Spin Transport Properties Research Articles

Probing Spin-Dependent Ballistic Charge Transport at Single-Nanometer Length Scales.

The coherent transport of charge and spin is a key requirement of future devices for quantum computing and communication. Scattering at defects or impurities may significantly reduce the coherence of quantum-mechanical states, thereby affecting the device functionality. While numerous methods exist to experimentally assess charge transport, the real-space detection of a material's ballistic spin transport properties with nanometer resolution remains a challenge. Here we report on a novel approach that utilizes a combination of spin-polarized scanning tunneling microscopy (SP-STM) and the recently introduced molecular nanoprobe (MONA) technique. It relies on the local injection of spin-polarized charge carriers from a magnetic STM tip and their detection by a single surface-deposited phthalocyanine molecule via reversible electron-induced tautomerization events. Based on the particular electronic structure of the Rashba alloy BiAg2, which is governed by a spin-momentum-locked surface state, we prove that the current direction inverses upon tip magnetization reversal.

Computations on platinum based ternary ferromagnetic half metals for spin valve diodes and green energy technology based thermoelectric

The ab initio method commonly known as wavefunction method is used to calculate the structural, mechanical, electronic, magnetic, lattice dynamical and thermoelectric properties of the half metallic half-Heusler alloys VPtZ (Z = Si, Ge, Sn). The equilibrium lattice constants and corresponding ground state energies of the studied alloys are computed in stable ferromagnetic γ-phase with a cubic structure. The electronic properties suggest that the compounds are half metals with indirect band gap having the band energies of 1.02 eV, 1.04 eV and 0.9 eV for VPtSi, VPtGe and VPtSn, respectively, in the spin down channel. The materials have spin magnetic moment of 1 μB, suggest that the materials are suitable for making spintronic devices such as spin valve diodes. Temperature dependent heat transport properties were studied by using classical Boltzmann theory. The materials have high power factor at high temperatures. Thermoelectric performance of the materials is calculated between the temperature range 100–1200 K respectively. The obtained thermoelectric figure of merit for p-type VPtSi, VPtGe and VPtSn are 0.4, 0.7 and 0.3, respectively, at 1200 K. Similarly, the figure of merit obtained for n-type VPtSi, VPtGe and VPtSn are 0.41, 0.75 and 0.3, respectively, at the same temperature. The favourable energy and spin transport properties of these alloys show that they are the potential candidates as thermoelectric for heat energy harvesting and spintronic for magneto electronics applications.

Tuning the spin transport properties of phosphorene superlattice under a uniform electric field and Rashba spin-orbit interaction

The spin-interdependent transport features for phosphorene superlattice, containing spin-flip and spin polarization are described under a constant electric field and extrinsic Rashba spin-orbit interaction utilizing the transfer matrix method. Our findings demonstrated the number of barriers at this structure acts the prominent role at the spin polarization and transmission probability, that can be applied in modeling optimum spintronic apparatus. Interestingly, the best outcomes for the transmission probability and spin polarization transpired in the five obstacles. Furthermore, an incoming electron with spin-up (down) can be transferred as an electron with spin-down (up) by adjusting the bias voltage and Rashba strength. Our findings can be utilized to create new nanostructures and maximize the performance of spintronic systems based on phosphorene nanoribbons.

Ultrafast spin-to-charge conversion in antiferromagnetic (111)-oriented L12-Mn3Ir

Antiferromagnetic L12-Mn3Ir combines outstanding spin-transport properties with magnons in the terahertz (THz) frequency range. However, the THz radiation emitted by ultrafast spin-to-charge conversion via the inverse spin Hall effect remains unexplored. In this study, we measured the THz emission and transmission of a Permalloy/(111)-oriented L12-Mn3Ir multilayer by THz time-domain spectroscopy. The spin Hall angle was determined to be approximately constant at 0.035 within a frequency range of 0.3–2.2 THz, in comparison with the THz spectroscopy of a Permalloy/Pt multilayer. Our results not only demonstrate the potential of L12-Mn3Ir as a spintronic THz emitter but also provide insights into the THz spin transport properties of L12-Mn3Ir.

