Significant Spin Polarization Research Articles

Paramagnetic Effects in NMR Spectroscopy of Transition-Metal Complexes: Principles and Chemical Concepts.

ConspectusMagnetic resonance techniques represent a fundamental class of spectroscopic methods used in physics, chemistry, biology, and medicine. Electron paramagnetic resonance (EPR) is an extremely powerful technique for characterizing systems with an open-shell electronic nature, whereas nuclear magnetic resonance (NMR) has traditionally been used to investigate diamagnetic (closed-shell) systems. However, these two techniques are tightly connected by the electron-nucleus hyperfine interaction operating in paramagnetic (open-shell) systems. Hyperfine interaction of the nuclear spin with unpaired electron(s) induces large temperature-dependent shifts of nuclear resonance frequencies that are designated as hyperfine NMR shifts (δHF).Three fundamental physical mechanisms shape the total hyperfine interaction: Fermi-contact, paramagnetic spin-orbit, and spin-dipolar. The corresponding hyperfine NMR contributions can be interpreted in terms of through-bond and through-space effects. In this Account, we provide an elemental theory behind the hyperfine interaction and NMR shifts and describe recent progress in understanding the structural and electronic principles underlying individual hyperfine terms.The Fermi-contact (FC) mechanism reflects the propagation of electron-spin density throughout the molecule and is proportional to the spin density at the nuclear position. As the imbalance in spin density can be thought of as originating at the paramagnetic metal center and being propagated to the observed nucleus via chemical bonds, FC is an excellent indicator of the bond character. The paramagnetic spin-orbit (PSO) mechanism originates in the orbital current density generated by the spin-orbit coupling interaction at the metal center. The PSO mechanism of the ligand NMR shift then reflects the transmission of the spin polarization through bonds, similar to the FC mechanism, but it also makes a substantial through-space contribution in long-range situations. In contrast, the spin-dipolar (SD) mechanism is relatively unimportant at short-range with significant spin polarization on the spectator atom. The PSO and SD mechanisms combine at long-range to form the so-called pseudocontact shift, traditionally used as a structural and dynamics probe in paramagnetic NMR (pNMR). Note that the PSO and SD terms both contribute to the isotropic NMR shift only at the relativistic spin-orbit level of theory.We demonstrate the advantages of calculating and analyzing the NMR shifts at relativistic two- and four-component levels of theory and present analytical tools and approaches based on perturbation theory. We show that paramagnetic NMR effects can be interpreted by spin-delocalization and spin-polarization mechanisms related to chemical bond concepts of electron conjugation in π-space and hyperconjugation in σ-space in the framework of the molecular orbital (MO) theory. Further, we discuss the effects of environment (supramolecular interactions, solvent, and crystal packing) and demonstrate applications of hyperfine shifts in determining the structure of paramagnetic Ru(III) compounds and their supramolecular host-guest complexes with macrocycles.In conclusion, we provide a short overview of possible pNMR applications in the analysis of spectra and electronic structure and perspectives in this field for a general chemical audience.

Highly Efficient Spin-Orbit Torque Switching in Bi2Se3/Fe3GeTe2 van der Waals Heterostructures.

Topological insulators (TIs) have shown promise as a spin-generating layer to switch the magnetization state of ferromagnets via spin-orbit torque (SOT) due to charge-to-spin conversion efficiency of the TI surface states that arises from spin-momentum locking. However, when TIs are interfaced with conventional bulk ferromagnetic metals, the combination of charge transfer and hybridization can potentially destroy the spin texture and hamper the possibility of accessing the TI surface states. Here, we fabricate an all van der Waals (vdW) heterostructure consisting of molecular beam epitaxy grown bulk-insulating Bi2Se3 and exfoliated 2D metallic ferromagnet Fe3GeTe2 (FGT) with perpendicular anisotropy. By detecting the magnetization state of the FGT via anomalous Hall effect and magneto-optical Kerr effect measurements, we determine the critical switching current density for magnetization switching to be Jc ≈ 1.2 × 106 A/cm2, the lowest reported for the switching of a perpendicular anisotropy ferromagnet using Bi2Se3. From second harmonic Hall measurements, we further determine the SOT efficiency (ξDL) to be in the range of 1.8 ± 0.3 and 1.4 ± 0.08 between 5 and 150 K, comparable to the highest values reported for Bi2Se3. Our density functional theory calculations find that the weak interlayer interactions at the Bi2Se3/FGT interface lead to a weakened dipole at the interface and suppress the proximity induced magnetic moment on Bi2Se3. This enables direct access to the TI surface states contributed by the first quintuple layer, where the spins are singly degenerate with significant net in-plane spin polarization. Our results highlight the clear advantage of all-vdW heterostructures with weak interlayer interactions that can enhance SOT efficiency and minimize critical current density, an important step toward realizing next generation low-power nonvolatile memory and spintronic devices.

