A van der Waals antiferromagnetic topological insulator with weak interlayer magnetic coupling

AbstractMagnetic topological insulators (TI) provide an important material platform to explore quantum phenomena such as quantized anomalous Hall effect and Majorana modes, etc. Their successful material realization is thus essential for our fundamental understanding and potential technical revolutions. By realizing a bulk van der Waals material MnBi4Te7 with alternating septuple [MnBi2Te4] and quintuple [Bi2Te3] layers, we show that it is ferromagnetic in plane but antiferromagnetic along the c axis with an out-of-plane saturation field of ~0.22 T at 2 K. Our angle-resolved photoemission spectroscopy measurements and first-principles calculations further demonstrate that MnBi4Te7 is a Z2 antiferromagnetic TI with two types of surface states associated with the [MnBi2Te4] or [Bi2Te3] termination, respectively. Additionally, its superlattice nature may make various heterostructures of [MnBi2Te4] and [Bi2Te3] layers possible by exfoliation. Therefore, the low saturation field and the superlattice nature of MnBi4Te7 make it an ideal system to investigate rich emergent phenomena.

Download Full-text

Natural van der Waals heterostructural single crystals with both magnetic and topological properties

Science Advances ◽

10.1126/sciadv.aax9989 ◽

2019 ◽

Vol 5 (11) ◽

pp. eaax9989 ◽

Cited By ~ 43

Author(s):

Jiazhen Wu ◽

Fucai Liu ◽

Masato Sasase ◽

Koichiro Ienaga ◽

Yukiko Obata ◽

...

Keyword(s):

Magnetic Properties ◽

Hall Effect ◽

Exchange Coupling ◽

Van Der Waals ◽

Surface States ◽

Anomalous Hall Effect ◽

Topological Quantum ◽

Magnetic Layers ◽

Topological Surface ◽

Ferromagnetic Hysteresis

Heterostructures having both magnetism and topology are promising materials for the realization of exotic topological quantum states while challenging in synthesis and engineering. Here, we report natural magnetic van der Waals heterostructures of (MnBi2Te4)m(Bi2Te3)n that exhibit controllable magnetic properties while maintaining their topological surface states. The interlayer antiferromagnetic exchange coupling is gradually weakened as the separation of magnetic layers increases, and an anomalous Hall effect that is well coupled with magnetization and shows ferromagnetic hysteresis was observed below 5 K. The obtained homogeneous heterostructure with atomically sharp interface and intrinsic magnetic properties will be an ideal platform for studying the quantum anomalous Hall effect, axion insulator states, and the topological magnetoelectric effect.

Download Full-text

The positive exchange bias property with hopping switching behavior in van der Waals magnet FeGeTe

2D Materials ◽

10.1088/2053-1583/ac456f ◽

2021 ◽

Author(s):

Shaojie Hu ◽

Xiaomin Cui ◽

Zengji Yue ◽

Pangpang Wang ◽

Lei Guo ◽

...

Keyword(s):

Exchange Bias ◽

Van Der Waals ◽

Magnetic Coupling ◽

Anomalous Hall Effect ◽

Switching Behavior ◽

Bias Effect ◽

Magnetic Exchange ◽

Exchange Bias Effect ◽

Coupling Energy ◽

Surface Magnetization

Abstract The magnetic exchange bias effect is one of the representative interlayer magnetic coupling phenomena and is widely utilized in numerous technological applications. However, its mechanism is still elusive even in a simple magnetic bilayered system because of the complex interface magnetic orders. Van der Waals layered magnetic materials may provide an essential platform for deeply understanding the detailed mechanism of the exchange bias owing to its ideal interface structure. Here we first observed the positive exchange-biased anomalous Hall effect (AHE) with a hopping switching behavior in the FeGeTe Van der Waals nano-flakes. After systemically studying the cooling field dependence properties of the exchange bias effect, we propose that the coexistence of stable and frustrated surface magnetization of the antiferromagnetic phase will modify the total interface coupling energy density between the ferromagnetic (FM) and antiferromagnetic (AFM) phases. This model could provide a consistent description for such unusual exchange bias effect based on microspin simulation.

Download Full-text

The Coexistence of Superconductivity and Topological Order in the Bi2Se3 Thin Films

Science ◽

10.1126/science.1216466 ◽

2012 ◽

Vol 336 (6077) ◽

pp. 52-55 ◽

Cited By ~ 302

Author(s):

Mei-Xiao Wang ◽

Canhua Liu ◽

Jin-Peng Xu ◽

Fang Yang ◽

Lin Miao ◽

...

