Tunable Spin-Polarized States in Graphene on a Ferrimagnetic Oxide Insulator

Spin-polarized two-dimensional (2D) materials with large and tunable spin-splitting energy promise the field of 2D spintronics. While graphene has been a canonical 2D material, its spin properties and tunability are limited. Here, this work demonstrates the emergence of robust spin-polarization in graphene with large and tunable spin-splitting energy of up to 132 meV at zero applied magnetic fields. The spin polarization is induced through a magnetic exchange interaction between graphene and the underlying ferrimagnetic oxide insulating layer, Tm3 Fe5 O12 , as confirmed by its X-ray magnetic circular dichroism (XMCD). The spin-splitting energies are directly measured and visualized by the shift in their Landau-fan diagram mapped by analyzing the measured Shubnikov-de-Haas (SdH) oscillations as a function of applied electric fields, showing consistent fit with the first-principles and machine learning calculations. Further, the observed spin-splitting energies can be tuned over a broad range between 98 and 166 meV by field cooling. The methods and results are applicable to other 2D (magnetic) materials and heterostructures, and offer great potential for developing next-generation spin logic and memory devices.

Optically Addressing Exciton Spin and Pseudospin in Nanomaterials for Spintronics Applications

Oriented exciton spins that can be generated and manipulated optically are of interest for a range of applications, including spintronics, quantum information science, and neuromorphic computing architectures. Although materials that host such excitons often lack practical coherence times for use on their own, strategic transduction of the magnetic information across interfaces can combine fast modulation with longer-term storage and readout. Several nanostructure systems have been put forward due to their interesting magneto-optical properties and their possible manipulation using circularly polarized light. These material systems are presented here, namely two-dimensional (2D) systems due to the unique spin-valley coupling properties and quantum dots for their exciton fine structure. 2D magnets are also discussed for their anisotropic spin behavior and extensive 2D magnetic states that are not yet fully understood but could pave the way for emergent techniques of magnetic control. This review also details the experimental and theoretical tools to measure and understand these systems along with a discussion on the progress of optical manipulation of spins and magnetic order transitions.

A Group-Theoretic Approach to the Origin of Chirality-Induced Spin-Selectivity in Nonmagnetic Molecular Junctions

Spin-orbit coupling gives rise to a range of spin-charge interconversion phenomena in nonmagnetic systems where certain spatial symmetries are reduced or absent. Chirality-induced spin-selectivity (CISS), a term that generically refers to a spin-dependent electron transfer in nonmagnetic chiral systems, is one such case, appearing in a variety of seemingly unrelated situations ranging from inorganic materials to molecular devices. In particular, the origin of CISS in molecular junctions is a matter of an intense current debate. Here, we derive a set of geometrical conditions for this effect to appear, hinting at the fundamental role of symmetries beyond otherwise relevant quantitative issues. Our approach, which draws on the use of point-group symmetries within the scattering formalism for transport, shows that electrode symmetries are as important as those of the molecule when it comes to the emergence of a spin-polarization and, by extension, to the possible appearance of CISS. It turns out that standalone metallic nanocontacts can exhibit spin-polarization when relative rotations which reduce the symmetry are introduced. As a corollary, molecular junctions with achiral molecules can also exhibit spin-polarization along the direction of transport, provided that the whole junction is chiral in a specific way. This formalism also allows the prediction of qualitative changes of the spin-polarization upon substitution of a chiral molecule in the junction with its enantiomeric partner. Quantum transport calculations based on density functional theory corroborate all of our predictions and provide further quantitative insight within the single-particle framework.

Spin-thermoelectric properties and giant tunneling magnetoresistance of boron-substituted graphene nanoribbon: a first principle study

In this study we have designed a spin caloritronic device based on boron doped armchair graphene nanoribbons (B2-7AGNR). In presence of ferromagnetic (FM) graphitic-carbon nitride (g-C₄N₃) electrodes the spin-thermoelectric features of the device, both for FM and antiferromagnetic (AFM) states, are studied using first principle calculations. The spin polarized transmission peaks and the presence of density of states near the Fermi level indicate that the system have large spin-thermoelectric figure of merit. In addition, it is observed that the system has a large tunneling magnetoresistance due to the difference in total current between FM and AFM configurations. Further studies reveal that the spin component of the Seebeck coefficient of the device is much higher than the other zigzag and armchair nanoribbons. When the spin magnetic moments of the electrodes are aligned in parallel manner, spin-thermoelectric figure of merit of the system becomes significantly high. It has also been found that on decreasing temperature the efficiency of the device increases. As a whole, the numerical results show thatg-C₄N₃-B2-7AGNR-g-C₄N₃system in FM configuration is an efficient low temperature thermoelectric device.