Spin-Selective Electron Transport Through Single Chiral Molecules.

The interplay between chirality and magnetism is a source of fascination among scientists for over a century. In recent years, chirality-induced spin selectivity (CISS) has attracted renewed interest. It is observed that electron transport through layers of homochiral molecules leads to a significant spin polarization of several tens of percent. Despite the abundant experimental evidence gathered through mesoscopic transport measurements, the exact mechanism behind CISS remains elusive. This study reports spin-selective electron transport through single helical aromatic hydrocarbons that are sublimed in vacuo onto ferromagnetic cobalt surfaces and examined with spin-polarized scanning tunneling microscopy (SP-STM) at a temperature of 5K. Direct comparison of two enantiomers under otherwise identical conditions revealed magnetochiral conductance asymmetries of up to 50% when either the molecular handedness is exchanged or the magnetization direction of the STM tip or Co substrate is reversed. Importantly, the results rule out electron-phonon coupling and ensemble effects as primary mechanisms responsible forCISS.

Excellent optical and thermoelectric features of two-dimensional half-metallic ferromagnet Zn1-x(TM)xO: A first principle investigation

The electronic, magneto-optical, and thermoelectric properties of graphene-like ZnO nanosheets doped with transition-metal TM (= Fe, Co, Ni, or Cu) were analyzed using spin-polarized first-principles computations. When a TM atom substitution with the Zn atom, the resultant material exhibits significant spin-polarization around the Fermi level and displays a half-metallic ferromagnet within the PBEsol-GGA. Doped ZnO nanosheets by Fe-, Co-, Ni-, and Cu-dopants generate the total magnetic moment of 4μB, 3μB, 2μB, and 1μB, respectively, which is produced primarily by the TM-3d atoms. The p-d hybridization explains the ferromagnetic interactions caused by the robust coupling among Co-, Ni-, or Cu-3d and anion O-2p orbitals. We have investigated the optical characteristics of TM-doped ZnO nanosheets in parallel (EǁX) and perpendicular (EǁZ) polarization directions. Our analysis demonstrates that the Zn1-x(TM)xO two-dimensional nanosheets can absorb visible sunlight in the (EǁX) polarization direction comparatively to pristine ZnONS. Along the x-axis, we studied the Seebeck coefficient, electrical, electronic thermal conductivities, and power factor for various temperatures. All the TM-doped ZnO (2D) nanosheets demonstrate superior electrical conductivity and high power factor in n-type doping than in p-type doping. According to these findings, TM-doped ZnO nanosheets could represent the next evolution in Spintronic, photovoltaic cells, and thermoelectric technologies.

The spin polarization of palladium on magneto-electric Cr2O3

While induced spin polarization of a palladium (Pd) overlayer on antiferromagnetic and magneto-electric Cr2O3(0001) is possible because of the boundary polarization at the Cr2O3(0001), in the single domain state, the Pd thin film appears to be ferromagnetic on its own, likely as a result of strain. In the conduction band, we find the experimental evidence of ferromagnetic spin polarized in Pd thin films on a Cr2O3(0001) single crystal, especially in the thin limit, Pd thickness of around 1–4 nm. Indeed there is significant spin polarization in 10 Å thick Pd films on Cr2O3(0001) at 310 K, i.e. above the Néel temperature of bulk Cr2O3. While Cr2O3(0001) has surface moments that tend to align along the surface normal, for Pd on Cr2O3, the spin polarization contains an in-plane component. Strain in the Pd adlayer on Cr2O3(0001) appears correlated to the spin polarization measured in spin polarized inverse photoemission spectroscopy. Further evidence for magnetization of Pd on Cr2O3 is provided by measurement of the exchange bias fields in Cr2O3/Pd(buffer)/[Co/Pd] n exchange bias systems. The magnitude of the exchange bias field is, over a wide temperature range, virtually unaffected by the Pd thickness variation between 1 and 2 nm.

