties of Fe, Co and Ni nanowires encapsulated into silicon carbide nanotube

A similar optimized structure, i.e. a near square cross-section shape for outside nanotube and a relative rotation between nanowire and its outside nanotube, is obtained for the transition-metal M13 (M = Fe, Co, Ni) nanowires with the FCC structure encapsulated inside the armchair (8, 8) silicon carbide nanotube [ M13@(8, 8)] . It is also found that the stabilities of M13 nanowires are enhanced by silicon carbide nanotube encapsulation. Although the spin polarization P of each hybrid system is slightly lowered with respect to the corresponding free-standing nanowire, the largest spin polarization value 71% of Co13@(8, 8) among the three hybrid systems suggests it could be utilized to construct efficient spin transport devices. As compared with the corresponding free-standing nanowire, the magnetic moments μ1 and μ2 for the peripheral M1 (especially) and M2 atoms are decreased, while the magnetic moments μ3 and μ4 for the interior M3 and M4 atoms are increased for each M13@(8, 8) hybrid system. In particular, different from the bulk FCC Fe that is antiferromagnetic, the minimum energy magnetic structure of FCC Fe13 free-standing nanowire is ferromagnetic. Furthermore, contrary to the cases of Co13 and Ni13 nanowires, the ferromagnetism is further enhanced after Fe13 nanowire is encapsulated inside (8, 8) silicon carbide nanotube.

Transport property of insulating barrier in a ferromagnet-semiconductor hybrid system

Two-dimensional electron gas layer is essential for a developing spin-FET because of its high mobility and large spin–orbit coupling. The junction properties between a ferromagnet (FM) and a 2-DEG system are the most important factor. Two types of a 2-DEG layer, an InAs and a GaAs channel heterostructures, are fabricated to compare the junction properties of the two systems. A GaAs-based channel 2-DEG layer with Al2O3 tunneling layer using a new oxidization method is prepared. During the heat treatment at the furnace, the arsenic gas was evaporated and the top AlAs layer was converted to the aluminum oxide layer. An InAs channel 2-DEG layer with a semiconductor-based barrier and FM junction shows the ohmic behavior. In the potentiometric measurement, a spin–orbit coupling of 2-DEG layer is observed in the interface between a FM and an InAs channel 2-DEG layer, which proves the efficient spin transport junction.

Spin injection and transport in magnetic nanostructures

We theoretically study the spin transport in a device consisting of a nonmagnetic metal (N) connected to ferromagnetic injector (F1) and detector (F2) electrodes. We solve the spin-dependent transport equations in a device with arbitrary interface resistance from a metallic-contact to tunneling regime, and obtain the conditions for efficient spin injection, accumulation, and transport in the device. In a spin-injection device containing a superconductor (F1/S/F2), we examine the effect of superconductivity on the spin transport. The spin-current induces the anomalous Hall effect in nonmagnetic conductors is also discussed.

Comparing small molecules and polymer for future organic spin-valves

We report spin polarized injection and transport in organic spin-valves made from both organic small molecules and polymers. The devices with the structure La 0.67Sr 0.33MnO 3 (LSMO)/organic spacer/Co showed inverse magnetoresistance and low temperature operation with the vacuum evaporated π-conjugated small molecule 8-hydroxyquinoline aluminium (Alq 3) while normal MR and high temperature operation was observed in devices with the π-conjugated polymer poly(3-hexylthiophene). Due to the proximate values of the work functions of LSMO and Co with the highest occupied molecular orbital (HOMO) energy of the polymers, spin polarized carrier injection is much more efficient in the polymeric spin-valves compared to the organic spin-valves with Alq 3 where carrier injection is hindered due to greater barrier height. Efficient spin transport is also observed in polymeric spin-valves and can be attributed to longer conjugation in the polymeric chains compared to the small molecules.

