Magnetic Surface States Research Articles

Analytical solutions for electromagnetic surface states at the interface between metal and gyromagnetic media

We systematically study the surface states at the interface between semi-infinite metal and gyromagnetic media, considering three orthogonal magnetization directions. By solving a full vectorial polarization-complete evanescent wave equation derived from Maxwell’s equations and all four boundary conditions, we obtain rigorous analytical solutions for the surface states. All analytical dispersion equations and field equations under the three configurations are solved. Simultaneously, the solutions also reveal the intriguing features of surface plasmon polaritons (SPPs) under the gyromagnetic effect. In the polar configuration (where the magnetization is perpendicular to the interface), the gyromagnetic effect uniquely results in candlelight-like field patterns in SPP. In the transversal configuration (where the magnetization is parallel to the interface and perpendicular to propagation), the unidirectional magnetic surface state is found. In the longitudinal configuration (where the magnetization is parallel to propagation), the SPP also exhibits candlelight-like field patterns but manifests a different polarization from the polar configuration. Our results reveal peculiar behaviors of SPPs under the gyromagnetic effect and would deepen the understanding of the magneto-optical effect on interfaces.

Spin magnetization of magnetic topological surface states in the presence of band asymmetry

In this work we investigate spin magnetization of surface electrons in magnetic topological insulators in the presence of electron–hole band asymmetry. The asymmetry is introduced by quadratic momentum term. The asymmetry in LL spectrum and unique zeroth LL behavior leads to unusual behavior of electron spin magnetization. Electronic magnetization behavior changes from positive to negative depending on zeroth Landau level being electron-like or hole-like. We show that the critical magnetic field at which discontinuity occurs depends both on intrinsic magnetic gap Δ0 and mass term. We show that plateau structure in magnetization depends on interplay of linear and quadratic momentum terms in electronic dispersion as chemical potential is tuned. It is also observed that the plateau structure in our system is unlike conventional oscillatory behavior in 2DEG or plateaus in particle hole symmetric surface states in the absence of mass term.

Planar magnetic texture on the surface of a topological insulator

We study the planar magnetic textures coupled to a two-dimensional Dirac surface state, where both the magnetic texture and surface state form in the magnetic topological insulator surface layer. It is shown that the radial vortex with winding number $w=\ifmmode\pm\else\textpm\fi{}1$ leads to the confinement of Dirac states, where a mapping to the Schr\"odinger equation of a two-dimensional hydrogen atom is found. The fully spin-polarized zero-energy bound state forms a flat band that resembles the zeroth Landau level of Dirac electrons in a uniform out-of-plane magnetic field. Remarkably, the number of the zero-energy state is topologically robust. Interestingly, when such a system is proximity coupled to an $s$-wave superconductor, the existence of Majorana zero modes at the Abrikosov vortex depends only on the relative value of the magnetic exchange coupling and the pairing strength. We conclude with a brief discussion on the physical realization with such magnetic textures.

Toward a systematic discovery of artificial functional magnetic materials

Although ferromagnets are found in all kinds of technological applications, only few substances are known to be intrinsically ferromagnetic at room temperature. In the past twenty years, a plethora of new artificial ferromagnetic materials have been found by introducing defects into non-magnetic host materials. In contrast to the intrinsic ferromagnetic materials, they offer an outstanding degree of material engineering freedom, provided one finds a type of defect to functionalize every possible host material to add magnetism to its intrinsic properties. Still, one controversial question remains: Are these materials really technologically relevant ferromagnets? To answer this question, in this work the emergence of a ferromagnetic phase upon ion irradiation is systematically investigated both theoretically and experimentally. Quantitative predictions are validated against experimental data from the literature of SiC hosts irradiated with high energy Ne ions and own experiments on low energy Ar ion irradiation of TiO$_2$ hosts. In the high energy regime, a bulk magnetic phase emerges, which is limited by host lattice amorphization, whereas at low ion energies an ultrathin magnetic layer forms at the surface and evolves into full magnetic percolation. Lowering the ion energy, the magnetic layer thickness reduces down to a bilayer, where a perpendicular magnetic anisotropy appears due to magnetic surface states.

