Efficient Spin Transport Research Articles

Probe of antiferromagnetic transition via thermal-excited incoherent spin current

Spin Seebeck effect (SSE) for an yttrium iron garnet (YIG)/nickel oxide (NiO)/platinum (Pt) trilayer device was studied in a wide temperature range. It is found that the SSE signal is nonmonotonic in the temperature domain and shows a maximum around the Néel temperature of the NiO layer, which is similar to the temperature dependence of the spin pumping signal in the same device. This result identified that the thermal-excited incoherent spin currents are also sensitive to the fluctuation of an antiferromagnetic system and can be used to probe the antiferromagnetic phase transition. Furthermore, the antiferromagnetic NiO layer is considered to be valuable for the optimization of spin transport efficiency at YIG/Pt interface, which enhances the SSE signal around room temperature.

Electrical spin transport in a GaAs (110) channel

A homoepitaxial GaAs (110) channel gives a great interest in the field of semiconductor spintronics due to the longer spin diffusion. By utilizing optimal temperature process and V/III flux ratio control, the GaAs layer is grown without a serious defect. In a ferromagnet/semiconductor hybrid device, Tb20Fe62Co18/Ru/Co40Fe40B20 films are deposited on the GaAs (110) channel as a spin source to investigate the spin transport in (110)-oriented channel. To measure the Hanle signal, an in-plane magnetic field is applied to the perpendicularly polarized spins which are injected from the Tb20Fe62Co18 layer. From the experimental results, the spin diffusion length in a GaAs (110) is longer than that in a GaAs (100) by up to 25%. The proper selection of crystalline growth direction for the spin transport channel is a viable solution for an efficient spin transport.

Simulating Spin Chains Using a Superconducting Circuit: Gauge Invariance, Superadiabatic Transport, and Broken Time‐Reversal Symmetry

AbstractSimulation of materials by using quantum processors is envisioned to be a major direction of development in quantum information science. Here, the mathematical analogies between a triangular spin lattice with Dzyaloshinskii–Moriya coupling on one edge and a three‐level system driven by three fields in a loop configuration are exploited to emulate spin‐transport effects. It is shown that the spin transport efficiency, seen in the three‐level system as population transfer, is enhanced when the conditions for superadiabaticity are satisfied. It is demonstrated experimentally that phenomena characteristic to spin lattices due to gauge invariance, non‐reciprocity, and broken time‐reversal symmetry can be reproduced in the three‐level system.

Graphene/Half-Metallic Heusler Alloy: A Novel Heterostructure toward High-Performance Graphene Spintronic Devices.

Graphene-based vertical spin valves (SVs) are expected to offer a large magnetoresistance effect without impairing the electrical conductivity, which can pave the way for the next generation of high-speed and low-power-consumption storage and memory technologies. However, the graphene-based vertical SV has failed to prove its competence due to the lack of a graphene/ferromagnet heterostructure, which can provide highly efficient spin transport. Herein, the synthesis and spin-dependent electronic properties of a novel heterostructure consisting of single-layer graphene (SLG) and a half-metallic Co2 Fe(Ge0.5 Ga0.5 ) (CFGG) Heusler alloy ferromagnet are reported. The growth of high-quality SLG with complete coverage by ultrahigh-vacuum chemical vapor deposition on a magnetron-sputtered single-crystalline CFGG thin film is demonstrated. The quasi-free-standing nature of SLG and robust magnetism of CFGG at the SLG/CFGG interface are revealed through depth-resolved X-ray magnetic circular dichroism spectroscopy. Density functional theory (DFT) calculation results indicate that the inherent electronic properties of SLG and CFGG such as the linear Dirac band and half-metallic band structure are preserved in the vicinity of the interface. These exciting findings suggest that the SLG/CFGG heterostructure possesses distinctive advantages over other reported graphene/ferromagnet heterostructures, for realizing effective transport of highly spin-polarized electrons in graphene-based vertical SV and other advanced spintronic devices.

Spin transport in insulators without exchange stiffness

The discovery of new materials that efficiently transmit spin currents has been important for spintronics and material science. The electric insulator Gd3Ga5O12 (GGG), a standard substrate for growing magnetic films, can be a spin current generator, but has never been considered as a superior conduit for spin currents. Here we report spin current propagation in paramagnetic GGG over several microns. Surprisingly, spin transport persists up to temperatures of 100 K ggTg = 180 mK, the magnetic glass-like transition temperature of GGG. At 5 K and 3.5 T, we find a spin diffusion length λGGG = 1.8 ± 0.2 μm and a spin conductivity σGGG = (7.3 ± 0.3) × 104 Sm−1 that is larger than that of the record quality magnet Y3Fe5O12 (YIG). We conclude that exchange stiffness is not required for efficient spin transport, which challenges conventional models and provides new material-design strategies for spintronic devices.

