Magnetic Topological Insulators Research Articles

Thickness-Driven Quantum Anomalous Hall Phase Transition in Magnetic Topological Insulator Thin Films.

The quantized version of the anomalous Hall effect realized in magnetic topological insulators (MTIs) has great potential for the development of topological quantum physics and low-power electronic/spintronic applications. Here we report the thickness-tailored quantum anomalous Hall (QAH) effect in Cr-doped (Bi,Sb)2Te3 thin films by tuning the system across the two-dimensional (2D) limit. In addition to the Chern number-related QAH phase transition, we also demonstrate that the induced hybridization gap plays an indispensable role in determining the ground magnetic state of the MTIs; namely, the spontaneous magnetization owing to considerable Van Vleck spin susceptibility guarantees the zero-field QAH state with unitary scaling law in thick samples, while the quantization of the Hall conductance can only be achieved with the assistance of external magnetic fields in ultrathin films. The modulation of topology and magnetism through structural engineering may provide useful guidance for the pursuit of other QAH-based phase diagrams and functionalities.

MnBi2Se4-Based Magnetic Modulated Heterostructures

Thin films of magnetic topological insulators (TIs) are expected to exhibit a quantized anomalous Hall effect when the magnetizations on the top and bottom surfaces are parallel and a quantized topological magnetoelectric effect when the magnetizations have opposite orientations. Progress in the observation of these quantum effects was achieved earlier in the films with modulated magnetic doping. On the other hand, the molecular-beam-epitaxy technique allowing the growth of stoichiometric magnetic van der Waals blocks in combination with blocks of topological insulator was developed. This approach should allow the construction of modulated heterostructures with the desired architecture. In the present paper, based on the first-principles calculations, we study the electronic structure of symmetric thin film heterostructures composed of magnetic MnBi2Se4 blocks (septuple layers, SLs) and blocks of Bi2Se3 TI (quintuple layers, QLs) in dependence on the depth of the magnetic SLs relative to the film surface and the TI spacer between them. Among considered heterostructures we have revealed those characterized by nontrivial band topology.

Layer-Number-Dependent Antiferromagnetic and Ferromagnetic Behavior in MnSb_{2}Te_{4}.

MnBi_{2}Te_{4}, an intrinsic magnetic topological insulator, has shown layer-number-correlated magnetic and topological phases. More interestingly, in the isostructural material MnSb_{2}Te_{4}, the antiferromagnetic (AFM) and ferromagnetic (FM) states have been both observed in the bulk counterparts, which are also predicted to be topologically nontrivial. Revealing the layer-number-dependent magnetic properties of MnSb_{2}Te_{4} down to a single septuple layer (SL) is of great significance for exploring the topological phenomena. However, this is still elusive. Here, using the polar reflective magnetic circular dichroism spectroscopy, both the A-type AFM and FM behaviors are observed and comprehensively studied in MnSb_{2}Te_{4} down to a single SL limit. In A-type AFM MnSb_{2}Te_{4} flakes, an obvious odd-even layer-number effect is observed. An additional surface spin-flop (SSF) transition occurs in even-SL flakes with the number of layers larger than 2. With the AFM linear-chain model, we identify that the even-SL flakes stabilize in a collinear state between the SSF transition and the spin-flop transition due to their appropriate energy ratio between the magnetic-field-scale anisotropy and interlayer interaction. In FM MnSb_{2}Te_{4} flakes, we observe very different magnetic behaviors with an abrupt spin-flipping transition and very small saturation fields, indicating a weakened interlayer interaction. By revealing the rich magnetic states of few-SL MnSb_{2}Te_{4} on the parameter space of the number of layers, external magnetic field, and temperature, our findings pave the way for further quantum transport studies of few-SL MnSb_{2}Te_{4}.

