[en] Symmetry protected edge states in 2D topological insulators are interesting both from the fundamental point of view as well as from the point of view of potential applications in nanoelectronics as perfectly conducting 1D channels and functional elements of circuits. Here using a simple tight-binding model and the Landauer-Büttiker formalism we explore local current distributions in a 2D topological insulator focusing on effects of non-magnetic impurities and vacancies as well as finite size effects. For an isolated edge state, we show that the local conductance decays into the bulk in an oscillatory fashion as explained by the complex band structure of the bulk topological insulator. We demonstrate that although the net conductance of the edge state is topologically protected, impurity scattering leads to intricate local current patterns. In the case of vacancies we observe vortex currents of certain chirality, originating from the scattering of current-carrying electrons into states localized at the edges of hollow regions. For finite size strips of a topological insulator we predict the formation of an oscillatory band gap in the spectrum of the edge states, the emergence of Friedel oscillations caused by an open channel for backscattering from an impurity and antiresonances in conductance when the Fermi energy matches the energy of the localized state created by an impurity. (paper)

[en] Spin–orbit coupling (SOC) links the spin degree of freedom to the orbital motion of electrons in a solid and plays an important role in the emergence of new physical phenomena. In non-centrosymmetric materials, the SOC locks the electron’s spin direction to its momentum resulting in non-trivial spin textures in the reciprocal space. Depending on the crystal symmetry, the spin texture may exhibit Rashba, Dresselhaus, persistent, or more intricate configurations. In ferroelectric materials these spin textures are coupled to the ferroelectric polarization and thus can be controlled by its orientation and magnitude. This provides a promising platform to explore the coupling between spin, orbital, valley, and lattice degrees of freedoms in solids and opens a new direction for nonvolatile spintronic devices, such as a spin-field-effect transistor and a valley spin valve. Here, we review the recent advances in spin-texture physics of ferroelectric materials and outline possible device implications. (topical review)

[en] Deterministic control of transport properties through manipulation of spin states is one of the paradigms of spintronics. Topological insulators offer a new playground for exploring interesting spin-dependent phenomena. Here, we consider a ferromagnetic ‘gate’ representing a magnetic adatom coupled to the topologically protected edge state of a two-dimensional (2D) topological insulator to modulate the electron transmission of the edge state. Due to the locked spin and wave vector of the transport electrons the transmission across the magnetic gate depends on the mutual orientation of the adatom magnetic moment and the current. If the Fermi energy matches an exchange-split bound state of the adatom, the electron transmission can be blocked due to the full back scattering of the incident wave. This antiresonance behavior is controlled by the adatom magnetic moment orientation so that the transmission of the edge state can be changed from 1 to 0. Expanding this consideration to a ferromagnetic gate representing a 1D chain of atoms shows a possibility to control the spin-dependent current of a strip of a 2D topological insulator by magnetization orientation of the ferromagnetic gate. (letter)

[en] Density-functional calculations are employed to investigate the effect of ferroelectric polarization of BaTiO₃ on the magnetocrystalline anisotropy of the Fe /BaTiO₃(001) interface. It is found that the interface magnetocrystalline anisotropy energy changes from 1.33 to 1.02 erg cm ^-2 when the ferroelectric polarization is reversed. This strong magnetoelectric coupling is explained in terms of the changing population of the Fe 3d orbitals at the Fe/BaTiO₃ interface driven by polarization reversal. Our results indicate that the electronically assisted magnetoelectric effects at the ferromagnetic/ferroelectric interfaces may be a viable alternative to the strain mediated coupling in related heterostructures and the electric field-induced effects on the interface magnetic anisotropy in ferromagnet/dielectric structures. (paper)

[en] Topological antiferromagnetic (AFM) spintronics is an emerging field of research, which involves the topological electronic states coupled to the AFM order parameter known as the Néel vector. The control of these states is envisioned through manipulation of the Néel vector by spin-orbit torques driven by electric currents. In this work, we present a different approach favorable for low-power AFM spintronics, where the control of the topological states in a two-dimensional material, such as graphene, is performed via the proximity effect by the voltage induced switching of the Néel vector in an adjacent magnetoelectric AFM insulator, such as chromia. Mediated by the symmetry protected boundary magnetization and the induced Rashba-type spin-orbit coupling at the interface between graphene and chromia, the emergent topological phases in graphene can be controlled by the Néel vector. Using density functional theory and tight-binding Hamiltonian approaches, we model a $\mathrm{graphene}/{\mathrm{Cr}}_2{\mathrm{O}}_3$ (0001) interface and demonstrate nontrivial band gap openings in the graphene Dirac bands asymmetric between the $K$ and $K^{\text{'}}$ valleys. This gives rise to an unconventional quantum anomalous Hall effect (QAHE) with a quantized value of $2e^2/h$ and an additional steplike feature at a value close to $e^2/2h$, and the emergence of the spin-polarized valley Hall effect (VHE). Additionally, depending on the Néel vector orientation, we predict the appearance and transformation of different topological phases in graphene across the 180° AFM domain wall, involving the QAHE, the valley-polarized QAHE, and the quantum VHE, and the emergence of the chiral edge states along the domain wall. These topological properties are controlled by voltage through magnetoelectric switching of the AFM insulator with no need for spin-orbit torques.