Ultrahigh spin filter efficiency and large spin Seebeck polarization of binuclear manganese phthalocyanine molecular junctions on nickel electrodes

In this paper, we investigated the spin transport properties of binuclear manganese phthalocyanine (Mn2Pc2) spintronic devices sandwiched between two nickel electrodes using the non-equilibrium Green's function method in combination with density functional theory. Based on the calculation results, the Mn2Pc2 device exhibited excellent spin-filtering capabilities, demonstrating an exceptionally high spin filter efficiency (SFE). Irrespective of the parallel or antiparallel orientation of magnetization in the electrodes, we observed that when both manganese atoms were in a spin-up state, the SFE of spin-resolved currents under finite bias and the thermoelectric currents induced by temperature gradients at fixed temperatures were both close to 100%. The large spin Seebeck polarization of the Mn2Pc2 device was also obtained at low reference temperatures. This study explores the potential for developing multifunctional spintronic single-molecule devices using Ni−Mn2Pc2.

Ballistic spin-transport properties of magnetic tunnel junctions with MnCr-based ferrimagnetic quaternary Heusler alloys

Ballistic spin-transport properties of magnetic tunnel junctions with MnCr-based ferrimagnetic quaternary Heusler alloys

Highly Efficient Spin Transport in a Paramagnetic Insulator at Room Temperature

AbstractThe exploration of new materials exhibiting excellent spin transport properties, particularly those capable of efficiently transmitting pure spin currents at room temperature, is a crucial aspect of spintronics. This work reports the observation of room‐temperature spin transport in a paramagnetic insulator Gd3Ga5O12 (GGG). By measuring the longitudinal spin Seebeck voltage, spin Hall magnetoresistance, and anomalous Hall resistance in a Y3Fe5O12 (YIG)/GGG/Pt heterostructure at room temperature, the spin current is found to propagate in GGG up to a distance of 7 nm through the paramagnons, which are the collective excitations of the local spin within the paramagnet. Remarkably, this spin propagation phenomenon occurs at a small magnetic field and irrespective of whether the spin current injected into GGG is thermally induced from YIG or electrically generated through the spin Hall effect of Pt. Moreover, the inclusion of a thin GGG spacer layer increases the thermal spin current from YIG to Pt by 31%. Calculations based on the diffusive magnon transport model indicate high spin conductance at the GGG/Pt interface. These findings highlight the potential of a wide range of paramagnetic materials in facilitating spin transport and advancing the development of spintronic devices.

Single spin magnetometry and relaxometry applied to antiferromagnetic materials

Despite the considerable interest for antiferromagnets that appeared with the perspective of using them for spintronics, their experimental study, including the imaging of antiferromagnetic textures, remains a challenge. To address this issue, quantum sensors, and, in particular, the nitrogen-vacancy (NV) defects in diamond have become a widespread technical solution. We review here the recent applications of single NV centers to study a large variety of antiferromagnetic materials, from quantitative imaging of antiferromagnetic domains and non-collinear states, to the detection of spin waves confined in antiferromagnetic textures and the non-perturbative measurement of spin transport properties. We conclude with recent developments improving further the magnetic sensitivity of scanning NV microscopy, opening the way to detailed investigations of the internal texture of antiferromagnetic objects.

Magnetoresistance in Organic Spin Valves Based on Acid‐Exfoliated 2D Covalent Organic Frameworks Thin Films

AbstractCovalent organic frameworks (COFs), as a burgeoning class of crystalline porous materials, have made significant progress in their application to optoelectronic devices such as field‐effect transistors, memristors, and photodetectors. However, the insoluble features of microcrystalline two‐dimensional (2D) COF powders limit development of their thin film devices. Additionally, the exploration of spin transport properties in this category of π‐conjugated skeleton materials remains vacant thus far. Herein, an imine‐linked 2D Py‐Np COF nanocrystalline powder was synthesized by Schiff base condensation of 4,4′,4′′,4′′′‐(pyrene‐1,3,6,8‐tetrayl)tetraaniline and naphthalene‐2,6‐dicarbaldehyde. Then, we prepared a large‐scale free‐standing Py‐Np COF film via a top‐down strategy of chemically assisted acid exfoliation. Moreover, high‐quality COF films acted as active layers were transferred onto ferromagnetic La0.67Sr0.33MnO3(LSMO) electrodes for the first attempt to fabricate organic spin valves (OSVs) based on 2D COF materials. This COF‐based OSV device with a configuration of LSMO/Py‐Np COF/Co/Au demonstrated a remarkable magnetoresistance (MR) value up to −26.5 % at 30 K. Meanwhile, the MR behavior of the COF‐based OSVs exhibited a highly temperature dependence and operational stability. This work highlights the enormous application prospects of 2D COFs in organic spintronics and provides a promising approach for developing electronic and spintronic devices based on acid‐exfoliated COF thin films.

