High Spin Polarization Research Articles

2D spin transport through graphene-MnBi2Te4 heterojunction

The development of two-dimensional (2D) magnetic semiconductors promotes the study of nonvolatile control of magnetoelectric nanodevices. MnBi2Te4 is the first realization of antiferromagnetic topological insulator. In semiconductor circuits, metal-semiconductor contacts are usually essential. In future all-carbon circuits, graphene is a promising material for 2D conductive connections. This work studies electronic transport through graphene-MnBi2Te4-graphene junctions. We find that graphene-MnBi2Te4 interfaces are perfect Ohmic contacts, which benefits the use of MnBi2Te4 in carbon circuits. The currents through MnBi2Te4 junctions possess high spin polarization. Compared with usual van der Waals junctions, lateral graphene-MnBi2Te4-graphene junctions present a lower barrier and much higher conductance to electrons. These findings may provide guidance for further study of 2D spin filtering.

Study on FeCr thin film for a spintronic material with negative spin polarization

Negative spin polarization materials generate a spin current whose spin direction is opposite to the magnetization direction. This characteristic is technologically important because it increases the freedom in the design of spintronic devices. In this paper, we studied FeCr from the viewpoints of band structure, atomic magnetic moments, and local atomic arrangement to explore its potentiality as a negative spin polarization material. First-principles calculations on the disordered FeCr alloy predicted a high negative spin polarization, which was attributed to the band matching of the minority spin. We experimentally fabricated Fe1-xCrx thin films in the Cr concentration (x) range from 0.2 to 0.4 by sputter deposition and demonstrated inverse magnetoresistance using current-perpendicular-to-plane giant magnetoresistance (GMR) devices, which provided evidence of negative spin polarization. The spin polarization of Fe0.7Cr0.3 estimated from the GMR device measurements was −0.28 and fell below the calculated value. Synchrotron X-ray magnetic circular dichroism measurements showed a ferrimagnetic configuration of the Fe and Cr magnetic moments. The Fe moment showed a large moment over a wide Cr concentration range, whereas the Cr moment decreased with the Cr concentration, which agreed with the calculation result. Synchrotron Mössbauer spectroscopy measurements indicated the inhomogeneity of the FeCr composition even in the as-deposited state. Fe-rich regions were formed after annealing, reflecting the phase separation nature of the Fe-Cr system. Inhomogeneity was not considered in the calculation and could be the origin of the smaller negative spin polarization observed in the experiment.

Large magnetocapacitance beyond 420% in epitaxial magnetic tunnel junctions with an MgAl2O4 barrier

Magnetocapacitance (MC) effect has been observed in systems where both symmetries of time-reversal and space-inversion are broken, for examples, in multiferroic materials and spintronic devices. The effect has received increasing attention due to its interesting physics and the prospect of applications. Recently, a large tunnel magnetocapacitance (TMC) of 332% at room temperature was reported using MgO-based (001)-textured magnetic tunnel junctions (MTJs). Here, we report further enhancement in TMC beyond 420% at room temperature using epitaxial MTJs with an MgAl2O4(001) barrier with a cation-disordered spinel structure. This large TMC is partially caused by the high effective tunneling spin polarization, resulted from the excellent lattice matching between the Fe electrodes and the MgAl2O4 barrier. The epitaxial nature of this MTJ system sports an enhanced spin-dependent coherent tunneling effect. Among other factors leading to the large TMC are the appearance of the spin capacitance, the large barrier height, and the suppression of spin flipping through the MgAl2O4 barrier. We explain the observed TMC by the Debye-Fröhlich modelled calculation incorporating Zhang-sigmoid formula, parabolic barrier approximation, and spin-dependent drift diffusion model. Furthermore, we predict a 1000% TMC in MTJs with a spin polarization of 0.8. These experimental and theoretical findings provide a deeper understanding on the intrinsic mechanism of the TMC effect. New applications based on large TMC may become possible in spintronics, such as multi-value memories, spin logic devices, magnetic sensors, and neuromorphic computing.