Occupancy disorder in the magnetic topological insulator candidate Mn1−x Sb2+x Te4

Abstract MnSb2Te4 is a candidate magnetic topological insulator exhibiting more pronounced cation intermixing than its predecessor MnBi2Te4. Investigating the cation intermixing and its possible implications on the magnetic order in MnSb2Te4 are currently hot topics in research on quantum materials for spintronics and energy-saving applications. Two single-crystal X-ray diffraction measurements of Mn1−x Sb2+x Te4 (x = 0.06 and x = −0.1) are presented alongside a detailed discussion of its crystal structure with a spotlight on the apparent occupancy disorder between the two cations. This disorder has been noted by other groups as well, yet never been analyzed in-depth with single-crystal X-ray diffraction. The latter is the tool of choice to receive a meaningful quantification of antisite disorder. Between the two synthesis procedures we find subtle differences in phases and/or alternation of the cation content which has implications on the magnetic order, as illustrated by bulk magnetometry. Understanding and assessing this disorder in magnetic topological insulators of the MnX2Te4 (X = Bi, Sb) type is crucial to gauge their applicability for modern spintronics. Furthermore, it opens new ways to tune the “chemical composition – physical property” relationship in these compounds, creating an alluring aspect also for fundamental science.

Novel π/2-Periodic Planar Hall Effect Due to Orbital Magnetic Moments in MnBi2Te4.

The Berry curvature and orbital magnetic moment (OMM) come from either inversion symmetry or time-reversal symmetry breaking in quantum materials. Here, we demonstrate the significance of OMMs and Berry curvature in planar Hall effect (PHE) in antiferromagnetic topological insulator MnBi2Te4 flakes. We observe a PHE with period of π and positive magnitude at low fields, resembling the PHE of the surface states in nonmagnetic topological insulators. Remarkably, a novel predominant PHE with period of π/2 and negative magnitude emerges below the Néel temperature with B > 10 T. Our theoretical calculations reveal that this unusual π/2-periodic PHE originates from the topological OMMs of bulk Dirac electrons. Moreover, the competition between the contributions from the bulk and the surface states leads to nontrivial evolutions of PHE and anisotropic magnetoresistance. Our results reveal intriguing electromagnetic response due to the OMMs and also provide insight into the potential applications of magnetic topological insulators in spintronics.

Pressure-densified new rhombohedral phase of EuSn2As2

The magnetic topological insulator ${\mathrm{EuSn}}_{2}{\mathrm{As}}_{2}$ is known to crystallize in a ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$-type layered structure in rhombohedral symmetry at ambient conditions and undergo a structural phase transition toward a monoclinic $\ensuremath{\beta}$ phase under increasing pressure. Here, we identify by ab initio calculations a new high-pressure rhombohedral structure (termed the $\ensuremath{\gamma}$ phase) in $R\overline{3}m$ symmetry comprising Sn-As-As-Sn linear chains along the $z$ axis via the formation of As-As bonds in the AsSn/SnAs bilayers with strong anisotropic lattice distortion along the [001] orientation. This pressure-densified new rhombohedral phase has a metallic A-type antiferromagnetic ground state and is energetically more favorable than the original rhombohedral structure and the monoclinic $\ensuremath{\beta}$ phase above 11 GPa. Simulated x-ray diffraction patterns provide an excellent match to the previously unexplained distinct diffraction peak around $14.{7}^{\ensuremath{\circ}}$ found in experiments for ${\mathrm{EuSn}}_{2}{\mathrm{As}}_{2}$ above 13 GPa. These findings offer key insights into a new type of high-pressure phase of layered magnetic topological insulators, laying a foundation for further exploration of this intriguing class of materials.

Epitaxial Growth and Structural Characterizations of MnBi2Te4 Thin Films in Nanoscale

