日本物理学会誌及びJournal of the Physical Society of Japanの月刊について

The isolation of graphene has triggered an avalanche of studies into the spin-dependent physical properties of this material, as well as graphene-based spintronic devices Here we review the experimental and theoretical state-of-art concerning spin injection and transport, defect-induced magnetic moments, spin-orbit coupling and spin relaxation in graphene Future research in graphene spintronics will need to address the development of applications such as spin transistors and spin logic devices, as well as exotic physical properties including topological states and proximity-induced phenomena in graphene and other 2D materials

Graphene Spintronics

We review the electronic properties of bilayer graphene, beginning with a description of the tight-binding model of bilayer graphene and the derivation of the effective Hamiltonian describing massive chiral quasiparticles in two parabolic bands at low energies. We take into account five tight-binding parameters of the Slonczewski–Weiss–McClure model of bulk graphite plus intra- and interlayer asymmetry between atomic sites which induce band gaps in the low-energy spectrum. The Hartree model of screening and band-gap opening due to interlayer asymmetry in the presence of external gates is presented. The tight-binding model is used to describe optical and transport properties including the integer quantum Hall effect, and we also discuss orbital magnetism, phonons and the influence of strain on electronic properties. We conclude with an overview of electronic interaction effects.

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The electronic properties of bilayer graphene.

This article reviews static and dynamic interfacial effects in magnetism, focusing on interfacially-driven magnetic effects and phenomena associated with spin-orbit coupling and intrinsic symmetry breaking at interfaces. It provides a historical background and literature survey, but focuses on recent progress, identifying the most exciting new scientific results and pointing to promising future research directions. It starts with an introduction and overview of how basic magnetic properties are affected by interfaces, then turns to a discussion of charge and spin transport through and near interfaces and how these can be used to control the properties of the magnetic layer. Important concepts include spin accumulation, spin currents, spin transfer torque, and spin pumping. An overview is provided to the current state of knowledge and existing review literature on interfacial effects such as exchange bias, exchange spring magnets, spin Hall effect, oxide heterostructures, and topological insulators. The article highlights recent discoveries of interface-induced magnetism and non-collinear spin textures, non-linear dynamics including spin torque transfer and magnetization reversal induced by interfaces, and interfacial effects in ultrafast magnetization processes.

https://link.aps.org/accepted/10.1103/RevModPhys.89.025006

Interface-induced phenomena in magnetism

A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented. Being semiconductors for one spin projection and metals for another, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMFs by the peculiarities of their electronic structure and chemical bonding is discussed. The effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Special attention is paid to the appearance of nonquasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-the-art electronic calculations for correlated d-systems are discussed, and results for specific HMFs (Heusler alloys, zinc-blende structure compounds, CrO2, and Fe3O4) are reviewed.

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Half-metallic ferromagnets: From band structure to many-body effects

We study the electronic structure of graphene with a single substitutional vacancy using a combination of the density-functional, tight-binding, and impurity Green's function approaches. Density functional studies are performed with the all-electron spin-polarized linear augmented plane wave (LAPW) method. The three $sp^2 \sigma$ dangling bonds adjacent to the vacancy introduce localized states (V$\sigma$) in the mid-gap region, which split due to the crystal field and a Jahn-Teller distortion, while the $p_z \pi$ states introduce a sharp resonance state (V$\pi$) in the band structure. For a planar structure, symmetry strictly forbids hybridization between the $\sigma$ and the $\pi$ states, so that these bands are clearly identifiable in the calculated band structure. As for the magnetic moment of the vacancy, the Hund's-rule coupling aligns the spins of the four localized V$\sigma_1 \uparrow \downarrow$, V$\sigma_2 \uparrow $, and the V$\pi \uparrow$ electrons resulting in a S=1 state, with a magnetic moment of $2 \mu_B$, which is reduced by about $0.3 \mu_B$ due to the anti-ferromagnetic spin-polarization of the $\pi$ band itinerant states in the vicinity of the vacancy. This results in the net magnetic moment of $1.7 \mu_B$. Using the Lippmann-Schwinger equation, we reproduce the well-known $\sim 1/r$ decay of the localized V$\pi$ wave function with distance and in addition find an interference term coming from the two Dirac points, previously unnoticed in the literature. The long-range nature of the V$\pi$ wave function is a unique feature of the graphene vacancy and we suggest that this may be one of the reasons for the widely varying relaxed structures and magnetic moments reported from the supercell band calculations in the literature.