The Role of Electron-Phonon Coupling in Spin Transport through FM-QD Molecular-FM in the Presence of Spin Accumulation in the Leads

The spin transport process through quantum dot molecular, embedded between two ferromagnetic leads in parallel configuration with the presence of spin accumulation, is studied by getting use of the non-equilibrium Keldysh – Green’s function technique. The electron-phonon coupling can be implicitly considered in the model, using the canonical transformation where a single phonon mode is considered in the strong electron – phonon coupling regime. Since the heat may interchange between the quantum dot molecular and the phonon bath coupled to it. And as the principle aim of our study is to determine the parameters that afford high spin (charge) heat generation, that must be avoid in the experimental applications, all the spin transport properties are investigated throughout the calculation of the spin and charge accumulation on the quantum dot molecular, the spin polarized currents, the spin and charge currents, the spin polarized heat generations, the spin heat generation and the charge heat generation. The calculations are accomplished as a function of the model calculation parameters that can be tuned experimentally. The spin blockade and the negative differential phenomena are investigated since the spin transport properties are studied extendedly in the case of a parallel configuration in the leads. It is concluded that the operative and functional values of the bias voltage can be determined by single phonon energy and the electron-phonon coupling values. Since all our calculations are accomplished for electron-phonon coupling equals 0.05eV. Finally, we must report the following: in the case of parallel configuration with high spin polarization, the spin heat generation equals the charge heat generation and both are relatively high where the intradot interaction has no role. While, as the spin polarization is lowered then the charge heat generation be greater than the spin heat generation when the correlation energy is relatively low.

Hybrid Chiral MoS2 Layers for Spin‐Polarized Charge Transport and Spin‐Dependent Electrocatalytic Applications

The chiral‐induced spin selectivity effect enables the application of chiral organic materials for spintronics and spin‐dependent electrochemical applications. It is demonstrated on various chiral monolayers, in which their conversion efficiency is limited. On the other hand, relatively high spin polarization (SP) is observed on bulk chiral materials; however, their poor electronic conductivities limit their application. Here, the design of chiral MoS2 with a high SP and high conductivity is reported. Chirality is introduced to the MoS2 layers through the intercalation of methylbenzylamine molecules. This design approach activates multiple tunneling channels in the chiral layers, which results in an SP as high as 75%. Furthermore, the spin selectivity suppresses the production of H2O2 by‐product and promotes the formation of ground state O2 molecules during the oxygen evolution reaction. These potentially improve the catalytic activity of chiral MoS2. The synergistic effect is demonstrated as an interplay of the high SP and the high catalytic activity of the MoS2 layer on the performance of the chiral MoS2 for spin‐dependent electrocatalysis. This novel approach employed here paves way for the development of other novel chiral systems for spintronics and spin‐dependent electrochemical applications.

Room-temperature spin-orbit torque switching in a manganite-based heterostructure

The oxide ferromagnet ${\mathrm{La}}_{0.67}{\mathrm{Sr}}_{0.33}\mathrm{Mn}{\mathrm{O}}_{3}$ (LSMO) possesses low saturation magnetization $({M}_{s})$, a low magnetic damping constant $(\ensuremath{\alpha})$, and high carrier spin polarization, which promise superior functionalities in spintronics devices. For applications, these devices are to be integrated into electronic circuits, which require the control of the magnetization of LSMO by electrical means. However, a reliable electrical switching method remains largely unexplored. Here, we study the current-induced spin-orbit torque (SOT) in an all-oxide $\mathrm{SrIr}{\mathrm{O}}_{3}/\mathrm{LSMO}$ bilayer. We found the magnetization of a 15 nm LSMO can be switched by electrical current, as confirmed from both the electrical transport measurement and the magnetic optical measurement. By taking advantage of the strain-mediated magnetic anisotropy, the magnetic easy axis of LSMO was controlled to be either perpendicular or parallel to the charge current direction such that a type-$y$ or a type-$x$ SOT switching was achieved, respectively. The dampinglike SOT efficiency is characterized to be 0.15 at room temperature according to the harmonic Hall voltage analysis. We also studied the temperature dependences of the SOT effective field and current-induced switching. Our demonstration of the electrical switching of LSMO magnetization at room temperature may stimulate SOT studies in a wide variety of all-oxide systems.