Magnetoresistance in Organic Spin Valves Based on Acid-Exfoliated 2D Covalent Organic Frameworks Thin Films.

Covalent organic frameworks (COFs), as a burgeoning class of crystalline porous materials, have made significant progress in their application to optoelectronic devices such as field-effect transistors, memristors, and photodetectors. However, the insoluble features of microcrystalline two-dimensional (2D) COF powders limit development of their thin film devices. Additionally, the exploration of spin transport properties in this category of π-conjugated skeleton materials remains vacant thus far. Herein, an imine-linked 2D Py-Np COF nanocrystalline powder was synthesized by Schiff base condensation of 4,4',4'',4'''-(pyrene-1,3,6,8-tetrayl)tetraaniline and naphthalene-2,6-dicarbaldehyde. Then, we prepared a large-scale free-standing Py-Np COF film via a top-down strategy of chemically assisted acid exfoliation. Moreover, high-quality COF films acted as active layers were transferred onto ferromagnetic La0.67 Sr0.33 MnO3 (LSMO) electrodes for the first attempt to fabricate organic spin valves (OSVs) based on 2D COF materials. This COF-based OSV device with a configuration of LSMO/Py-Np COF/Co/Au demonstrated a remarkable magnetoresistance (MR) value up to -26.5 % at 30 K. Meanwhile, the MR behavior of the COF-based OSVs exhibited a highly temperature dependence and operational stability. This work highlights the enormous application prospects of 2D COFs in organic spintronics and provides a promising approach for developing electronic and spintronic devices based on acid-exfoliated COF thin films.

Excellent spin filtering and significant magnetoresistance of rhenium phthalocyanine molecular junctions on Au(100)

Spin transport properties of single rhenium phthalocyanine (RePc) and double-rhenium phthalocyanine (Re2Pc2) molecular devices with gold electrodes have been investigated by combining Density Functional theory and nonequilibrium Green’s functions. It was found that the RePc molecular devices exhibited good spin filtration efficiency in the small bias range even in the absence of magnetic electrodes. The Re2Pc2 molecular device exhibited a large magnetoresistance and excellent spin-filtering properties at a finite bias, which can only be achieved by altering the magnetization orientation of the rhenium atom at the center of the phthalocyanine molecule. These conclusions of the rhenium phthalocyanine molecule may have potential application in magnetoresistive devices and spin filters.

Chirality induced spin selectivity in chiral hybrid organic-inorganic perovskites.

Chiral materials exhibit many interesting physical properties including circular dichroism, circularly polarized photoluminescence, and spin selectivity. Since its discovery, chirality-induced spin selectivity (CISS) has been demonstrated in many chiral material systems, which indicates promising applications in spintronic devices. Thus, searching for compounds that possess both sizable chirality and excellent spin transport properties is in order. Hybrid organic-inorganic perovskites have attracted intensive research interest due to their long carrier lifetime, high carrier mobility, chemically tunable electronic properties, and long spin lifetime, which make this emerging class of semiconductors promising candidate for spintronics. Moreover, hybrid perovskites integrate inorganic octahedral framework and organic ligands, which may introduce chirality into the materials, especially in quasi-two-dimensional structures. Recently, CISS has been observed in 2D chiral hybrid perovskites, showing the spin filtering effect. Studies of CISS in chiral hybrid perovskites not only help deepen our understanding of CISS mechanism but also shed new light on designing novel spintronic devices. In this review, we summarize the state-of-the-art studies of CISS effect in 2D chiral hybrid organic-inorganic perovskites system. We also discuss the remaining challenges and research opportunities of employing CISS in next-generation spintronic devices.

Impact of growth conditions on magnetic anisotropy and magnon Hanle effect in α-Fe2O3