Coherent spin transport in a DNA molecule

Coherent spin transport in a DNA molecule has been examined using a first principles calculation. In the calculated model, a B-form double strand (ds) (5′-CGCGAATTCGCG-3′) (Dickerson-Drew duplex) was located between ferromagnetic nickel electrodes and spin transport was calculated under two relative magnetic configurations of electrodes: parallel and antiparallel alignments. As a result, the molecule was too long to demonstrate an efficient coherent spin transport. Short DNA molecules of ds(5′-CGCG-3′) and ds(5′-AATT-3′), which were cut out from the Dickerson-Drew duplex, showed similar results. On the contrary, stacking nucleic acids (π-stacking) of ds(5′-CGCG-3′) or ds(5′-AATT-3′) resulted in an efficient coherent spin transport sufficient to cause spin polarization and a certain difference in conductance by varying the magnetic configuration of ferromagnetic electrodes. This result suggested the possibility of tunnel magnetoresistance in the short π-stacking part of a DNA molecule.

Two-Dimensional Flexible High Diffusive Spin Circuits.

Owing to their unprecedented electronic properties, graphene and two-dimensional (2D) crystals have brought fresh opportunities for advances in planar spintronic devices. Graphene is an ideal medium for spin transport while being an exceptionally resilient material for flexible nanoelectronics. However, these extraordinary traits have never been combined to create flexible graphene spin circuits. Realizing such circuits could lead to bendable strain-spin sensors, as well as a unique platform to explore pure spin current based operations and low-power 2D flexible nanoelectronics. Here, we demonstrate graphene spin circuits on flexible substrates for the first time. Despite the rough topography of the flexible substrates, these circuits prepared with chemical vapor deposited monolayer graphene reveal an efficient room temperature spin transport with distinctively large spin diffusion coefficients ∼0.2 m2 s-1. Compared to earlier graphene devices on Si/SiO2 substrates, such values are up to 20 times larger, leading to one order higher spin signals and an enhanced spin diffusion length ∼10 μm in graphene-based nonlocal spin valves fabricated using industry standard systems. This high performance arising out of a characteristic substrate terrain shows promise of a scalable and flexible platform towards flexible 2D spintronics. Our innovation is a key step for the exploration of strain-dependent 2D spin phenomena and paves the way for flexible graphene spin memory-logic units and planar spin sensors.

Polarized neutron diffraction using a novel high-T c superconducting magnet on the single-crystal diffractometer POLI at MLZ

The polarized single-crystal diffractometer POLI is the first neutron scattering instrument routinely using 3He spin filters both to produce and to analyse neutron polarization. The instrument, with a non-magnetic goniometer, was designed to perform two types of polarized neutron diffraction experiment: spherical neutron polarimetry, also known as full three-dimensional polarization analysis in zero magnetic field, and classical polarized neutron diffraction, also called the flipping-ratio (FR) method, in high applied magnetic fields. Reported here is the implementation of the FR setup for short-wavelength neutrons on POLI using a new high-T c superconducting magnet with a maximal field of 2.2 T. The complete setup consists of a 3He polarizer, a nutator, a Mezei-type flipper, guide fields and dedicated pole pieces, together with the magnet. Each component, as well as the whole setup, was numerically simulated, optimized, built and finally successfully tested under real experimental conditions on POLI. The measured polarized neutron spin transport efficiency is about 99% at different wavelengths, e.g. as short as 0.7 Å, and up to the maximal available field of the magnet. No further depolarization of the 3He cells due to stray fields of the magnet occurs. The additional use of the available 3He analyser allows uniaxial polarization analysis experiments in fields up to 1.2 T. The results of the first experiment on the field-dependent distribution of the trigonal antiferromagnetic domains in haematite (α-Fe2O3) are presented and compared with the literature data.

Thermoelectric spin voltage in graphene.