Keyword(s):

Thin Films ◽

Three Dimensional ◽

Surface States ◽

Scanning Tunneling ◽

Majorana Fermions ◽

Topological Order ◽

Tunneling Microscopy ◽

Angle Resolved Photoemission Spectroscopy ◽

Topological Surface ◽

Quantum Phenomena

Three-dimensional topological insulators (TIs) are characterized by their nontrivial surface states, in which electrons have their spin locked at a right angle to their momentum under the protection of time-reversal symmetry. The topologically ordered phase in TIs does not break any symmetry. The interplay between topological order and symmetry breaking, such as that observed in superconductivity, can lead to new quantum phenomena and devices. We fabricated a superconducting TI/superconductor heterostructure by growing dibismuth triselenide (Bi2Se3) thin films on superconductor niobium diselenide substrate. Using scanning tunneling microscopy and angle-resolved photoemission spectroscopy, we observed the superconducting gap at the Bi2Se3 surface in the regime of Bi2Se3 film thickness where topological surface states form. This observation lays the groundwork for experimentally realizing Majorana fermions in condensed matter physics.

Download Full-text

Realization of an intrinsic ferromagnetic topological state in MnBi8Te13

Science Advances ◽

10.1126/sciadv.aba4275 ◽

2020 ◽

Vol 6 (30) ◽

pp. eaba4275 ◽

Cited By ~ 2

Author(s):

Chaowei Hu ◽

Lei Ding ◽

Kyle N. Gordon ◽

Barun Ghosh ◽

Hung-Ju Tien ◽

...

Keyword(s):

Rational Design ◽

Anomalous Hall Effect ◽

Charge Carriers ◽

Photoemission Spectroscopy ◽

First Principles Calculations ◽

Angle Resolved Photoemission Spectroscopy ◽

Topological Materials ◽

Emergent Phenomena ◽

Ideal System ◽

Topological State

Novel magnetic topological materials pave the way for studying the interplay between band topology and magnetism. However, an intrinsically ferromagnetic topological material with only topological bands at the charge neutrality energy has so far remained elusive. Using rational design, we synthesized MnBi8Te13, a natural heterostructure with [MnBi2Te4] and [Bi2Te3] layers. Thermodynamic, transport, and neutron diffraction measurements show that despite the adjacent [MnBi2Te4] being 44.1 Å apart, MnBi8Te13 manifests long-range ferromagnetism below 10.5 K with strong coupling between magnetism and charge carriers. First-principles calculations and angle-resolved photoemission spectroscopy measurements reveal it is an axion insulator with sizable surface hybridization gaps. Our calculations further demonstrate the hybridization gap persists in the two-dimensional limit with a nontrivial Chern number. Therefore, as an intrinsic ferromagnetic axion insulator with clean low-energy band structures, MnBi8Te13 serves as an ideal system to investigate rich emergent phenomena, including the quantized anomalous Hall effect and quantized magnetoelectric effect.

Download Full-text

Detection of Magnetic Gap in Topological Surface States of MnBi2Te4

Chinese Physics Letters ◽

10.1088/0256-307x/38/10/107404 ◽

2021 ◽

Vol 38 (10) ◽

pp. 107404

Author(s):

Hao-Ran Ji ◽

Yan-Zhao Liu ◽

He Wang ◽

Jia-Wei Luo ◽

Jia-Heng Li ◽

...

Keyword(s):

Materials Science ◽

Theoretical Calculations ◽

Point Contact ◽

Surface States ◽

Anomalous Hall Effect ◽

Sample Surface ◽

Quantum Phases ◽

Out Of Plane ◽

Topological Surface ◽

Increasing Temperature

Recently, intrinsic antiferromagnetic topological insulator MnBi2Te4 has drawn intense research interest and leads to plenty of significant progress in physics and materials science by hosting quantum anomalous Hall effect, axion insulator state, and other quantum phases. An essential ingredient to realize these quantum states is the magnetic gap in the topological surface states induced by the out-of-plane ferromagnetism on the surface of MnBi2Te4. However, the experimental observations of the surface gap remain controversial. Here, we report the observation of the surface gap via the point contact tunneling spectroscopy. In agreement with theoretical calculations, the gap size is around 50 meV, which vanishes as the sample becomes paramagnetic with increasing temperature. The magnetoresistance hysteresis is detected through the point contact junction on the sample surface with an out-of-plane magnetic field, substantiating the surface ferromagnetism. Furthermore, the non-zero transport spin polarization coming from the ferromagnetism is determined by the point contact Andreev reflection spectroscopy. Combining these results, the magnetism-induced gap in topological surface states of MnBi2Te4 is revealed.

Download Full-text

Grain separation enhanced magnetic coercivity in Pt/Co multilayers.