Magnetoelastic anisotropy in Heusler-type Mn2−δCoGa1+δ films

Perpendicular magnetization is essential for high-density memory application using magnetic materials. High-spin polarization of conduction electrons is also required for realizing large electric signals from spin-dependent transport phenomena. Heusler alloy is a well-known material class showing the half-metallic electronic structure. However, its cubic lattice nature favors in-plane magnetization and thus minimizes the perpendicular magnetic anisotropy (PMA), in general. This study focuses on an inverse-type Heusler alloy, Mn$_{2-\delta}$CoGa$_{1+\delta}$ (MCG) with a small off-stoichiometry ($\delta$) , which is expected to be a half-metallic material. We observed relatively large uniaxial magnetocrystalline anisotropy constant ($K_\mathrm{u}$) of the order of 10$^5$ J/m$^3$ at room temperature in MCG films with a small tetragonal distortion of a few percent. A positive correlation was confirmed between the $c/a$ ratio of lattice constants and $K_\mathrm{u}$. Imaging of magnetic domains using Kerr microscopy clearly demonstrated a change in the domain patterns along with $K_\mathrm{u}$. X-ray magnetic circular dichroism (XMCD) was employed using synchrotron radiation soft x-ray beam to get insight into the origin for PMA. Negligible angular variation of orbital magnetic moment ($\Delta m_\mathrm{orb}$) evaluated using the XMCD spectra suggested a minor role of the so-called Bruno's term to $K_\mathrm{u}$. Our first principles calculation reasonably explained the small $\Delta m_\mathrm{orb}$ and the positive correlation between the $c/a$ ratio and $K_\mathrm{u}$. The origin of the magnetocrystalline anisotropy was discussed based on the second-order perturbation theory in terms of the spin--orbit coupling, claiming that the mixing of the occupied $\uparrow$- and the unoccupied $\downarrow$-spin states is responsible for the PMA of the MCG films.

A First‐Principles Study of the Structural, Magnetic, Optical Properties and Doping Effect in Chromium Arsenide

Pristine and doped chromium arsenide (CrAs) in six different crystal structures is systematically studied to investigate the structural, magnetic, and optical properties for real applications by first‐principles calculations. First, it is found that the ground‐state structure is an orthorhombic MnP‐type structure with antiferromagnetic spin order. The rocksalt structure is a low‐energy metastable phase and a ferromagnetic metal with high spin polarization at the Fermi level. Second, the NiAs structure and MnP structure have a higher absorption coefficient than other structures in the infrared region and ultraviolet region, respectively. In the visible region, the wurtzite and zincblende structures are more transparent than other structures. At last, the substitution of Cr by Ti and the substitution of As by Te can lead to a phase transition in ground‐state structure and ground‐state magnetic order, respectively. These results can promote the application of the CrAs system into spintronics and optoelectronics.

Effect of Co-doping on the structure, magnetic and electronic properties of Heusler alloys Mn2Fe1-xCoxGa (x = 0–1)

A series of Mn2Fe1-xCoxGa (x = 0–1) Heusler alloys was synthesized by melt-spinning method. The crystal structure, magnetic properties and electronic structure of them were investigated experimentally and theoretically. In undoped Mn2FeGa, a pure FCC phase is identified. The substitution of Co for Fe in Mn2Fe1-xCoxGa tends to stabilize the BCC Heusler phase, with the help of melt-spinning technique, single BCC Heusler phase was obtained within the range of × = 0.5–1.0. The FCC Mn2Fe1-xCoxGa are antiferromagnetic and their Néel temperatures TN decrease with Co-doping. But in BCC Mn2Fe1-xCoxGa, a ferromagnetic character is observed and the saturated magnetization Ms at 5 K increases rapidly with increasing Co content. First-principles calculations suggest that Co-doping can make the formation energy of Mn2Fe1-xCoxGa BCC phase more negative and increase the phase stability, which agrees well with the appearance of the BCC phase in the XRD pattern when Co content is high. This effect can be explained from the change of electronic structure with Co-doping. The calculated total spin moments of Mn2Fe1-xCoxGa increase from 1.04μB / f.u. for × = 0 to 2.01 μB / f.u. for × = 1, following the Slater-Pauling curve of M = Z −24. The Ms at 5 K agrees well with the theoretical results when × = 0.75–1.0. Calculations also suggest that Mn2Fe1-xCoxGa alloys all have 100% or quite high spin polarization ratio. All this makes them promising candidates for spintronic applications when Co content is high.