Electronic structure of the substitutional vacancy in graphene: Density-functional and Green's function studies

Density-functional electronic structure studies of a prototype interface between a paramagnetic metal and an antiferromagnetic (AFM) insulator (${\mathrm{CaRuO}}_{3}/{\mathrm{CaMnO}}_{3}$) reveal the exponential leakage of the metallic electrons into the insulator side. The leaked electrons in turn control the magnetism at the interface via the ferromagnetic (FM) Anderson-Hasegawa double exchange, which competes with the AFM superexchange of the bulk ${\mathrm{CaMnO}}_{3}$. The competition produces a FM interfacial ${\mathrm{CaMnO}}_{3}$ layer (possibly canted); but beyond this layer, the electron penetration is insufficient to alter the bulk magnetism.

Electron leakage and double-exchange ferromagnetism at the interface between a metal and an antiferromagnetic insulator: CaRuO3/CaMnO3.

We acknowledge support of this work by the U.S.
Department of Energy through Grant No. DE-FG02-
00ER45818.

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Spin-polarized two-dimensional electron gas at oxide interfaces.

We have studied in detail the electronic structure and magnetism in M (Mn and Cr)-doped semiconducting half-Heusler compounds FeVSb, CoTiSb and NiTiSn (XYxM1−xZ) in a wide concentration range using the local-spin density functional method in the framework of the tight-binding linearized muffin tin orbital method (TB-LMTO) and supercell approach. Our calculations indicate that some of these compounds are not only ferromagnetic but also half-metallic and may be useful for spintronics applications. The electronic structure of the doped systems is analysed with the aid of a simple model where we have considered the interaction between the dopant transition metal (M) and the valence band X–Z hybrid. We have shown that the strong X-d–M-d interaction places the M-d states close to the Fermi level with the M-t2g states lying higher in energy in comparison to the M-eg states. Depending on the number of available d electrons, ferromagnetism is realized provided that the d manifold is partially occupied. The tendencies toward ferromagnetic (FM) or antiferromagnetic (AFM) behaviour are discussed within Anderson–Hasegawa models of super-exchange and double-exchange. In our calculations for Mn-doped NiTiSn, the strong preference for FM over AFM ordering suggests a possible high Curie temperature for these systems.

https://iopscience.iop.org/article/10.1088/0953-8984/17/33/008/pdf

Electronic structure and magnetism in doped semiconducting half-Heusler compounds

We show how epitaxially grown Cu–Rh and Cu–Ni heterostructures exploit the strain effect, due to the lattice mismatch, and the ligand effect, arising from the electronic interaction between the heterolayers, to achieve improved CO2 electroreduction. In this study we have performed density functional calculations on Cui/Mj/Cu(211) sandwiched surfaces and Cui/M(211) overlayers (where M = Rh or Ni, with varied i and j monolayers). We examined the free energy profiles of the reaction mechanisms for CO2 reduction to CO and CH4. We find that in Cu1/M1/Cu(211), in which the Cu monolayer experiences only a pure ligand effect, the influence of Ni is weaker than Rh and it decreases the overpotential for CO2 reduction by ∼10–20 mV. A larger decrease (33–64 mV) in the overpotential is predicted for other sandwiched surfaces: Cu1/Ni2/Cu(211), Cu2/Rh1/Cu(211), and Cu2/Rh2/Cu(211) in which the ligand effect is weaker. In the Cu1/M(211) overlayer, Cu is affected by both the strain and ligand effects, of which the latter ...

Birabar Nanda

Papers

Electronic structure of the substitutional vacancy in graphene: Density-functional and Green's function studies

Electron leakage and double-exchange ferromagnetism at the interface between a metal and an antiferromagnetic insulator: CaRuO3/CaMnO3.

Spin-polarized two-dimensional electron gas at oxide interfaces.

Electronic structure and magnetism in doped semiconducting half-Heusler compounds

Enhancing CO2 Electroreduction by Tailoring Strain and Ligand Effects in Bimetallic Copper–Rhodium and Copper–Nickel Heterostructures