The antiferromagnetic insulator α-Fe2O3 (hematite), widely used in spintronics and magnonics, features a spin-reorientation transition (Morin transition) at 263 K. Thin films, however, often lack this Morin transition, limiting their potential applications. Here, we investigate the impact of different growth conditions on the magnetic anisotropy in α-Fe2O3 films to tune the Morin transition temperature. To this end, we compare the structural, magnetic, and magnon-based spin transport properties of α-Fe2O3 films with different thicknesses grown by pulsed laser deposition in molecular and atomic oxygen atmospheres. We observe a finite Morin transition for those grown by atomic-oxygen-assisted deposition, interestingly even down to 19 nm thickness, where we find a Morin transition at 125 K. In easy-plane antiferromagnets, the nature and time-evolution of the elementary excitations of the spin system are captured by the orientation and precession of the magnon pseudospin around its equilibrium pseudofield, manifesting itself in the magnon Hanle effect. We characterize this effect in these α-Fe2O3 films via all-electrical magnon transport measurements. The films grown with atomic oxygen show a markedly different magnon spin signal from those grown in molecular oxygen atmospheres. Most importantly, the maximum magnon Hanle signal is significantly enhanced, and the Hanle peak is shifted to lower magnetic field values for films grown with atomic oxygen, suggesting changes in the magnetic anisotropy due to an increased oxygen content in these films. Our findings provide new insights into the possibility to fine-tune the magnetic anisotropy in α-Fe2O3 and thereby to engineer the magnon Hanle effect.

High-efficiency spin and valley filter in a monolayer WSe2 superlattice modulated by an anti-handed off-resonance circularly polarized light

We have theoretically studied spin and valley transport properties in an optically-controlled electromagnetic superlattice based on monolayer WSe2. Owing to the regulation of the anti-handed off-resonance circularly polarized light, we find that spin- and valley-dependent line-type resonant peaks appear in the transport forbidden region for small incident angles, so that high-efficiency spin-valley polarization can be obtained. The result indicates that the high-efficiency spin and valley filter can be more conveniently implemented based on the 2D transition metal dichalcogenides superlattice. In addition, the number of spin- and valley-dependent line-type resonant peaks is tunable using an incident angle, gate voltage and the width of the modulated regions.

Spin transport properties of spinel vanadate-based heterostructures

Spin–orbit coupling and breaking of inversion symmetry are necessary ingredients to enable a pure spin current-based manipulation of the magnetization via the spin–orbit torque effect. Currently, magnetic insulator oxides with non-dissipative characteristics are being explored. When combined with non-magnetic heavy metals, known for their large spin–orbit coupling, they offer promising potential for energy-efficient spin-orbitronics applications. The intrinsic electronic correlations characterizing those strongly correlated oxides hold the promises to add extra control-knobs to the desired efficient spin-wave propagation and abrupt magnetization switching phenomena. Spinel vanadate FeV2O4 (FVO) exhibits several structural phase transitions, which are accompanied by an intricate interplay of magnetic, charge, and orbital orderings. When grown as a thin film onto SrTiO3, the compressive strain state induces a perpendicular magnetic anisotropy, making FVO-based heterostructures desirable for spin-orbitronics applications. In this study, we have optimized the deposition of stoichiometric and epitaxial Pt/FVO heterostructures by pulsed laser deposition and examined their spin-related phenomena. From angle-dependent magnetotransport measurements, we observed both anisotropic magnetoresistance and spin Hall magnetoresistance (SMR) effects. Our findings show the SMR component as the primary contributor to the overall magnetoresistance, whose high value of 0.12% is only comparable to properly optimized oxide-based systems.

Layer- and barrier-dependent spin filtering effect and high tunnel magnetoresistance in FeCl2 based van der Waals junctions

2D magnetic van der Waals (vdW) junctions have attracted intensive attention due to their easily controllable thickness and clear interface compared to conventional magnetic multilayer films, which provide a perfect platform to control the performance of spintronic devices. Herein, based on the experimentally fabricated FeCl2 flaks with interlayer antiferromagnetism and intralayer ferromagnetism, we explore the spin transport properties of two classes of vdW junctions with an Au electrode, Au/FeCl2/Au (FeCl2 as the tunnel barrier) and Au/FeCl2/barrier/FeCl2/Au (MoS2 or graphene as the tunnel barrier), and focus on the effects of different barriers and the number of layers. It is found that from monolayer to bilayer FeCl2 in Au/FeCl2/Au, the spin filtering effect is considerably increased due to the weakened interface effect, and almost complete spin polarized current can be obtained. For Au/FeCl2/MoS2/FeCl2/Au, whether the number of layers of MoS2 or FeCl2 is increased from monolayer to bilayer, the tunnel magnetoresistance (TMR) becomes higher due to the high spin polarization of FeCl2, which can reach 1 374 000%. The high TMR of 763 000% can also be achieved for Au/FeCl2/graphene/FeCl2/Au. This work suggests potential applications for FeCl2 flaks in 2D vdW spin filters and spin valves and will stimulate broad studies on layer- and barrier-controllable vdW spintronic devices. All calculations are performed by using the first-principles combined with non-equilibrium Green's function method.