Perpendicular Manganite Magnetic Tunnel Junctions Induced by Interfacial Coupling.

The half-metallic manganite oxide La2/3Sr1/3MnO3 (LSMO) has a very high spin polarization of ∼100%, making it ideal for ferromagnetic electrodes to realize tunneling magnetoresistance (TMR). Because of the in-plane magnetic anisotropy of the ferromagnetic LSMO electrode, which leads to the density limit of memory, realizing perpendicular tunneling in manganite-based magnetic tunnel junctions (MTJ) is critical for future applications. Here, we design and fabricate manganite-based MTJs composed of alternately stacked cobaltite and manganite layers that demonstrate strong perpendicular magnetic anisotropy (PMA) induced by interfacial coupling. Moreover, spin-dependent tunneling behaviors with an out-of-plane magnetic field were observed in the perpendicular MTJs. We found that the direct tunneling effect plays a dominant role in the low bias region during the transport behavior of devices, which is associated with thermionic emission of electrons or oxygen vacancies in the high bias region. Our works of realizing perpendicular tunneling in manganite-based MTJs lead to new approaches for designing and developing all-oxide spintronic devices.

Effect of stoichiometry and film thickness on the structural and magnetization dynamics behavior of Co2MnAl thin films cosputtered on Si(1 0 0)

Obtaining low gilbert damping (α) and high spin polarization (P) in Heusler alloy thin films is one of the key requirements from the device application point of view in spintronics. Both α and P are correlated, closely related to the density of states (DOS) at the Fermi level and need to be controlled for engineered material. We investigated the effect of stoichiometry (different Co to Mn atomic percent ratios, viz. 2.21, 4.53, 5.43, and 9.19) and film-thickness {(t) in the range of 24–90 nm} on the structural and magnetization dynamics behavior in co-sputtered Co2MnAl thin films grown over Si(100) substrates. While the stoichiometric (∼50 at.% of Co) films with t ≥ 38 nm were found to exhibit B2 order with low α, the off-stoichiometric films (t ∼ 50 nm) were found to stabilize in partially ordered A2 phase and exhibited higher α. The change in structural ordering due to change in stoichiometry and film-thickness in polycrystalline Co2MnAl (CMA) thin films helped in tuning α in the range of (4.11 ± 0.06) × 10-3 – (11.95 ± 0.28) × 10-3. The minimum value of α ∼ (4.11 ± 0.06) × 10-3 was achieved for stoichiometric CMA (t = 70 nm) thin films which is close to the literature value of epitaxially grown L21 and B2 ordered CMA thin films. The study shows that we can tune and attain low damping in polycrystalline CMA Heusler thin films by controlling the stoichiometry.

Heusler‐Based Cylindrical Micro‐ and Nanowires

Wire‐shaped materials are ideal form for rapid, cheap, and scalable production and application. Herein, the most recent results on Heusler‐based cylindrical micro‐ and nanowires are reviewed, emphasizing spintronic, magnetocaloric, and shape memory applications. Herein, how the Heusler alloy's composition can be tuned to achieve a material with repeatable magnetic and mechanical properties is showed. A wide range of possible Heusler compositions allows us to tune the structural transformation temperatures by alloying the original composition with a few percent of metalloid. Wire shape brings additional effect—well‐defined easy axis change due to a structural transformation that enhances the magnetocaloric effect at low‐magnetic fields. The unique wire's shape properties can be downscaled to nanoscale range by electrodeposition. It is showed that such a method allows the production of a highly ordered array of nanowires. As a result, the magnetocaloric effect of an array of nanowires can be one order higher than that obtained by other production methods, where the nanowires are oriented randomly. Finally, some possible applications of shape memory or magnetocaloric actuators and the possibility of implementing high spin polarization for the construction of spintronic devices are showed.