In recent years, new spin-dependent thermal effects have been discovered in ferromagnets, stimulating a growing interest in spin caloritronics, a field that exploits the interaction between spin and heat currents 1,2 . Amongst the most intriguing phenomena is the spin Seebeck effect 3-5 , in which a thermal gradient gives rise to spin currents that are detected through the inverse spin Hall effect 6-8 . Non-magnetic materials such as graphene are also relevant for spin caloritronics, thanks to efficient spin transport 9-11 , energy-dependent carrier mobility and unique density of states 12,13 . Here, we propose and demonstrate that a carrier thermal gradient in a graphene lateral spin valve can lead to a large increase of the spin voltage near to the graphene charge neutrality point. Such an increase results from a thermoelectric spin voltage, which is analogous to the voltage in a thermocouple and that can be enhanced by the presence of hot carriers generated by an applied current 14-17 . These results could prove crucial to drive graphene spintronic devices and, in particular, to sustain pure spin signals with thermal gradients and to tune the remote spin accumulation by varying the spin-injection bias.

Observation of spin dependent electrochemical potentials at room temperature in a quantum well structure

Spin transport with a strong Rashba spin splitting is observed in a semiconductor channel. Using the current injection into the side of the quantum well, the potentiometric signal and the spin-torque-induced magnetization change are simultaneously detected. In this geometry, the spin current is directly injected from the ferromagnetic electrode into the quantum well channel and the electrochemical potential is monitored by the detection ferromagnet. For the side injection instead of vertical injection, the milling of the top cladding layer is not necessary, so the efficient spin transport and the undamaged Rashba spin splitting can be realized. The ferromagnetic detector senses the Rashba-effect-induced electrochemical potential shift brought by the charge current and this potentiometric signal is also modified by the magnetization rotation induced by the spin current. The large potential changes of 1.03 Ω and 0.96 Ω are observed at 1.8 K and 300 K, respectively.

Long-distance spin transport in a disordered magnetic insulator

Spin transport through magnetic insulators via magnons has recently been explored for a growing variety of magnetic systems with long-range order and well-understood spin excitation spectra. Here we show dramatic effects of spin transport through an amorphous magnetic insulator, which is both magnetically and structurally disordered. We generate and detect spin flow though amorphous yttrium-iron-garnet ($a$-YIG) thin films in a non-local geometry by use of the spin Hall and inverse spin Hall effects in platinum strips separated by 10 or more microns. By comparing non-local spin transport in $a$-YIG on suspended micromachined thermal isolation platforms to the same experiment performed on a bulk substrate, we show strong effects of in-plane thermal gradients on spin transport in the disordered magnetic insulator. The resulting non-local voltage signals are orders of magnitude larger than those seen in crystalline magnetic insulators, with easily measurable spin signals seen even at distances in excess of $100$ microns. In analogy to heat transport, where disordered materials support a range of vibrational excitations that can allow large thermal conductivities, we suggest that efficient spin transport in disordered magnetic systems can occur via a similar spectrum of excitations that relies on strong local exchange interactions and does not require long-range order. This work not only opens a new area for fundamental experimental and theoretical studies of spin transport, but also sets a new direction in materials science for magnonic and spintronic devices.

Energy consumption analysis of graphene based all spin logic device with voltage controlled magnetic anisotropy

All spin logic device (ASLD) is a promising option to realize the ultra-low power computing systems. However, the low spin transport efficiency and the non-local switching of the detector have become two key challenges of the ASLD. In this paper, we analyze the energy consumption of a graphene based ASLD with the ferromagnetic layer switching assistance by voltage control magnetic anisotropy (VCMA) effect. This structure has significant potential towards ultra-low power consumption: the applied voltage can not only shorten switching time of the ferromagnetic layer, but also decreases the critical injection current; the graphene channel enhances greatly the spin transport efficiency. By applying the approximate circuit model, the impact of material configurations, interfaces and geometry can be synthetically studied. An accurate physic model was also developed, based on which, we carry out the micro-magnetic simulations to analyze the magnetization dynamics. Combining these electrical and magnetic investigations, the energy consumption of the proposed ASLD can be estimated. With the optimizing parameters, the energy consumption can be reduced to 2.5 pJ for a logic operation.

Efficient spin transport through polyaniline

By using the spin pumping process, we show that it is possible to transport a pure spin current across layers of conducting polyaniline (PANI) with several hundred nanometers sandwiched between a film of the ferrimagnetic insulator yttrium iron garnet (YIG) and a thin layer of platinum. The spin current generated by microwave-driven ferromagnetic resonance of the YIG film, injected through the YIG/PANI interface, crosses the whole PANI layer and then is injected into the Pt layer. By means of the inverse spin Hall effect in the Pt, the spin current is converted into charge current and electrically detected as a dc voltage. We measured a spin diffusion length in PANI of 590 \ifmmode\pm\else\textpm\fi{} 40 nm, which is very large compared with normal metals, demonstrating that PANI can be used as an efficient spin current conductor and poor charge current conductor, opening the path towards spintronics applications based in this very attractive material.