The intrinsic magnetic topological insulator MnBi2Te4 has attracted much attention due to its special magnetic and topological properties. To date, most reports have focused on bulk or flake samples. For material integration and device applications, the epitaxial growth of MnBi2Te4 film in nanoscale is more important but challenging. Here, we report the growth of self-regulated MnBi2Te4 films by the molecular beam epitaxy. By tuning the substrate temperature to the optimal temperature for the growth surface, the stoichiometry of MnBi2Te4 becomes sensitive to the Mn/Bi flux ratio. Excessive and deficient Mn resulted in the formation of a MnTe and Bi2Te3 phase, respectively. The magnetic measurement of the 7 SL MnBi2Te4 film probed by the superconducting quantum interference device (SQUID) shows that the antiferromagnetic order occurring at the Néel temperature 22 K is accompanied by an anomalous magnetic hysteresis loop along the c-axis. The band structure measured by angle-resolved photoemission spectroscopy (ARPES) at 80 K reveals a Dirac-like surface state, which indicates that MnBi2Te4 has topological insulator properties in the paramagnetic phase. Our work demonstrates the key growth parameters for the design and optimization of the synthesis of nanoscale MnBi2Te4 films, which are of great significance for fundamental research and device applications involving antiferromagnetic topological insulators.

Superexchange dominates in magnetic topological insulators

It has been suggested that the enlarged spin susceptibility in topological insulators, described by Van Vleck's formalism, accounts for the ferromagnetism of bismuth-antimony topological chalcogenides doped with transition metal impurities. In contrast, earlier studies of HgTe and related topological systems pointed out that the interband analog of the Ruderman-Kittel-Kasuya-Yosida interaction (the Bloembergen-Rowland mechanism) leads to antiferromagnetic coupling between pairs of localized spins. Here, we critically revisit these two approaches, show their shortcomings, and elucidate why the magnitude of the interband contribution is small even in topological systems. From the proposed theoretical approach and our computational studies of magnetism in Mn-doped HgTe and CdTe, we conclude that, in the absence of band carriers, the superexchange dominates, and its sign depends on the coordination and charge state of magnetic impurities rather than on the topological class of the host material.

Charge carrier mediation and ferromagnetism induced in MnBi6Te10 magnetic topological insulators by antimony doping

A new kind of intrinsic magnetic topological insulator (MTI) MnBi2Te4 family has shed light on the observation of novel topological quantum effects such as the quantum anomalous Hall effect (QAHE). However, strong anti-ferromagnetic (AFM) coupling and high carrier concentration in the bulk hinder practical applications. In closely related materials MnBi4Te7 and MnBi6Te10, the interlayer magnetic coupling is greatly suppressed by Bi2Te3 layer intercalation. However, AFM is still the ground state in these compounds. Here, by magnetic and transport measurements, we demonstrate that a Sb substitutional dopant plays a dual role in MnBi6Te10, which can not only adjust the charge carrier type and concentration, but also induces the solid into a ferromagnetic (FM) ground state. The AFM ground state region, which is also close to the charge neutral point, can be found in the phase diagram of Mn(Sb x Bi1−x )6Te10 when x ∼ 0.25. An intrinsic FM-MTI candidate is thus demonstrated, which may take us a step closer to realizing a high-quality and high-temperature QAHE and related topological quantum effects in the future.

Coexistence of Quantum Hall and Quantum Anomalous Hall Phases in Disordered MnBi2Te4

In most cases, to observe quantized Hall plateaux, an external magnetic field is applied in intrinsic magnetic topological insulators $\mathrm{MnBi_2Te_4}$. Nevertheless, whether the nonzero Chern number ($C\neq 0$) phase is a quantum anomalous Hall (QAH) state, or a quantum Hall (QH) state, or a mixing state of both is still a puzzle, especially for the recently observed $C=2$ phase [Deng \textit{et al}., Science \textbf{367}, 895 (2020)]. In this Letter, we propose a physical picture based on the Anderson localization to understand the observed Hall plateaux in disordered $\mathrm{MnBi_2Te_4}$. Rather good consistency between the experimental and numerical results confirms that the bulk states are localized in the absence of a magnetic field and a QAH edge state emerges with $C=1$. However, under a strong magnetic field, the lowest Landau band formed with the localized bulk states, survives disorder, together with the QAH edge state, leading to a $C=2$ phase. Eventually, we present a phase diagram of a disordered $\mathrm{MnBi_2Te_4}$ which indicates more coexistence states of QAH and QH to be verified by future experiments.