First principles study of the electronic structure of the Ni2MnIn/InAs and Ti2MnIn/InSb interfaces

We present a first principles study of the electronic and magnetic properties of epitaxial interfaces between the Heusler compounds, ${\mathrm{Ti}}_{2}\mathrm{MnIn}$ and ${\mathrm{Ni}}_{2}\mathrm{MnIn}$, and the III-V semiconductors, InSb and InAs, respectively. We use density functional theory (DFT) with a machine-learned Hubbard $U$ correction determined by Bayesian optimization. We evaluate these interfaces for prospective applications in Majorana-based quantum computing and spintronics. In both interfaces, states from the Heusler penetrate into the gap of the semiconductor, decaying within a few atomic layers. The magnetic interactions at the interface are weak and local in space and energy. Magnetic moments of less than $0.1{\textmu{}}_{B}$ are induced in the two atomic layers closest to the interface. The induced spin polarization around the Fermi level of the semiconductor decays within a few atomic layers. The decisive factor for the induced spin polarization around the Fermi level of the semiconductor is the spin polarization around the Fermi level in the Heusler, rather than the overall magnetic moment. As a result, the ferrimagnetic narrow-gap semiconductor ${\mathrm{Ti}}_{2}\mathrm{MnIn}$ induces a more significant spin polarization in the InSb than the ferromagnetic metal ${\mathrm{Ni}}_{2}\mathrm{MnIn}$ induces in the InAs. This is explained by the position of the transition metal $d$ states in the Heusler with respect to the Fermi level. Based on our results, these interfaces are unlikely to be useful for Majorana devices but could be of interest for spintronics.

Spin polarization through a molecular junction based on nuclear Berry curvature effects

We explore the effects of spin-orbit coupling on nuclear wave packet motion near an out-of-equilibrium molecular junction, where nonzero Berry curvature emerges as the antisymmetric part of the electronic friction tensor. The existence of nonzero Berry curvature mandates that different nuclear wave packets (associated with different electronic spin states) experience different nuclear Berry curvatures, i.e., different pseudomagnetic fields. Furthermore, for a generic, two-orbital two-lead model (representing the simplest molecular junction), we report significant spin polarization of the electronic current with decaying and oscillating signatures in the large voltage limit---all as a result of nuclear motion. These results are consistent with magnetic AFM chiral-induced spin selectivity experiments. Altogether, our results highlight an essential role for Berry curvature in condensed phase dynamics, where spin separation survives dissipation to electron-hole pair creation and emerges as one manifestation of nuclear Berry curvature.

Optical readout of singlet fission biexcitons in a heteroacene with photoluminescence detected magnetic resonance.

Molecular spin systems based on photoexcited triplet pairs formed via singlet fission (SF) are attractive as carriers of quantum information because of their potentially pure and controllable spin polarization, but developing systems that offer optical routes to readout as well as initialization is challenging. Herein, we characterize the electron spin magnetic resonance change in the photoluminescence intensity for a tailored organic molecular crystal while sweeping a microwave drive up to 10GHz in a broadband loop structure. We observe resonant transitions for both triplet and quintet spin sublevel populations showing their optical sensitivity and revealing the zero-field parameters for each. We map the evolution of these spectra in both microwave frequency and magnetic field, producing a pattern of optically detected magnetic resonance (ODMR) peaks. Fits to these data using a suitable model suggest significant spin polarization in this system with orientation selectivity. Unusual excitation intensity dependence is also observed, which inverts the signof the ODMR signal for the triplet features, but not for the quintet. These observations demonstrate optical detection of the spin sublevel population dictated by SF and intermolecular geometry, and highlight anisotropic and multi-scale dynamics of triplet pairs.