Tuning the spin transport properties of a magnetic MoS2 superlattice

In this study, the author theoretically investigates the effect of the external Rashba interaction on the properties of spin transport in a magnetic MoS2 superlattice. The results show that the spin conductance of the magnetic MoS2 superlattice heavily relied on the applied external magnetic field in the presence of more than one barrier. Moreover, the spin conductances with and without the spin-flip exhibited a gap, the size of which increased with the number of the barriers in the superlattice. The spin-dependent conductance in the magnetic MoS2 superlattice could be controlled by changing the number of barriers to the superlattice and the strength of the Rashba interaction. The spin polarization exhibited oscillating behavior in terms of the energy of the incident electron and the strength of the Rashba interaction. Remarkably, full-spin polarization could be achieved by using an MoS2 superlattice under an external magnetic field. The results of this study confirm that the magnetic MoS2 under the Rashba interaction can be used in spintronics and electronics devices in the future.

Deterministic Current‐Induced Perpendicular Switching in Epitaxial Co/Pt Layers without an External Field

AbstractCurrent‐induced spin‐orbit torques (SOTs) have emerged as a powerful tool to control magnetic elements and non‐uniform magnetic textures such as domain walls and skyrmions. SOT‐induced switching of perpendicular magnetization generally requires an external field to break the rotational symmetry of the spin‐orbit effective fields responsible for the deterministic reversal. The proposed mechanisms to eliminate this requirement often rely on complex multilayer structures that necessitate laborious optimization in the material and spin transport properties, making them less attractive for applications. Herein, current‐induced, external field‐free switching of an epitaxial MgO/Pt/Co trilayer with an extremely large perpendicular anisotropy in excess of 3 Tesla is reported. It is found that switching occurs due to the interplay of strong SOTs, local anisotropy fluctuations, and the Dzyaloshinkii‐Moriya interaction inherent to this epitaxial system. Given that these layers constitute the base stack of a magnetic tunnel junction, this switching mechanism offers the most technologically viable path toward devices such as field‐free SOT‐based magnetic random‐access memories.

Antiferromagnetic spintronics: From metals to functional oxides

Antiferromagnetic spintronics exploits unique properties of antiferromagnetic materials to create new and improved functionalities in future spintronic applications. Here, we briefly review the experimental efforts in our group to unravel spin transport properties in antiferromagnetic materials. Our investigations were initially focused on metallic antiferromagnets, where the first evidence of antiferromagnetic spin-transfer torque was discovered. Because of the lack of metallic antiferromagnets, we then shifted towards antiferromagnetic Mott insulators, where a plethora of transport phenomena was found. For instance, we observed a very large anisotropic magnetoresistance, which can be used to detect the magnetic state of an antiferromagnet. We also observed reversible resistive switching and now provide unequivocal evidence that the resistive switching is associated with structural distortions driven by an electric field. Our findings support the potential of electrically controlled functional oxides for various memory technologies.

Spin Transport Properties of MnBi2Te4-Based Magnetic Tunnel Junctions

The van der Waals heterojunctions, stacking of different two-dimensional materials, have opened unprecedented opportunities to explore new physics and device concepts. Here, combining the density functional theory with non-equilibrium Green's function technique, we systematically investigate the spin-polarized transport properties of van der Waals magnetic tunnel junctions (MTJs), Cu/MnBi2Te4/MnBi2Te4/Cu and Cu/MnBi2Te4/h-BN/n⋅MnBi2Te4/Cu (n = 1, 2, 3). It is found that the maximum tunnel magnetoresistance of Cu/MnBi2Te4/h-BN/3⋅MnBi2Te4/Cu MTJs can reach 162.6%, exceeding the system with only a single layer MnBi2Te4. More interestingly, our results indicate that Cu/MnBi2Te4/h-BN/n⋅MnBi2Te4/Cu (n = 2, 3) MTJs can realize the switching function, while Cu/MnBi2Te4/h-BN/3⋅MnBi2Te4/Cu MTJs exhibit the negative differential resistance. The Cu/MnBi2Te4/h-BN/3⋅MnBi2Te4/Cu in the parallel state shows a spin injection efficiency of more than 83.3%. Our theoretical findings of the transport properties will shed light on the possible experimental studies of MnBi2Te4-based van der Waals magnetic tunneling junctions.