Pressure-driven electronic and structural phase transition in intrinsic magnetic topological insulator MnSb2Te4

Intrinsic magnetic topological insulators provide an ideal platform to achieve various exciting physical phenomena. However, this kind of materials and related research are still very rare. In this work, we reported the electronic and structural phase transitions in the intrinsic magnetic topological insulator $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ driven by hydrostatic pressure. Electric transport results revealed that temperature dependent resistance showed a minimum value near short-range antiferromagnetic (AFM) ordering temperature ${{T}_{N}}^{\ensuremath{'}}$, which declines with pressure. The short-range AFM ordering was sensitive to pressure and fully suppressed above 11.5 GPa. The intensity of three Raman vibration modes in $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ declined quickly starting from 7.5 GPa and these modes become undetectable above 9 GPa, suggesting possible insulator-metal transition, which is further confirmed by theoretical calculation. In situ x-ray diffraction demonstrated that an extra diffraction peak appears near 9.1 GPa and $\mathrm{Mn}{\mathrm{Sb}}_{2}{\mathrm{Te}}_{4}$ started to enter an amorphouslike state above 16.6 GPa, suggesting the structural origin of suppressed AFM ordering and metallization. This work has demonstrated the correlation among interlayer interaction, magnetic ordering, and electric behavior, which could be benefit the understanding of the fundamental properties of this kind of materials and devices.

Magnetic-field tuning of the spin dynamics in the magnetic topological insulators(MnBi2Te4)(Bi2Te3)n

We report a high frequency/high magnetic field electron spin resonance (HF-ESR) spectroscopy study in the sub-THz frequency domain of the two representatives of the family of magnetic topological insulators (MnBi$_{2}$Te$_{4}$)(Bi$_{2}$Te$_{3}$)$_{n}$ with $n = 0$ and 1. The HF-ESR measurements in the magnetically ordered state at a low temperature of $T = 4$ K combined with the calculations of the resonance modes showed that the spin dynamics in MnBi$_{\text{4}}$Te$_{\text{7}}$ is typical for an anisotropic easy-axis type ferromagnet (FM) whereas MnBi$_{\text{2}}$Te$_{\text{4}}$ demonstrates excitations of an anisotropic easy-axis type antiferromagnet (AFM). However, by applying the field stronger than a threshold value $\sim 6$ T we observed in MnBi$_{\text{2}}$Te$_{\text{4}}$ a crossover from the AFM resonance modes to the FM modes which properties are very similar to the ferromagnetic response of MnBi$_{\text{4}}$Te$_{\text{7}}$. We attribute this remarkably unusual effect unexpected for a canonical easy-axis AFM, which, additionally, can be accurately reproduced by numerical calculations of the excitation modes, to the closeness of the strength of the AFM exchange and magnetic anisotropy energies which appears to be a very specific feature of this compound. Our experimental data evidences that the spin dynamics of the magnetic building blocks of these compounds, the Mn-based septuple layers (SLs), is inherently ferromagnetic featuring persisting short-range FM correlations far above the magnetic ordering temperature as soon as the SLs get decoupled either by introducing a nonmagnetic quintuple interlayer, as in MnBi$_{\text{4}}$Te$_{\text{7}}$, or by applying a moderate magnetic field, as in MnBi$_{\text{2}}$Te$_{\text{4}}$, which may have an effect on the surface topological band structure of these compounds.

High-temperature ideal Weyl semimetal phase and Chern insulator phase in ferromagnetic BaEuNiOsO6 and its (111) (BaEuNiOsO6)/(BaTiO3)10 superlattice