Proceedings, annual meeting, Electron Microscopy Society of America ◽

10.1017/s0424820100151027 ◽

1993 ◽

Vol 51 ◽

pp. 1038-1039 ◽

Cited By ~ 1

Author(s):

G.A. Bertero ◽

R. Sinclair

Keyword(s):

Remanent Magnetization ◽

Strong Influence ◽

Uniaxial Anisotropy ◽

Magnetic Coupling ◽

Recording Media ◽

Magnetic Media ◽

Signal To Noise ◽

Out Of Plane ◽

Longitudinal Recording ◽

The Relationship

Pt/Co multilayers displaying perpendicular (out-of-plane) magnetic anisotropy and 100% perpendicular remanent magnetization are strong candidates as magnetic media for the next generation of magneto-optic recording devices. The magnetic coercivity, Hc, and uniaxial anisotropy energy, Ku, are two important materials parameters, among others, in the quest to achieving higher recording densities with acceptable signal to noise ratios (SNR). The relationship between Ku and Hc in these films is not a simple one since features such as grain boundaries, for example, can have a strong influence on Hc but affect Ku only in a secondary manner. In this regard grain boundary separation provides a way to minimize the grain-to-grain magnetic coupling which is known to result in larger coercivities and improved SNR as has been discussed extensively in the literature for conventional longitudinal recording media.We present here results from the deposition of two Pt/Co/Tb multilayers (A and B) which show significant differences in their coercive fields.

Download Full-text

First-Principles Study of a MoS2-PbS van der Waals Heterostructure Inspired by Naturally Occurring Merelaniite

Materials ◽

10.3390/ma14071649 ◽

2021 ◽

Vol 14 (7) ◽

pp. 1649

Author(s):

Gemechis D. Degaga ◽

Sumandeep Kaur ◽

Ravindra Pandey ◽

John A. Jaszczak

Keyword(s):

Density Functional ◽

Van Der Waals ◽

Hole Mobility ◽

Functional Theory ◽

Mos2 Monolayer ◽

Naturally Occurring ◽

Weak Interlayer ◽

Bulk Structure ◽

P Type ◽

Interlayer Bonding

Vertically stacked, layered van der Waals (vdW) heterostructures offer the possibility to design materials, within a range of chemistries and structures, to possess tailored properties. Inspired by the naturally occurring mineral merelaniite, this paper studies a vdW heterostructure composed of a MoS2 monolayer and a PbS bilayer, using density functional theory. A commensurate 2D heterostructure film and the corresponding 3D periodic bulk structure are compared. The results find such a heterostructure to be stable and possess p-type semiconducting characteristics. Due to the heterostructure’s weak interlayer bonding, its carrier mobility is essentially governed by the constituent layers; the hole mobility is governed by the PbS bilayer, whereas the electron mobility is governed by the MoS2 monolayer. Furthermore, we estimate the hole mobility to be relatively high (~106 cm2V−1s−1), which can be useful for ultra-fast devices at the nanoscale.

Download Full-text

Colossal Anomalous Hall Effect in Ferromagnetic van der Waals CrTe2

ACS Nano ◽

10.1021/acsnano.1c00488 ◽

2021 ◽

Author(s):

Meng Huang ◽

Shanshan Wang ◽

Zhaohao Wang ◽

Ping Liu ◽

Junxiang Xiang ◽

...

Keyword(s):

Hall Effect ◽

Van Der Waals ◽

Anomalous Hall Effect

Download Full-text

Intercorrelated ferroelectrics in 2D van der Waals materials

Materials Horizons ◽

10.1039/d1mh00446h ◽

2021 ◽

Author(s):

Yan Liang ◽

Shiying Shen ◽

Baibiao Huang ◽

Ying Dai ◽

Yandong Ma

Keyword(s):

Van Der Waals ◽

Out Of Plane ◽

Fundamental Phenomenon ◽

Van Der Waals Materials

2D intercorrelated ferroelectrics, exhibiting a coupled in-plane and out-of-plane ferroelectricity, is a fundamental phenomenon in the field of condensed-mater physics. The current research is based on the paradigm of bi-directional...

Download Full-text

Consecutive Vibrational Redistribution and Predissociation Processes in s-Tetrazine–Argon Van Der Waals Complexes

Laser Chemistry ◽

10.1155/lc.2.125 ◽

1983 ◽

Vol 2 (3-4) ◽

pp. 125-135 ◽

Cited By ~ 19

Author(s):

J. J. F. Ramaekers ◽

L. B. Krijnen ◽

H. J. Lips ◽

J. Langelaar ◽

R. P. H. Rettschnick

Keyword(s):

Decay Time ◽

Van Der Waals ◽

Stagnation Pressure ◽

Van Der Waals Complexes ◽

Supersonic Expansion ◽

Vibrational Predissociation ◽

Out Of Plane ◽

Spectrally Resolved ◽

Plane Mode

s-Tetrazine argon complexes T−Arn (n = 1, 2) are formed in a supersonic expansion of argon seeded with s-tetrazine. The expansion was conducted through a nozzle of 50 or 100 μm with an argon stagnation pressure between 1 and 1.5 bar. From spectrally resolved measurements it is clear that vibrational redistribution processes as well as vibrational predissociation processes take place after SVL excitation within the complex.From rise and decay time experiments it can be concluded, that after excitation of the 6a1 complex level, the above mentioned processes are consecutive and not parallel. It appears that the out of plane mode 16a couples with the Van der Waals stretching mode. The predissociation rate of the 16a2 complex is observed to be 2.3 × 109 s−1.

Download Full-text