[en] The phenomenon of electron tunnelling has been known since the advent of quantum mechanics, but continues to enrich our understanding of many fields of physics, as well as creating sub-fields on its own. Spin-dependent tunnelling (SDT) in magnetic tunnel junctions (MTJs) has recently aroused enormous interest and has developed in a vigorous field of research. The large tunnelling magnetoresistance (TMR) observed in MTJs garnered much attention due to possible applications in non-volatile random-access memories and next-generation magnetic field sensors. This led to a number of fundamental questions regarding the phenomenon of SDT. In this review article we present an overview of this field of research. We discuss various factors that control the spin polarization and magnetoresistance in MTJs. Starting from early experiments on SDT and their interpretation, we consider thereafter recent experiments and models which highlight the role of the electronic structure of the ferromagnets, the insulating layer, and the ferromagnet/insulator interfaces. We also discuss the role of disorder in the barrier and in the ferromagnetic electrodes and their influence on TMR. (topical review)

[en] The surface termination and the nominal valence states for hexagonal LuFeO_3 thin films grown on Al_2O_3(0 0 0 1) substrates were characterized by angle resolved x-ray photoemission spectroscopy. The Lu 4f, Fe 2p and O 1s core level spectra indicate that both the surface termination and the nominal valence depend on surface preparation, but the stable surface terminates in a Fe–O layer. This is consistent with the results of density functional calculations which predict that the Fe–O termination of LuFeO_3(0 0 0 1) surface is energetically favorable and stable over a broad range of temperatures and oxygen partial pressures when it is reconstructed to eliminate surface polarity. (paper)

[en] We demonstrate that electronic and magnetic properties of graphene can be tuned via proximity of multiferroic substrate. Our first-principles calculations performed both with and without spin–orbit coupling clearly show that by contacting graphene with bismuth ferrite BiFeO₃ (BFO) film, the spin-dependent electronic structure of graphene is strongly impacted both by the magnetic order and by electric polarization in the underlying BFO. Based on extracted Hamiltonian parameters obtained from the graphene band structure, we propose a concept of six-resistance device based on exploring multiferroic proximity effect giving rise to significant proximity electro- (PER), magneto- (PMR), and multiferroic (PMER) resistance effects. This finding paves a way towards multiferroic control of magnetic properties in two dimensional materials. (paper)

[en] A high degree of spin polarization in electron transport is one of the most sought-after properties of a material which can be used in spintronics—an emerging technology utilizing a spin degree of freedom in electronic devices. An ideal candidate to exhibit highly spin-polarized current would be a room temperature half-metal, a material which behaves as an insulator for one spin channel and as a conductor for the other spin channel. In this paper, we explore a semi-Heusler compound, IrMnSb, which has been reported to exhibit pressure induced half-metallic transition. We confirm that the bulk IrMnSb is a spin-polarized metal, with dominant contribution to electronic states at the Fermi energy from majority-spin electrons. Application of a uniform pressure shifts the Fermi level into the minority-spin energy gap, thus demonstrating pressure induced half-metallic transition. This behavior is explained by the reduction of the exchange splitting of the spin bands consistent with the Stoner model for itinerant magnetism. We find that the half-metallic transition is suppressed when instead of uniform pressure the bulk IrMnSb is exposed to biaxial strain. This suppression of half-metallicity is driven by the epitaxial strain induced tetragonal distortion, which lifts the degeneracy of the Mn 3 d t_2g and e_g orbitals and reduces the minority-spin band gap under compressive strain, thus preventing half-metallic transition. Our calculations also indicate that in thin film geometry, surface states emerge in the minority-spin band gap, which has detrimental for practical applications impact on the spin polarization of IrMnSb. (paper)

[en] One of the exceptional features of the van der Waals (vdW) ferroelectrics is the existence of stable polarization at a level of atomically thin monolayers. This ability to withstand a detrimental effect of the depolarization fields gives rise to complex domain configurations characterized, among others, by the presence of layered "antipolar" head-to-head (H-H) or tail-to-tail (T-T) dipole arrangements. In this study, tomographic piezoresponse force microscopy (TPFM) is employed to study the 3D polarization arrangement in vdW ferroelectric α-In${}_2$Se${}_3$. Sequential removal of thin layers from the polar surface using the PFM tip reveals a complex 3D profile of the domain walls in the α-In${}_2$Se${}_3$ crystals. Antiparallel domain layers stacked along the polar direction are also observed by PFM imaging of the non-polar surfaces showing that H-H and T-T domain boundaries are commonly present in α-In${}_2$Se${}_3$. Application of TPFM to the electrically written domains allows evaluation of their geometrical lateral-to-vertical size aspect ratio, which shows a strong prevalence for the sidewise expansion in comparison to the forward growth. Local I-V measurements reveal a strong polarization direction dependence of conductivity due to the modulation of the energy barrier height as corroborated by theoretical modeling. (© 2024 The Author(s). Advanced Functional Materials published by Wiley‐VCH GmbH)