Stimulating and Manipulating Robust Circularly Polarized Photoluminescence in Achiral Hybrid Perovskites

Circularly polarized light (CPL) is essential for optoelectronic and chiro-spintronic applications. Hybrid perovskites, as star optoelectronic materials, have demonstrated CPL activity, which is, however, mostly limited to chiral perovskites. Here, we develop a simple, general, and efficient strategy to stimulate CPL activity in achiral perovskites, which possess rich species, efficient luminescence, and tunable bandgaps. With the formation of van der Waals heterojunctions between chiral and achiral perovskites, a nonequilibrium spin population and thus CPL activity are realized in achiral perovskites by receiving spin-polarized electrons from chiral perovskites. The polarization degree of room-temperature CPL in achiral perovskites is at least one order of magnitude higher than in chiral ones. The CPL polarization degree and emission wavelengths of achiral perovskites can be flexibly designed by tuning chemical compositions, operating temperature, or excitation wavelengths. We anticipate that unlimited types of achiral perovskites can be endowed with CPL activity, benefiting their applications in integrated CPL sources and detectors.

Atomic Intercalation Induced Spin-Flip Transition in Bilayer CrI3

The recent discovery of 2D magnets has induced various intriguing phenomena due to the modulated spin polarization by other degrees of freedoms such as phonons, interlayer stacking, and doping. The mechanism of the modulated spin-polarization, however, is not clear. In this work, we demonstrate theoretically and computationally that interlayer magnetic coupling of the CrI3 bilayer can be well controlled by intercalation and carrier doping. Interlayer atomic intercalation and carrier doping have been proven to induce an antiferromagnetic (AFM) to ferromagnetic (FM) phase transition in the spin-polarization of the CrI3 bilayer. Our results revealed that the AFM to FM transition induced by atom intercalation was a result of enhanced superexchange interaction between Cr atoms of neighboring layers. FM coupling induced by O intercalation mainly originates from the improved superexchange interaction mediated by Cr 3d-O 2p coupling. FM coupling induced by Li intercalation was found to be much stronger than that by O intercalation, which was attributed to the much stronger superexchange by electron doping than by hole doping. This comprehensive spin exchange mechanism was further confirmed by our results of the carrier doping effect on the interlayer magnetic coupling. Our work provides a deep understanding of the underlying spin exchange mechanism in 2D magnetic materials.

Study of structural, electronic and magnetic properties of Ti doped Co2FeGe Heusler alloy: Co2Fe1-xTixGe (x= 0, 0.5, and 0.75)

Tunability of structural, magnetic and electronic properties of Co₂FeGe Heusler alloy is experimentally demonstrated by doping Ti in the Fe site (i.e. Co₂Fe_1-xTi_xGe), followed by in-depth first principle calculations. Co₂FeGe in its pure phase shows very high saturation magnetization, Curie temperature and spin-wave stiffness constant which were reported in our earlier work. With gradual increase in Ti doping concentration (x= 0.5 and 0.75), the experimental saturation magnetization is found to be decreased to 4.3 μ_B/f.u. and 3.1 μ_B/f.u. respectively as compared to the parent alloy (x= 0) having the saturation magnetization of 6.1 μ_B/f.u. Variation of spinwave stiffness constant is also studied for differentxand found to be decreasing from peak value of 10.4 nm² meV (forx= 0) to the least value of 2.56 nm² meV forx= 0.5. Justification of the experimental results is given with first principle calculations. Computational phase diagram of the alloys is found in terms of formation energy showing that the doping in Fe site (i.e. Co₂Fe_1-xTi_xGe) is more stable rather than in Co site (i.e. Co_2-xFeTi_xGe). The change in magnetic moment and half-metallicity with Ti doping concentration is better explained under GGA +Uapproach as compared to GGA approach signifying that the electron-electron correlation (U) has a distinct role to play in the alloys. Effect of variation ofUfor Ti atom is studied and optimized with reference to the experimental results. The dynamical stability of the Co₂Fe_1-xTi_xGe alloy crystal structure is explained in terms of phonon dispersion relations and the effect ofUon the phonon density of states is also explored. Close agreement between the experimental and theoretical results is observed.