Efficient spin transport through native oxides of nickel and permalloy with platinum and gold overlayers

We present measurements of spin pumping detected by the inverse spin Hall effect voltage and ferromagnetic resonance spectroscopy in a series of metallic ferromagnet/normal metal thin film stacks. We compare heterostructures grown in situ to those where either a magnetic or nonmagnetic oxide is introduced between the two metals. The heterostructures, either nickel with a platinum overlayer (Ni/Pt) or the nickel-iron alloy permalloy (Py) with a gold overlayer (Py/Au), were also characterized in detail using grazing-incidence x-ray reflectivity, Auger electron spectroscopy, and both SQUID and alternating-gradient magnetometry. We verify the presence of oxide layers, characterize layer thickness, composition, and roughness, and probe saturation magnetization, coercivity, and anisotropy. The results show that while the presence of a nonmagnetic oxide at the interface suppresses spin transport from the ferromagnet to the nonmagnetic metal, a thin magnetic oxide (here the native oxide formed on both Py and Ni) somewhat enhances the product of the spin-mixing conductance and the spin Hall angle. We also observe clear evidence of an out-of-plane component of magnetic anisotropy in Ni/Pt samples that is enhanced in the presence of the native oxide, resulting in perpendicular exchange bias. Finally, the dc inverse spin Hall voltages generated at ferromagnetic resonance in our Py/Au samples are large, and suggest values for the spin Hall angle in gold of $0.04l{\ensuremath{\alpha}}_{\mathrm{SH}}l0.22$, in line with the highest values reported for Au. This is interpreted as resulting from Fe impurities. We present indirect evidence that the Au films described here indeed have significant impurity levels.

Magnetic anisotropy, damping, and interfacial spin transport in Pt/LSMO bilayers

We report ferromagnetic resonance measurements of magnetic anisotropy and damping in epitaxial La0.7Sr0.3MnO3 (LSMO) and Pt capped LSMO thin films on SrTiO3 (001) substrates. The measurements reveal large negative perpendicular magnetic anisotropy and a weaker uniaxial in-plane anisotropy that are unaffected by the Pt cap. The Gilbert damping of the bare LSMO films is found to be low α = 1.9(1) × 10−3, and two-magnon scattering is determined to be significant and strongly anisotropic. The Pt cap increases the damping by 50% due to spin pumping, which is also directly detected via inverse spin Hall effect in Pt. Our work demonstrates efficient spin transport across the Pt/LSMO interface.

Current-limiting challenges for all-spin logic devices.

All-spin logic device (ASLD) has attracted increasing interests as one of the most promising post-CMOS device candidates, thanks to its low power, non-volatility and logic-in-memory structure. Here we investigate the key current-limiting factors and develop a physics-based model of ASLD through nano-magnet switching, the spin transport properties and the breakdown characteristic of channel. First, ASLD with perpendicular magnetic anisotropy (PMA) nano-magnet is proposed to reduce the critical current (Ic0). Most important, the spin transport efficiency can be enhanced by analyzing the device structure, dimension, contact resistance as well as material parameters. Furthermore, breakdown current density (JBR) of spin channel is studied for the upper current limitation. As a result, we can deduce current-limiting conditions and estimate energy dissipation. Based on the model, we demonstrate ASLD with different structures and channel materials (graphene and copper). Asymmetric structure is found to be the optimal option for current limitations. Copper channel outperforms graphene in term of energy but seriously suffers from breakdown current limit. By exploring the current limit and performance tradeoffs, the optimization of ASLD is also discussed. This benchmarking model of ASLD opens up new prospects for design and implementation of future spintronics applications.

Oxygen deficiency induced deterioration in microstructure and magnetic properties at Y3Fe5O12/Pt interface

Transport efficiency of pure spin current across the ferromagnetic films adjacent with a nonmagnetic metal is strongly dependent on the spin mixing conductance, which is very sensitive to atomic-level interface conditions. Here, by the means of advanced electron microscopy techniques, atomic structure, electronic structure, and magnetic properties at Y3Fe5O12 (YIG)/Pt interface are detailed characterized to correlate the microstructure and magnetic properties with interfacial transport properties. It is found that the order-disorder structure transformation at the interface is accompanied with oxygen deficiency, thus the reduced iron valence and the break of magnetic atom-O-magnetic atom bridges, which is responsible for superexchange interaction and magnetic order. It is also found that the magnetic moment of interfacial iron ions is decreased. The disorder interfacial layer with suppressed magnetism finally contributes to the declined spin transport efficiency. Our results provide the knowledge to control and manipulate the interfacial structure and properties in order to obtain higher spin transport efficiency.