Disorder-mediated quenching of magnetization in NbVTiAl: Theory and experiment

In this paper, we present the structural, electronic, magnetic and transport properties of a equiatomic quaternary alloy NbVTiAl. The absence of (111) and (200) peaks in X-ray diffraction (XRD) data confirms the A2-type structure. Magnetization measurements indicate a high Curie temperature and a negligibly small magnetic moment (∼10−3 μB/f.u.) These observations are indicative of fully compensated ferrimagnetism in the alloy. Temperature dependent resistivity indicate metallic nature. Ab-initio calculation of fully ordered NbVTiAl structure confirms a nearly half metallic behavior with a high spin polarization (∼90 %) and a net magnetic moment of 0.8 μB/f.u. (in complete contrast to the experimental observation). One of the main objective of the present paper is to resolve and explain the long-standing discrepancy between theoretical prediction and experimental observation of magnetization for V-based quaternary Heusler alloys, in general. To gain an in-depth understanding, we modeled various disordered states and its subsequent effect on the magnetic and electronic properties. The discrepancy is attributed to the A2 disorder present in the system, as confirmed by our XRD data. The presence of disorder also causes the emergence of finite states at the Fermi level, which impacts the spin polarization of the system.

Hyperpolarized Solution-State NMR Spectroscopy with Optically Polarized Crystals.

Nuclear spin hyperpolarization provides a promising route to overcome the challenges imposed by the limited sensitivity of nuclear magnetic resonance. Here we demonstrate that dissolution of spin-polarized pentacene-doped naphthalene crystals enables transfer of polarization to target molecules via intermolecular cross-relaxation at room temperature and moderate magnetic fields (1.45 T). This makes it possible to exploit the high spin polarization of optically polarized crystals, while mitigating the challenges of its transfer to external nuclei. With this method, we inject the highly polarized mixture into a benchtop NMR spectrometer and observe the polarization dynamics for target 1H nuclei. Although the spectra are radiation damped due to the high naphthalene magnetization, we describe a procedure to process the data to obtain more conventional NMR spectra and extract the target nuclei polarization. With the entire process occurring on a time scale of 1 min, we observe NMR signals enhanced by factors between -200 and -1730 at 1.45 T for a range of small molecules.

Polarizing electron spins with a superconducting flux qubit

Electron spin resonance (ESR) is a useful tool to investigate properties of materials in magnetic fields where high spin polarization of target electron spins is required in order to obtain high sensitivity. However, the smaller magnetic fields becomes, the more difficult high polarization is passively obtained by thermalization. Here, we propose to employ a superconducting flux qubit (FQ) to polarize electron spins actively. We have to overcome a large energy difference between the FQ and electron spins for efficient energy transfer among them. For this purpose, we adopt a spin-lock technique on the FQ where the Rabi frequency associated with the spin-locking can match the resonance (Larmor) one of the electron spins. We find that adding dephasing on the spins is beneficial to obtain high polarization of them, because otherwise the electron spins are trapped in dark states that cannot be coupled with the FQ. We show that our scheme can achieve high polarization of electron spins in realistic experimental conditions.

Current spin polarization of a platform molecule with compression effect

Using the first-principles method, the spin-dependent transport properties of a novel platform molecule containing a freestanding molecular wire is investigated by simulating the spin-polarized scanning tunneling microscope experiment with Ni tip and Au substrate electrodes. Transport calculations show that the total current increases as the tip gradually approaches to the substrate, which is consistent with the conductance obtained from previous experiment. More interestingly, the spin polarization (SP) of current modulated by compression effect has the completely opposite trend to the total current. Transmission analyses reveal that the reduction of SP of current with compression process originates from the promotion of spin-down electron channel, which is controlled by deforming the molecule wire. In addition, the density of states shows that the SP of current is directly affected by the organic–ferromagnetic spinterface. The weak orbital hybridization between the Ni tip and propynyl of molecule results in high interfacial SP, whereas the breaking of the C≡C triple of propynyl in favor of the Ni–C–C bond induces the strong orbital hybridization and restrains the interfacial SP. This work proposes a new way to control and design the SP of current through organic–ferromagnetic spinterface using functional molecular platform.

Room-temperature magnetoresistance in Ni78Fe22/C8-BTBT/Ni78Fe22 nanojunctions fabricated from magnetic thin-film edges using a novel technique.