Weyl semimetals (WSMs) have recently stimulated intensive interest because they exhibit fascinating physical properties and also promise exciting technological applications. So far, however, the few confirmed magnetic WSMs generally have a large number of Weyl points either located away from the Fermi level $({E}_{F})$ or shrouded by nontopological Fermi surface pockets. Based on first-principles density functional theory calculations, we establish cubic double perovskite ${\mathrm{BaEuNiOsO}}_{6}$ to be a high Curie temperature $({T}_{c})$ ferromagnetic (FM) WSM with magnetization along the [111] direction, just two pairs of Weyl points at the ${E}_{F}$ and ${T}_{c}=325$ K. The strong FM is attributed to the strong FM Ni $3d$-Eu $4f$-Os $5d$ coupling induced by the substitution of half of Ba atoms with Eu atoms in double perovskite ${\mathrm{Ba}}_{2}{\mathrm{NiOsO}}_{6}$. Moreover, the momentum separation of one Weyl point pair is large, thus giving rise to not only a long (001) surface Fermi arc but also large anomalous Hall conductivity. Intriguingly, as a unique physical result of bulk ${\mathrm{BaEuNiOsO}}_{6}$ being a FM WSM, the (111) ${\mathrm{BaEuNiOsO}}_{6}$ monolayer (ML) superlattice $({\mathrm{BaEuNiOsO}}_{6})/{({\mathrm{BaTiO}}_{3})}_{10}$ is found to be a high-temperature (${T}_{c}=210$ K) Chern insulator with a large band gap of $\ensuremath{\sim}90$ meV. Therefore, cubic double perovskite ${\mathrm{BaEuNiOsO}}_{6}$ will provide a superior high-temperature material platform for exploring fundamental physics of Weyl fermions, and its (111) ML superlattices will offer a high-temperature magnetic topological insulator for studying exotic quantum phenomena such as the quantum anomalous Hall effect.

An integrated quantum material testbed with multi-resolution photoemission spectroscopy

We present the development of a multi-resolution photoemission spectroscopy (MRPES) setup, which probes quantum materials in energy, momentum, space, and time. This versatile setup integrates three light sources in one photoemission setup and can conveniently switch between traditional angle-resolved photoemission spectroscopy (ARPES), time-resolved ARPES (trARPES), and micrometer-scale spatially resolved ARPES. It provides a first-time all-in-one solution to achieve an energy resolution of <4 meV, a time resolution of <35 fs, and a spatial resolution of ∼10 μm in photoemission spectroscopy. Remarkably, we obtain the shortest time resolution among the trARPES setups using solid-state nonlinear crystals for frequency upconversion. Furthermore, this MRPES setup is integrated with a shadow-mask assisted molecular beam epitaxy system, which transforms the traditional photoemission spectroscopy into a quantum device characterization instrument. We demonstrate the functionalities of this novel quantum material testbed using FeSe/SrTiO3 thin films and MnBi4Te7 magnetic topological insulators.

Unusual dynamic susceptibility arising from soft ferromagnetic domains in MnBi8Te13 and Sb-doped MnBiTe (n = 2, 3)

MnBiTe (MBT) is the first intrinsic magnetic topological insulator and is promising to host emergent phenomena such as quantum anomalous Hall effect. They can be made ferromagnetic by having or with Sb doping. We studied the magnetic dynamics in a few selected ferromagnetic (FM) MBT compounds, including MnBi8Te13 and Sb doped MnBiTe ( using AC susceptibility and magneto-optical imaging. Slow relaxation behavior is observed in all three compounds, suggesting its universality among FM MBT. We attribute the origin of the relaxation behavior to the irreversible domain movements since they only appear below the saturation fields when ferromagnetic domains form. The very soft ferromagnetic domain nature is revealed by the low-field fine-structured domains and high-field sea-urchin-shaped remanent-state domains imaged via our magneto-optical measurements. Finally, we ascribe the rare ‘double-peak’ behavior observed in the AC susceptibility under small DC bias fields to the very soft ferromagnetic domain formations.