Proximity-induced anisotropic magnetoresistance in magnetized topological insulators

Topological insulators (TIs) host spin-momentum locked surface states that are inherently susceptible to magnetic proximity modulations, making them promising for nano-electronic, spintronic, and quantum computing applications. While much effort has been devoted to studying (quantum) anomalous Hall effects in magnetic magnetically doped TIs, the inherent magnetoresistance (MR) properties in magnetic proximity-coupled surface states remain largely unexplored. Here, we directly exfoliate Bi2Se3 TI flakes onto a magnetic insulator, yttrium iron garnet, and measure the MR at various temperatures. We experimentally observe an anisotropic magnetoresistance that is consistent with a magnetized surface state. Our results indicate that the TI has magnetic anisotropy out of the sample plane, which opens an energy gap between the surface states. By applying a magnetic field along any in-plane orientation, the magnetization of the TI rotates toward the plane and the gap closes. Consequently, we observe a large (∼6.5%) MR signal that is attributed to an interplay between coherent rotation of magnetization within a topological insulator and abrupt switching of magnetization in the underlying magnetic insulator.

Spin Magnetization of Magnetic Topological Surface States in the Presence of Band Asymmetry

Spin Magnetization of Magnetic Topological Surface States in the Presence of Band Asymmetry

Proximity‐Induced Magnetism Enhancement Emerged in Chiral Magnet MnSi/Topological Insulator Bi2Se3 Bilayer

AbstractA proximity effect between magnetic materials and topological surface states can generate and modulate the localized spins without complicated material structures, but its origin is not clearly verified. MnSi single layer and MnSi/Bi2Se3 bilayer on Al2O3(001) substrates are fabricated by magnetron co‐sputtering and molecular beam epitaxy systems, in which a large proximity effect between the chiral magnetic structure and the topological surface states is manifested. The magnetic and electronic properties of both samples are meticulously compared and the proximity‐induced magnetism enhancement in the MnSi/Bi2Se3 bilayer is found. Interestingly, this effect persists up to temperatures above 300 K. Furthermore, for the MnSi/Bi2Se3 bilayer, the increase of charge carrier density and the decrease of carrier mobility near the Curie temperature TC = 40 K are observed, which can mediate the ferromagnetic exchange interaction enhancing the magnetization. The finding provides insight into a new platform to consist of materials with distinct topological phases for future spintronic devices.

Electronic properties of Bi2Se3 dopped by 3d transition metal (Mn, Fe, Co, or Ni) ions

Topological insulators are characterized by the existence of band inversion and the possibility of the realization of surface states. Doping with a magnetic atom, which is a source of the time-reversal symmetry breaking, can lead to realization of novel magneto-electronic properties of the system. In this paper, we study effects of substitution by the transition metal ions (Mn, Fe, Co and Ni) into Bi2Se3 on its electric properties. Using the ab inito supercell technique, we investigate the density of states and the projected band structure. Under such substitution the shift of the Fermi level is observed. We find the existence of nearly dispersionless bands around the Fermi level associated with substituted atoms, especially, in the case of the Co and Ni. Additionally, we discuss the modification of the electron localization function as well as charge and spin redistribution in the system. Our study shows a strong influence of the transition metal-Se bond on local modifications of the physical properties. The results are also discussed in the context of the interplay between energy levels of the magnetic impurities and topological surface states.

Hypothetical P63/mmc-Type CsCrCl3 Ferromagnet: Half-Metallic Property and Nodal Surface State

This work is focusing on the electronic, magnetic, half-metallic behaviors, and interesting surface state of hypothetical CsCrCl3 ferromagnet with P63/mmc space group that are investigated under the spin-polarized Generalized Gradient Approach (GGA) and spin-polarized GGA plus Hubbard U correction within the framework of the Density Functional Theory (DFT). The calculated total magnetic moment of P63/mmc CsCrCl3 is ~7.397 µB, and the mainly contribution to the total magnetism is coming from the Cr atoms. Based on obtained spin-polarized band structures, we confirmed that this hypothetical CsCrCl3 is a half-metal with 100% spin-polarization. Without spin-orbit coupling, i.e., the spin and the orbit degrees of freedom are dependents, we found that there should be a interesting nodal surface state at kz = plane due to P63/mmc type lattice structure enjoys a two-fold screw axis S2z. Our study can be seen as a strong evidence to exhibit that the protected nodal surface state can be achieved in magnetic materials.