Molecular spintronic devices are gaining popularity because the organic semiconductors with long spin relaxation times are expected to have long spin diffusion lengths. A typical molecular spintronic device consists of organic molecules sandwiched between two magnetic layers, which exhibits magnetoresistance (MR) effect. Nanosized devices are also expected to have a high spin polarization, leading to a large MR effect owing to effective orbital hybridization. However, most studies on nanosized molecular spintronic devices have investigated the MR effect at low temperatures because of the difficulty in observing the MR effect at room temperature. Here we focus on high-mobility molecules expected to show long spin diffusion lengths, which lead to the observation of the MR effect in nanoscale junctions at room temperature. In this study, we fabricate magnetic nanojunctions consisting of high-mobility molecules, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), sandwiched between two Ni78Fe22 thin films with crossed edges. Transmission electron microscopy (TEM) images reveal that C8-BTBT molecular layers with smooth and clear interfaces can be deposited on the Ni78Fe22 thin-film edges. Consequently, we observe a clear positive MR effect, that is, R P < R AP, where R P and R AP are the resistances in the parallel (P) and antiparallel (AP) configurations, respectively, of two magnetic electrodes in the Ni78Fe22/C8-BTBT/Ni78Fe22 nanojunctions at room temperature. The obtained results indicate that the spin signal through the C8-BTBT molecules can be successfully observed. The study presented herein provides a novel nanofabrication technique and opens up new opportunities for research in high-mobility molecular nano-spintronics.

Spin transport properties of boron nitride nanotubes: A DFT study

In this work, we present the spin transport properties of a (10, 0) boron nitride nanotube (BNNT). To induce high spin polarization, we doped transitional metals (Fe, Ni, and Co). Calculations show doping Ni at N site causes to have a pure spin down current and doping Fe on N site causes to have pure spin up current. All dopants make pure spin-polarized levels among the band gap. The Doping TM metal on B sites induced more magnetic dipole moment in comparison to N sites.

Half-metallic properties of Zr2CrAl ferrimagnetic full-Heusler compound, investigated in tetragonal, orthorhombic and rhombohedral crystal structures

The research efforts focused on the implementation of Heusler materials into spintronic heterostructures triggered the investigations on the half-metallic and magnetic properties on Zr2CrAl ferrimagnetic compound. The theoretical investigations based on Density Functional Theory were performed on the tetragonal, orthorhombic and rhombohedral distortions of the typical inverse-Heusler cubic crystalline structure, previously reported. The compound exhibits half-metallic characteristics for all deformations considered. The slight variations in volume of the unit cell do not destroy the high spin polarization state. The band gap from minority spin channel slightly decreases in orthorhombic and rhombohedral deformations. The total magnetic moment calculated per formula unit is integer (equal to 1μB/f.u.) and follows the Slater-Pauling rule for all cases investigated. The antiferromagnetic coupling between the Zr and Cr atoms was preserved. The functionality of ferrimagnetic full-Heusler alloys to be used as overlayers in the production of epitaxially deposited heterostructures on substrates with crystalline structures other than cubic was proven.

Origin of Perpendicular Magnetic Anisotropy Enhancement in Co/Ni Bilayer Due to Plasma Oxidation

In spintronics and magnonics, materials that offer tunable perpendicular magnetic anisotropy (PMA) along with low Gilbert damping and high spin polarization are particularly important. Therefore, a lot of studies are performed on Co/Ni films, that aim at satisfying all these requirements. Moreover, the Dzyaloshinskii−Moriya interaction is induced in them by surrounding the magnetic layers with heavy metal or oxide layers. For this reason, the study of the oxidation of Co/Ni is of particular interest. Therefore, the magnetic properties of Co/Ni bilayers after plasma oxidation (PO) are investigated. It is shown that the magnetic anisotropy of these bilayers can be tuned, not only by the thicknesses of Co and Ni layers, but also by oxidation time varied in the range between 15 and 220 s. After PO, the effective thickness of ferromagnetic Ni is reduced, but it does not oxidize more than ≈2 nm, even for longer oxidation. However, the contribution to PMA increases for the entire range of oxidation times. This indicates that the thickness reduction of the Ni layer is not the only source of PMA enhancement. This additional contribution is attributed to the exchange bias coupling between the ferromagnetic (Co/Ni) and the antiferromagnetic NiO layers.