Na⋯B bond in NaBH 3 - : An induced spin-polarized bond

The nature of the Na⋯B bond, in the recently synthesized NaBH 3 - adduct, is analyzed on the light of the Na^- propensity to polarize along the bond axis as a consequence of the electric field produced by the BH₃ fragment. The observed induced polarization has two consequences: (i) the energetic stabilization of the Na^- , and (ii) the split of its valence electrons into two opposite lobes along the bond axis. Additionally, an analysis of the electron localization is presented using the information content of the correlated conditional pair density that reveals a significant delocalization between one lobe of the polarized Na^- anion and the BH₃ fragment at the equilibrium distance. Our findings reported here complement previous works on this system.

A New Spin-Correlated Plasmon in Novel Highly Oriented Single-Crystalline Gold Quantum Dots

A concept of spin plasmon, a collective mode of spin-density, in strongly correlated electron systems has been proposed since the 1930s. It is expected to bridge between spintronics and plasmonics by strongly confining the photon energy in the subwavelength scale within single magnetic-domain to enable further miniaturizing devices. However, spin plasmon in strongly correlated electron systems is yet to be realized. Herein, we present a new spin correlated-plasmon at room temperature in novel Mott-like insulating highly oriented single-crystalline gold quantum-dots (HOSG-QDs). Interestingly, the spin correlated-plasmon is tunable from the infrared to visible, accompanied by spectral weight transfer yielding a large quantum absorption midgap state, disappearance of low-energy Drude response, and transparency. Supported with theoretical calculations, it occurs due to an interplay of surprisingly strong electron-electron correlations, s-p hybridization and quantum confinement in the s band. The first demonstration of the high sensitivity of spin correlated-plasmon in surface-enhanced Raman spectroscopy is also presented.

Charge Redistribution and Spin Polarization Driven by Correlation Induced Electron Exchange in Chiral Molecules

Chiral induced spin selectivity is a phenomenon that has been attributed to chirality, spin-orbit interactions, and nonequilibrium conditions, while the role of electron exchange and correlations have been investigated only marginally until very recently. However, as recent experiments show that chiral molecules acquire a finite spin-polarization merely by being in contact with a metallic surface, these results suggest that electron correlations play a more crucial role for the emergence of the phenomenon than previously thought. Here, it is demonstrated that molecular vibrations give rise to molecular charge redistribution and accompany spin-polarization when coupling a chiral molecule to a nonmagnetic metal. The presented theory opens up new routes to construct a comprehensive picture of enantiomer separation.

Computational Insights Into the Electronic Structure and Magnetic Properties of Rhombohedral Type Half-Metal GdMnO3 With Multiple Dirac-Like Band Crossings

In spintronics, half-metallic materials (HMMs) with Dirac-like cones exhibit interesting physical properties such as massless Dirac fermions and full spin polarization. We combined first-principles calculations with the quasi-harmonic Debye model, and we proposed that the rhombohedral GdMnO₃ is an HMM with multiple linear band crossings. The physical properties of GdMnO₃ were studied thoroughly. Moreover, the changes of multiple linear band crossings and 100% spin polarization under spin-orbit coupling as well as the electron and hole doping were also investigated. It is noted that such spin-polarized HMMs with linear band crossings are still very rare in two-dimensional and three-dimensional materials.

Two Dimensional Antiferromagnetic Chern Insulator: NiRuCl6

Density functional theory (DFT) and Berry curvature calculations show that quantum anomalous Hall effect (QAHE) can be realized in two-dimensional(2D) antiferromagnetic (AFM) NiRuCl₆. The results indicate that NiRuCl₆ behaves as an AFM Chern insulator and its spin-polarized electronic structure and strong spin-orbit coupling (SOC) are responsible for the QAHE. By tuning SOC, we found that the topological property of NiRuCl₆ arises from its energy band inversion. Considering the compatibility between the AFM and insulators, AFM Chern insulator provides a new way to archive high temperature QAHE in experiments due to its different magnetic coupling mechanism from that of ferromagnetic (FM) Chern insulator.

Picosecond electron bunches from GaAs/GaAsP strained superlattice photocathode

GaAs/GaAsP strained superlattices are excellent candidates for use as spin-polarized electron sources. In the present study, picosecond electron bunches were successfully generated from such a superlattice photocathode. However, electron transport in the superlattice was much slower than in bulk GaAs. Transmission electron microscopy observations revealed that a small amount of variations in the uniformity of the layers was present in the superlattice. These variations lead to fluctuations in the superlattice mini-band structure and can affect electron transport. Thus, it is expected that if the periodicity of the superlattice can be improved, much faster electron bunches can be produced.