Field-effect-driven half-metallic multilayer graphene

Rhombohedral stacked multilayer graphene displays the occurrence of a magnetic surface state at low temperatures. Recent angular resolved photoemission experiments demonstrate the robustness of the magnetic state in long sequences of ABC graphene. Here, by using first-principles calculations, we show that field-effect doping of these graphene multilayers induces a perfect half-metallic behaviour with 100% of spin current polarization already at dopings attainable in conventional field effect transistors with solid state dielectrics. Our work demonstrates the realisability of a new kind of spintronic devices where the transition between the low resistance and the high resistance state is driven only by electric fields.

Quantification of Magnetic Surface and Edge States in an FeGe Nanostripe by Off-Axis Electron Holography.

Whereas theoretical investigations have revealed the significant influence of magnetic surface and edge states on Skyrmonic spin texture in chiral magnets, experimental studies of such chiral states remain elusive. Here, we study chiral edge states in an FeGe nanostripe experimentally using off-axis electron holography. Our results reveal the magnetic-field-driven formation of chiral edge states and their penetration lengths at 95 and 240K. We determine values of saturation magnetization M_{S} by analyzing the projected in-plane magnetization distributions of helices and Skyrmions. Values of M_{S} inferred for Skyrmions are lower by a few percent than those for helices. We attribute this difference to the presence of chiral surface states, which are predicted theoretically in a three-dimensional Skyrmion model. Our experiments provide direct quantitative measurements of magnetic chiral boundary states and highlight the applicability of state-of-the-art electron holography for the study of complex spin textures in nanostructures.

Chemical Disorder in Topological Insulators: A Route to Magnetism Tolerant Topological Surface States.

We show that the chemical inhomogeneity in ternary three-dimensional topological insulators preserves the topological spin texture of their surface states against a net surface magnetization. The spin texture is that of a Dirac cone with helical spin structure in the reciprocal space, which gives rise to spin-polarized and dissipation-less charge currents. Thanks to the nontrivial topology of the bulk electronic structure, this spin texture is robust against most types of surface defects. However, magnetic perturbations break the time-reversal symmetry, enabling magnetic scattering and loss of spin coherence of the charge carriers. This intrinsic incompatibility precludes the design of magnetoelectronic devices based on the coupling between magnetic materials and topological surface states. We demonstrate that the magnetization coming from individual Co atoms deposited on the surface can disrupt the spin coherence of the carriers in the archetypal topological insulator Bi2Te3, while in Bi2Se2Te the spin texture remains unperturbed. This is concluded from the observation of elastic backscattering events in quasiparticle interference patterns obtained by scanning tunneling spectroscopy. The mechanism responsible for the protection is investigated by energy resolved spectroscopy and ab initio calculations, and it is ascribed to the distorted adsorption geometry of localized magnetic moments due to Se-Te disorder, which suppresses the Co hybridization with the surface states.

Chiral magnetic conductivity and surface states of Weyl semimetals in topological insulator ultra-thin film multilayer

We investigate an ultra-thin film of topological insulator (TI) multilayer as a model for a three-dimensional (3D) Weyl semimetal. We introduce tunneling parameters tS, , and tD, where the former two parameters couple layers of the same thin film at small and large momenta, and the latter parameter couples neighbouring thin film layers along the z-direction. The Chern number is computed in each topological phase of the system and we find that for , the tunneling parameter changes from positive to negative as the system transits from Weyl semi-metallic phase to insulating phases. We further study the chiral magnetic effect (CME) of the system in the presence of a time dependent magnetic field. We compute the low-temperature dependence of the chiral magnetic conductivity and show that it captures three distinct phases of the system separated by plateaus. Furthermore, we propose and study a 3D lattice model of Porphyrin thin film, an organic material known to support topological Frenkel exciton edge states. We show that this model exhibits a 3D Weyl semi-metallic phase and also supports a 2D Weyl semi-metallic phase. We further show that this model recovers that of 3D Weyl semimetal in topological insulator thin film multilayer. Thus, paving the way for simulating a 3D Weyl semimetal in topological insulator thin film multilayer. We obtain the surface states (Fermi arcs) in the 3D model and the chiral edge states in the 2D model and analyze their topological properties.