Electron spin filtering in ferromagnet/semiconductor heterostructures

Circularly polarized light was used to generate spin-polarized electrons at room temperature in ferromagnet (FM)/GaAs Schottky diode structures. A change in the helicity-dependent photocurrent was obtained when the ferromagnetic layer magnetization was realigned from perpendicular to parallel to the photon helicity. This effect is attributed to spin filtering of photoexcited electrons generated in the GaAs due to the spin-split density of states at the Fermi level in the FM which occurs when the magnetization is aligned with the photon helicity. Significant spin filtering effects were observed in NiFe/GaAs and Fe/GaAs structures, increasing with increasing applied magnetic field. Antiferromagnetic Cr/GaAs showed no spin-dependent effects as expected. As the photon energy approaches the energy gap of the GaAs, the effects associated with the optically induced spin polarization in the GaAs become larger, confirming that polarized electrons are first excited in the semiconductor (SC) and then filtered by the ferromagnetic layer. The spin filtering effects in all cases increase with increasing ferromagnetic layer thickness, and are much larger than the estimated magneto-circular dichroism in NiFe. Our combined results unambiguously indicate that highly efficient spin transport from the SC to the FM occurs at room temperature.

Room temperature ferromagnetism in GaMnN and GaMnP

Results on the synthesis of ferromagnetic GaMnN and GaMnP by both Molecular Beam Epitaxy or implantation of Mn directly into p-GaN or GaP(C) at elevated temperatures to avoid amorphization will be described. There is a relatively broad range of growth conditions under which single-crystal, single-phase material may be obtained with significant Mn concentrations. The effects of background doping level are discussed, along with a comparison of results on direct implantation of Fe and Ni instead of Mn. Essential requirements for utilizing the spin of the electron in device structures include the ability to achieve efficient electrical spin injection and transport of spin-polarized carriers, along with effective detection of these carriers.

Ballistic spin filtering across ferromagnet/semiconductor interfaces at room temperature

Circularly polarized light was used to generate spin-polarized electrons at room temperature in ferromagnet/GaAs Schottky diode structures (with spin polarization perpendicular to the film plane). The Schottky barrier dependence of the helicity-dependent photocurrent was observed using various ferromagnetic materials (NiFe, Co, and Fe) and GaAs doping densities. A change in the helicity-dependent photocurrent was obtained in all cases in reverse bias when the ferromagnetic layer magnetization was realigned from perpendicular to parallel to the photon helicity. This effect is attributed to spin filtering of photoexcited electrons generated in the GaAs due to the spin split density of states at the Fermi level in the ferromagnet which occurs when the magnetization is aligned with the photon helicity. NiFe shows significant spin filtering, Fe shows either strong or weak spin filtering according to the Schottky barrier strength, while Co shows almost none. Antiferromagnetic Cr/GaAs shows no spin-dependent effects as expected. These spin transport effects in all cases vanish for very high doping due to the collapse of the Schottky barrier. As the photon energy approaches the energy gap of the GaAs, the effects associated with the optically induced spin polarization in the GaAs become larger, confirming that polarized electrons are first excited in the semiconductor and then filtered by the ferromagnetic layer. The spin filtering effects in all cases increase with increasing ferromagnetic layer thickness, and are much larger than the estimated magnetocircular dichroism in permalloy. These results unambiguously indicate that highly efficient spin transport from the semiconductor to the ferromagnet occurs at room temperature and that strong spin filtering occurs in reverse bias.

Anisotropic electrical spin injection in ferromagnetic semiconductor heterostructures

A fourteen-fold anisotropy in the spin transport efficiency parallel and perpendicular to the charge transport is observed in a vertically biased (Ga, Mn)As-based spin-polarized light emitting diode. The spin polarization is determined by measuring the polarization of electroluminescence from an (In, Ga)As quantum well placed a distance d (20–420 nm) below the p-type ferromagnetic (Ga, Mn)As contact. In addition, a monotonic increase (from 0.5% to 7%) in the polarization is measured as d decreases for collection parallel to the growth direction, while the in-plane polarization from the perpendicular direction (∼0.5%) remains unchanged.