Proposal for All-Electrical Spin Manipulation and Detection for a Single Molecule on Boron-Substituted Graphene.

All-electrical writing and reading of spin states attract considerable attention for their promising applications in energy-efficient spintronics devices. Here we show, based on rigorous first-principles calculations, that the spin properties can be manipulated and detected in molecular spinterfaces, where an iron tetraphenyl porphyrin (FeTPP) molecule is deposited on boron-substituted graphene (BG). Notably, a reversible spin switching between the S=1 and S=3/2 states is achieved by a gate electrode. We can trace the origin to a strong hybridization between the Fe-d_{z^{2}} and B-p_{z} orbitals. Combining density functional theory with nonequilibrium Green's function formalism, we propose an experimentally feasible three-terminal setup to probe the spin state. Furthermore, we show how the in-plane quantum transport for the BG, which is non-spin polarized, can be modified by FeTPP, yielding a significant transport spin polarization near the Fermi energy (>10% for typical coverage). Our work paves the way to realize all-electrical spintronics devices using molecular spinterfaces.

Valley and spin accumulation in ballistic and hydrodynamic channels

A theory of the valley and spin Hall effects and resulting accumulation of the valley and spin polarization is developed for ultraclean channels made of two-dimensional semiconductors where the electron mean free path due to the residual disorder or phonons exceeds the channel width. Both ballistic and hydrodynamic regimes of the electron transport are studied. The polarization accumulation is determined by interplay of the anomalous velocity, side-jump and skew scattering effects. In the hydrodynamic regime, where the electron–electron scattering is dominant, the valley and spin current generation and dissipation by the electron–electron collisions are taken into account. The accumulated polarization magnitude and its spatial distribution depend strongly on the transport regime. The polarization is much larger in the hydrodynamic regime as compared to the ballistic one. Significant valley and spin polarization arises in the immediate vicinity of the channel edges due to the side-jump and skew scattering mechanisms.

Spin Fano Resonances in Chiral Molecules: An Alternative Mechanism for the CISS Effect and Experimental Implications

Experiments on spin transport through a chiral molecule demonstrated the attainment of significant spin polarization, demanding a theoretical explanation. We report the emergence of spin Fano resonances as a mechanism in the chiral-induced spin-selectivity (CISS) effect associated with transport through a chiral polyacetylene molecule. Initializing electrons through optical excitation, we derive the Fano resonance formula for the spin polarization. Computations reveal that quasidegeneracy is common in this complex molecular system. A remarkable phenomenon is the generation of pronounced spin Fano resonances due to the contributions of two near-degeneracy states. We also find that the Fano resonance width increases linearly with the coupling strength between the molecule and the lead. Our findings provide another mechanism to explain the experimental observations and lead to new insights into the role of the CISS effect in complex molecules from the perspective of transport and spin polarization resonance, paving the way for chiral molecule-based spintronics applications.

Regulating Electronic Spin Moments of Single-Atom Catalyst Sites via Single-Atom Promoter Tuning on S-Vacancy MoS2 for Efficient Nitrogen Fixation.

The electrocatalytic activity of transition-metal (TM)-based catalysts is correlated with the spin states of metal atoms. However, developing a way to manipulate spin remains a great challenge. Using first-principles calculations, we first report the crucial role of the spin of exposed Mo atoms around an S-vacancy in the electrocatalytic dinitrogen reduction reaction on defective MoS2 nanosheets and propose a novel strategy for regulating the electronic spin moments by tuning a single-atom promoter (SAP). Single TM atoms adsorbed on a defective MoS2 basal plane serve as SAPs via a noncontact interaction with an exposed Mo active site, inducing a significant spin polarization that promotes N2 adsorption and activation. Interestingly, by changing only the adsorption site of the TM atom, we are able to change the spin moments of the Mo atom, over a wide range of tunable values. The spin moments can be tuned to largely improve the catalytic activity of MoS2 toward the reduction of N2 to NH3.