New spiral state and skyrmion lattice in 3D model of chiral magnets

We present the phase diagram of magnetic states for films of isotropic chiral magnets (ChMs) calculated as function of applied magnetic field and thickness of the film. We have found a novel magnetic state driven by the natural confinement of the crystal, localized at the surface and stacked on top of the conical bulk phase. This magnetic surface state has a three-dimensional (3D) chiral spin-texture described by the superposition of helical and cycloidal spin spirals. This surface state exists for a large range of applied magnetic fields and for any film thickness beyond a critical one. We also identified the whole thickness and field range for which the skyrmion lattice becomes the ground state of the system. Below a certain critical thickness the surface state and bulk conical phase are suppressed in favor of the skyrmion lattice. Unraveling of those phases and the construction of the phase diagram became possible using advanced computational techniques for direct energy minimization applied to a basic 3D model for ChMs. Presented results provide a comprehensive theoretical description for those effects already observed in experiments on thin films of ChMs, predict new effects important for applications and open perspectives for experimental studies of such systems.

Massive and massless Dirac fermions in Pb1-xSnxTe topological crystalline insulator probed by magneto-optical absorption.

Dirac fermions in condensed matter physics hold great promise for novel fundamental physics, quantum devices and data storage applications. IV-VI semiconductors, in the inverted regime, have been recently shown to exhibit massless topological surface Dirac fermions protected by crystalline symmetry, as well as massive bulk Dirac fermions. Under a strong magnetic field (B), both surface and bulk states are quantized into Landau levels that disperse as B1/2, and are thus difficult to distinguish. In this work, magneto-optical absorption is used to probe the Landau levels of high mobility Bi-doped Pb0.54Sn0.46Te topological crystalline insulator (111)-oriented films. The high mobility achieved in these thin film structures allows us to probe and distinguish the Landau levels of both surface and bulk Dirac fermions and extract valuable quantitative information about their physical properties. This work paves the way for future magnetooptical and electronic transport experiments aimed at manipulating the band topology of such materials.

Spontaneous gap generation on the surface of weakly interacting topological insulators using nonmagnetic impurities

Strong nonmagnetic impurities on the surface of three-dimensional topological insulators (TIs) generate localized resonance peaks close to the Dirac point. We show that this results in a strongly reduced critical Coulomb interaction strength to reach a magnetic surface state, following a Stoner-like criterion. Thus even weakly interacting TIs host a finite (local) magnetization around strong nonmagnetic impurities. The local magnetization gives rise to a global energy gap, linearly dependent on the maximum value of the magnetization but decreasing with reduced impurity concentration.

ChemInform Abstract: Magnetic Surface States in High Polarization Materials

AbstractReview: 110 refs.

Design of the Magnetic Separation Column Overflow Liquid Surface Monitoring System Based on Digital Image

The design has developed a the digital image processing system used to identify the magnetic separation column overflow liquid surface state, it identifies overflow liquid surface of normal situation and iron ore powder leak situation, and the system is partly implemented with visual C++ programming. The system consists of image processing and pattern recognition of two parts. The system can be calculated using magnetic separation column overflow the liquid surface texture parameters related, through these texture parameter and image recognition experiment set up some samples and, thereby, to identify parameter variation of the overflow the liquid surface provide reliable data basis.

Exchange bias behavior of monodisperse Fe3O4/γ-Fe2O3 core/shell nanoparticles

We have carried out systematic studies on well-characterized monodisperse Fe3O4/γ-Fe2O3 core/shell nanoparticles of 2–30 nm having a very narrow size distribution and possessing a uniquely mono-layer of surface γ-Fe2O3. This unique core-shell structure, probably having a disordered magnetic surface state, leads us to three key observations of unusual magnetic properties: i) a very large magnetic exchange anisotropy reaching over 7 × 106 erg/cm3 for the smaller particles, ii) exchange bias behavior in the magnetization data of the core/shell Fe3O4/γ-Fe2O3 nanoparticles, and iii) the temperature dependence of the coercive field following an unusual exponential behavior.

States near Dirac points of a rectangular graphene dot in a magnetic field

In neutral graphene dots, the Fermi level coincides with the Dirac points. We have investigated in the presence of a magnetic field several unusual properties of single electron states near the Fermi level of such a rectangular-shaped graphene dot with two zigzag and two armchair edges. We find that a quasidegenerate level forms near zero energy and the number of states in this level can be tuned by the magnetic field. The wave functions of states in this level are all peaked on the zigzag edges with or without some weight inside the dot. Some of these states are magnetic field-independent surface states while the others are field-dependent. We have found a scaling result from which the number of magnetic field-dependent states of large dots can be inferred from those of smaller dots.