(Invited) Importance of Interface Engineering in Organic Spintronics

Organic semiconductors provide a route to revolutionary changes in many areas of device technology with their unique electronic properties and functionalities, which can be easily tailored by chemical synthesis and molecular engineering.[1] In particular, an emerging research area referred to as organic spintronics has initiated a range of device concepts by exploiting spin-related properties in organic semiconductors.[2] Currently, there are two major interests in organic spintronics technology.[3] One of them is to realize a system enabling the injection/transport/detection of spin polarized charge carriers in organic semiconductors.[3] The other one is to understand and explore the magnetic field effects (MFEs) of organic semiconductors for the development and improvement of organic-based devices such as organic light emitting diodes and magnetic sensors.[3] It has been known that in organic-based devices, the interfaces between active materials play a critical role in their performance and functionality because the interfaces greatly affect the injection/transport/extraction of charge carriers in organic semiconductors.[4] Recent findings suggest that the interface engineering is also very crucial in organic spintronics technology such as enabling efficient spin-polarized charge carrier injection and tuning of organic MFEs.[5-7] In this presentation, we will discuss our efforts towards the realization of efficient spin injection and transport in organic semiconductors as well as the modification of organic MFEs by interface engineering. We experimentally show the importance of band alignment between a metal electrode and an organic semiconductor at their interface for efficient spin injection/detection.[6] We will also show how one can utilize a molecular self-assembled monolayer by inserting it between a metal electrode and an organic semiconductor in order to change the characteristics of spin injection into an organic semiconductor from a ferromagnetic metal as well as in order to tune the organic MFEs.[7] 1. Editorial, Nat. Mater. 12, 591 (2013). 2. C. Boehme and J. M. Lupton, Nat. Nano. 8, 612 (2013). 3. W. Wagemans, et al., SPIN, 1, 93 (2011). 4. H. Ma, et al., Adv. Funct. Mater. 20, 1371 (2010). 5. S. Sanvito, Nat. Phys. 6, 562 (2010). 6. H.-J. Jang, et al., Appl. Phys. Lett. 101, 102412 (2012). 7. H.-J. Jang, et al., ACS Nano 8, 7192 (2014).

Is spin transport through molecules really occurring in organic spin valves? A combined magnetoresistance and inelastic electron tunnelling spectroscopy study

Molecular and organic spintronics is an emerging research field which combines the versatility of chemistry with the non-volatility of spintronics. Organic materials have already proved their potential as tunnel barriers (TBs) or spacers in spintronics devices showing sizable spin valve like magnetoresistance effects. In the last years, a large effort has been focused on the optimization of these organic spintronics devices. Insertion of a thin inorganic tunnel barrier (Al2O3 or MgO) at the bottom ferromagnetic metal (FM)/organic interface seems to improve the spin transport efficiency. However, during the top FM electrode deposition, metal atoms are prone to diffuse through the organic layer and potentially short-circuit it. This may lead to the formation of a working but undesired FM/TB/FM magnetic tunnel junction where the organic plays no role. Indeed, establishing a protocol to demonstrate the effective spin dependent transport through the organic layer remains a key issue. Here, we focus on Co/Al2O3/Alq3/Co junctions and show that combining magnetoresistance and inelastic electron tunnelling spectroscopy measurements one can sort out working “organic” and short-circuited junctions fabricated on the same wafer.

Enhancement of Pure Spin Currents in Spin PumpingY3Fe5O12/Cu/MetalTrilayers through Spin Conductance Matching

Spin transport efficiency in heterostructures depends on the spin conductances of each constituent and their interfaces. We report a comparative study of spin pumping in Y3Fe5O12/Cu/Pt and Y3Fe5O12/Cu/W trilayers. Surprisingly, the insertion of a Cu interlayer between Y3Fe5O12 and W substantially improves (over a factor of 4) the spin current injection into W while similar insertion between Y3Fe5O12 and Pt degrades the spin current. This is a consequence of a much improved Y3Fe5O12/Cu spin mixing conductance relative to that for Y3Fe5O12/W. This result implies the possibility of engineered heterostructures with matching spin conductances to enable optimal spin transport efficiency.