Density Functional Theory Investigation of Physical Properties of KCrZ (Z = S, Se, Te) Half‐Heusler Alloys

A theoretical investigation of the electronic structure and the magnetic and thermoelectric (TE) properties of KCrS, KCrSe, and KCrTe half‐Heusler alloys is studied. The most stable structural−magnetic configuration is identified, taking into consideration three types of possible atomic arrangements. The results indicate that KCrZ (Z = S, Se, Te) alloys are completely spin‐polarized half‐metallic ferromagnets in their ground state. Band structure calculations demonstrate that all compounds exhibit large bandgaps in the localized minority spin channel with a significant magnetic moment and high spin polarization (100%). The temperature dependence of the TE properties, such as the Seebeck coefficient and the electrical and thermal conductivity coefficients, is discussed and investigated using the semi‐local Boltzmann transport theory in the temperature range 250–1000 K. The quasiharmonic model is implemented to investigate and analyze the thermal parameters. In particular, thermal expansion coefficient, thermal conductivity, Debye temperature, specific heat capacity at constant volume, and entropy of the three novel half‐Heusler alloys are calculated and interpreted.

Huge tunneling magnetoresistance in magnetic tunnel junction with Heusler alloy Co2MnSi electrodes

Magnetic tunnel junction with a large tunneling magnetoresistance has attracted great attention due to its importance in the spintronics applications. By performing extensive density functional theory calculations combined with the nonequilibrium Green’s function method, we explore the spin-dependent transport properties of a magnetic tunnel junction, in which a non-polar SrTiO3 barrier layer is sandwiched between two Heusler alloy Co2MnSi electrodes. Theoretical results clearly reveal that the near perfect spin-filtering effect appears in the parallel magnetization configuration. The transmission coefficient in the parallel magnetization configuration at the Fermi level is several orders of magnitude larger than that in the antiparallel magnetization configuration, resulting in a huge tunneling magnetoresistance (i.e. > 106), which originates from the coherent spin-polarized tunneling, due to the half-metallic nature of Co2MnSi electrodes and the significant spin-polarization of the interfacial Ti 3d orbital.

A joint experimental and theoretical study on structural, electronic, and magnetic properties of MnGen - (n = 3-14) clusters.

A systematic structure and property investigation of MnGen - (n = 3-14) was conducted by means of density functional theory coupled with mass-selected anion photoelectron spectroscopy. This combined theoretical and experimental study allows global minimum and coexistence structures to be identified. It is found that the pentagonal bipyramid shape is the basic framework for the nascent growth process of MnGen - (n = 3-10), and from n = 10, the endohedral structures can be found. For n = 12, the anion MnGe12 - cluster probably includes two isomers: a major isomer with a puckered hexagonal prism geometry and a minor isomer with a distorted icosahedron geometry. Specifically, the puckered hexagonal prism isomer follows the Wade-Mingos rules and can be suggested as a new kind of superatom with the magnetic property. Furthermore, the results of adaptive natural density partitioning and deformation density analyses suggest a polar covalent interaction between Ge and Mn for endohedral clusters of MnGe12 -. The spin density and natural population analysis indicate that MnGen - clusters have high magnetic moments localized on Mn. The density of states diagram visually shows the significant spin polarization for endohedral structures and reveals the weak interaction between the Ge 4p orbital and the 4s, 3d orbitals of Mn.

Spin polarization in lateral two-dimensional heterostructures

In this work, we study the spin polarization in the MoS(Se)2–WS(Se)2 transition metal dichalcogenide heterostructures by using the non-equilibrium Green’s function method and a three-band tight-binding model near the edges of the first Brillouin zone. Although it has been shown that the structures have no significant spin polarization in a specific range of energy of electrons, by applying a transverse electric field in the sheet of the metal atoms, shedding light on the sample, and under a small bias voltage, a significant spin polarization in the structure could be created. Besides, by applying a suitable bias voltage between leads and applying the electric field, a noticeable spin polarization can be found even without shedding the light on the heterostructures.

Tuning MoSO monolayer properties for optoelectronic and spintronic applications: effect of external strain, vacancies and doping.