Evolution of Berry curvature and reentrant quantum anomalous Hall effect in an intrinsic magnetic topological insulator

Recently, the magnetic topological insulator MnBi$_2$Te$_4$ emerged as a competitive platform to realize quantum anomalous Hall (QAH) states. We report a Berry-curvature splitting mechanism to realize the QAH effect in the disordered magnetic TI multilayers when switching from an antiferromagnetic order to a ferromagnetic order. We reveal that the splitting of spin-resolved Berry curvature, originating from the separation of the mobility edge during the magnetic switching, can give rise to a QAH insulator even \emph{without} closing the band gap. We present a global phase diagram, and also provide a phenomenological picture to elucidate the Berry curvature splitting mechanism by the evolution of topological charges. At last, we predict that the Berry curvature splitting mechanism will lead to a reentrant QAH effect, which can be detected by tuning gate voltage. Our theory will be instructive for the studies of the QAH effect in MnBi$_2$Te$_4$ in future experiments.

Topological hybrid semimetal phases and anomalous Hall effects in a three dimensional magnetic topological insulator

In this work, we propose a ferromagnetic Bi2Se3 as a candidate to hold the coexistence of Weyl- and nodal-line semimetal phases, which breaks the time reversal symmetry. We demonstrate that the type-I Weyl semimetal phase, type-I-, type-II- and their hybrid nodal-line semimetal phases can arise by tuning the Zeeman exchange field strength and the Fermi velocity. Their topological responses under U(1) gauge field are also discussed. Our results raise a new way for realizing Weyl and nodal-line semimetals and will be helpful in understanding the topological transport phenomena in three-dimensional material systems.

Giant Topological Hall Effect in the Noncollinear Phase of Two-Dimensional Antiferromagnetic Topological Insulator MnBi4Te7.

Magnetic topological insulators provide an important platform for realizing several exotic quantum phenomena, such as the axion insulating state and the quantum anomalous Hall effect, owing to the interplay between topology and magnetism. MnBi4Te7 is a two-dimensional Z2 antiferromagnetic (AFM) topological insulator with a Néel temperature of ∼13 K. In AFM materials, the topological Hall effect (THE) is observed owing to the existence of nontrivial spin structures. A material with noncollinearity that develops in the AFM phase rather than at the onset of the AFM order is particularly important. In this study, we observed that such an unanticipated THE starts to develop in a MnBi4Te7 single crystal when the magnetic field is rotated away from the easy axis (c-axis) of the system. Furthermore, the THE resistivity reaches a giant value of ∼7 μΩ-cm at 2 K when the angle between the magnetic field and the c-axis is 75°. This value is significantly higher than the values for previously reported systems with noncoplanar structures. The THE can be ascribed to the noncoplanar spin structure resulting from the canted state during the spin-flip transition in the ground AFM state of MnBi4Te7. The large THE at a relatively low applied field makes the MnBi4Te7 system a potential candidate for spintronic applications.

Large Spin‐to‐Charge Conversion at Room Temperature in Extended Epitaxial Sb2Te3 Topological Insulator Chemically Grown on Silicon

AbstractSpin‐charge interconversion phenomena at the interface between magnetic materials and topological insulators (TIs) are attracting enormous interest in the research effort toward the development of fast and ultra‐low power devices for future information and communication technology. A large spin‐to‐charge (S2C) conversion efficiency in Au/Co/Au/Sb2Te3/Si(111) heterostructures based on Sb2Te3 TIs grown by metal–organic chemical vapor deposition on 4″ Si(111) substrates is reported. By conducting room temperature spin pumping ferromagnetic resonance, a 250% enhanced charge current due to spin pumping in the Sb2Te3‐containing system is measured when compared to the reference Au/Co/Au/Si(111). The corresponding inverse Edelstein effect length λIEE ranges from 0.28 to 0.61 nm, depending on the adopted methodological analysis, with the upper value being so far the largest observed for the second generation of 3D chalcogenide‐based TIs. These results open the path toward the use of chemical methods to produce TIs on large area Si substrates and characterized by highly performing S2C conversion, thus marking a milestone toward future technology‐transfer.

Regulation of Te atomic vacancy defects in the intrinsic magnetic topological insulator $$\hbox {MnBi}_{{6}}\hbox {Te}_{{10}}$$

Regulation of Te atomic vacancy defects in the intrinsic magnetic topological insulator $$\hbox {MnBi}_{{6}}\hbox {Te}_{{10}}$$