Since the successful synthesis of the MoSSe monolayer, two-dimensional (2D) Janus materials have attracted huge attention from researchers. In this work, the MoSO monolayer with tunable electronic and magnetic properties is comprehensively investigated using first-principles calculations based on density functional theory (DFT). The pristine MoSO single layer is an indirect gap semiconductor with energy gap of 1.02(1.64) eV as predicted by the PBE(HSE06) functional. This gap feature can be efficiently modified by applying external strain presenting a decrease in its value upon switching the strain from compressive to tensile. In addition, the effects of vacancies and doping at Mo, S, and O sites on the electronic structure and magnetic properties are examined. Results reveal that Mo vacancies, and Al and Ga doping yield magnetic semiconductor 2D materials, where both spin states are semiconductors with significant spin-polarization at the vicinity of the Fermi level. In contrast, single S and O vacancies induce a considerable gap reduction of 52.89% and 58.78%, respectively. Doping the MoSO single layer with F and Cl at both S and O sites will form half-metallic 2D materials, whose band structures are generated by a metallic spin-up state and direct gap semiconductor spin-down state. Consequently, MoV, MoAl, MoGa, SF, SCl, OF, and OCl are magnetic systems, and the magnetism is produced mainly by the Mo transition metal that exhibits either ferromagnetic or antiferromagnetic coupling. Our work may suggest the MoSO Janus monolayer as a prospective candidate for optoelectronic applications, as well as proposing an efficient approach to functionalize it to be employed in optoelectronic and spintronic devices.

Tunable spin-photovoltaic effect in zigzag MoS2 nanoribbons

This article introduces a method for generating spin-polarized photocurrent through light radiation on zigzag MoS2 nanoribbons. In this regard, we investigate local manipulation by exposing edge states to (i) a ferromagnetic, (ii) an antiferromagnetic exchange field, (iii) a parallel and an antiparallel electric field. These local manipulations at the edges with intrinsic spin-orbit interaction could produce a spin-polarized photocurrent and manipulate optical properties. Even at one edge of the nanoribbon, applying the exchange field could induce a fully spin polarization by light irradiation. Efficient quantum efficiency, significant optical spin polarization, and spin-polarized photocurrent make zigzag MoS2 nanoribbon an effective structure for improving the performance of spin-photovoltaic devices.

Chemical Reaction Rates for Systems with Spin-Orbit Coupling and an Odd Number of Electrons: Does Berry's Phase Lead to Meaningful Spin-Dependent Nuclear Dynamics for a Two State Crossing?

Within the context of a simple avoided crossing, we investigate the effect of a complex-valued diabatic coupling in determining spin-dependent rate constants and scattering states. We find that, if the molecular geometry is not linear and the Berry force is not zero, one can find significant spin polarization of the products. This study emphasizes that, when analyzing nonadiabatic reactions with spin orbit coupling (and a complex-valued Hamiltonian), one must consider how Berry force affects nuclear motion-at least in the context of gas phase reactions. Work is currently ongoing as far as extrapolating these conclusions to the condensed phase, where interesting spin selection has been observed in recent years.

Probing the structural and electronic properties of anionic europium-doped silicon clusters by density functional theory and comparison of experimental photoelectron spectroscopy

Density functional theory (DFT) calculations combined with “stochastic kicking” (SK) global search technique were performed to investigate the structural properties of EuSin-(1≤n≤13). It was found that the Eu atom in EuSin- preferred to occupy the surface position up to a size of 11Si atoms. However, starting fromn = 12, the endohedral structures were found, as depicted by the Eu atom inserting into the center of Si frame. The global minimum structures for each size was determined by comparing with the experimental photoelectron spectroscopy (PES). Natural population analysis (NPA) coupled with spin density isosurfaces were utilized to further investigate the electronic and magnetic properties of EuSin-. The observed magnetic moments significantly concentrates among the Eu atom, ascribed to the Eu 4f-electrons remaining largely as before encapsulating the Sin clusters. Meanwhile, the representative caged cluster EuSi12- was subjected to density of states (DOS) and HOMO-LUMO analysis, and the results showed a significant spin polarization.