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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Picosecond Ultrasonics in Magnetic Topological Insulator MnBi 2 Te 4

MnBi 2 Te 4 is a magnetic topological insulator with layered A-type antiferromagnetic order. It exhibits a rich layer- and magnetic-state dependent topological phase diagram; however, much about the coupling between spin, charge, and lattice remains to be explored. Here, in this work, we report that MnBi 2 Te 4 is an excellent acoustic phonon cavity by realizing phonon frequency combs using picosecond ultrasonics. With the generated acoustic phonon wavepackets, we demonstrate that the timing and phase of acoustic echoes can be used to detect the presence of stacking faults between van der Waals layers buried deep within the crystal. Furthermore, by implementing this nondestructive ultrafast optical measurement in conjunction with time-resolved magneto-optical Kerr effect experiments, we uncover that out-of-plane vibrations in MnBi 2 Te 4 do not couple to the magnetic order, i.e. there is no appreciable magnetostriction. Our work points out how a well-developed technique can probe the structural defects and phonon pulse engineering in layered topological insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Exploring Quantized Axion Electrodynamics in Magnetic Topological Insulator Multilayer Heterostructures (Final Technical Report)

The current program is focusing on investigating the properties of the high Chern number quantum anomalous Hall (QAH) effect in magnetic topological insulator (TI) multilayer heterostructures and the exploration of the topological magnetoelectric effect in thick magnetic TI films/heterostructures. Our program includes both experimental and theoretical efforts on this topic.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Probing intrinsic magnetization dynamics of the Y 3⁢ Fe 5 ⁢O 12 /Bi 2 ⁢Te 3 interface at low temperature

Topological insulator–magnetic insulator (TI–MI) heterostructures hold significant promise in the field of spintronics, offering the potential for manipulating magnetization through topological surface state–enabled spin-orbit torque. However, many TI–MI interfaces are plagued by issues such as contamination within the magnetic insulator layer and the presence of a low-density transitional region of the topological insulator. These interfacial challenges often obscure the intrinsic behavior of the TI–MI system. In this study, we addressed these challenges by depositing sputtered Bi 2 ⁢Te 3 on liquid phase epitaxy grown Y 3 ⁢Fe 5 ⁢O 12 /Gd 3 ⁢Ga 5 ⁢O 12 . The liquid phase epitaxy grown Y 3 ⁢Fe 5 ⁢O 12 has been previously shown to have exceptional interface quality, without an extended transient layer derived from interdiffusion processes of the substrate or impurity ions, thereby eliminating rare-earth impurity-related losses in the MI at low temperatures. At the TI–MI interface, high-resolution depth-sensitive polarized neutron reflectometry confirmed the absence of a low-density transitional growth region of the TI. By overcoming these undesirable interfacial effects, we isolate and probe the intrinsic low-temperature magnetization dynamics and transport properties of the TI–MI interface. In conclusion, our findings revealed strong spin pumping at low temperatures, accompanied by an additional in-plane anisotropy. The enhanced spin pumping at low temperatures is correlated with the observed suppression of bulk conduction and the weak antilocalization in the TI film, highlighting the interplay between the transport and spin pumping behavior in the TI–MI system.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Antiferromagnetic order in the layered magnetic topological insulator MnBi 2 ⁢Se 4 probed by resonant soft x-ray scattering

The quasi-two-dimensional magnetic topological insulator MnBi 2 Se 4 , stabilized via nonequilibrium molecular beam epitaxy, is investigated by resonant soft x-ray scattering. Kiessig fringes are observed, confirming a high sample quality and a thin film thickness of 10 septuple layers (∼13 nm). An antiferromagnetic Bragg peak is observed at the structurally forbidden reflection, whose magnetic nature is validated by studying its temperature, energy, and polarization dependence. Through a detailed analysis, an A-type antiferromagetic order with in-plane moments is implied. This alternative spin structure in MnBi 2 Se 4 , in contrast to the Ising antiferromagnetic states in other magnetic topological insulators, might be relevant for hosting new topological states.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2025

Single-Layer Magnet Phase in Intrinsic Magnetic Topological Insulators, [MnTe][Bi 2 Te 3 ] n , Far beyond the Thermodynamic Limit

The intrinsic magnetic topological insulator (IMTI) family [MnTe][Bi 2 Te 3 ]n has demonstrated magneto-topological properties dependent on n, making it a promising platform for advanced electronics and spintronics. However, due to technical barriers in sample synthesis, their properties in the large n limit remain unknown. To overcome this, we utilized the atomic layer-by-layer molecular beam epitaxy (ALL-MBE) technique and achieved IMTIs with n as large as 15, far beyond that previously reported in bulk crystals or thin films. Then, we discover that the “single-layer magnet (SLM)” phase, primarily determined by intralayer ferromagnetic coupling, emerges for n > ∼4 and remains little affected up to n = 15. Nonetheless, still, nonzero, interlayer ferromagnetic coupling is necessary to stabilize the SLM phase, suggesting that the SLM phase eventually disappears in the n → ∞ limit. This study uncovers the secrets of IMTIs beyond the thermodynamic limit and opens a door to diverse magneto-topological applications.

2D ferromagnets↗
DOE OSTI2024

Nanoscale Control of Intrinsic Magnetic Topological Insulator MnBi 2 Te 4 Using Molecular Beam Epitaxy: Implications for Defect Control

Intrinsic magnetic topological insulators have emerged as a promising platform to study the interplay between the topological surface states and ferromagnetism. This unique interplay can give rise to a variety of exotic quantum phenomena, including the quantum anomalous Hall effect and axion insulating states. Here, in this study, utilizing molecular beam epitaxy (MBE), we present a comprehensive study of the growth of MnBi 2 Te 4 thin films on Si (111), epitaxial graphene, and highly ordered pyrolytic graphite substrates. By combining a suite of in situ characterization techniques, we obtain critical insights into the nanoscale control of MnBi 2 Te 4 epitaxial growth. First, we extract the free energy landscape for the epitaxial relationship as a function of the in-plane angular distribution. Then, by employing an optimized layer-by-layer growth, we determine the chemical potential and Dirac point of the thin film at different thicknesses and how this quantity is manifested by the dopant compensation from different antisite defects. Overall, these results establish a foundation for understanding the growth kinetics of MnBi 2 Te 4 and pave the way for future applications of MBE-grown thin films in emerging topological quantum materials.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Realizing gapped surface states in the magnetic topological insulator Mn Bi 2 - x Sb x Te 4

The interplay between magnetism and nontrivial topology in magnetic topological insulators (MTIs) is expected to give rise to exotic topological quantum phenomena like the quantum anomalous Hall effect and the topological axion states. A key to assessing these novel properties is to realize gapped topological surface sates. MnBi 2 Te 4 possesses nontrivial band topology with an intrinsic antiferromagnetic state. However, the highly electron-doped nature of the MnBi 2 Te 4 crystals obstructs the exhibition of the surface band gap. Here, we tailor the material through Sb substitution to reveal the gapped surface states in MnBi 2-x Sb x Te 4 . By shifting the Fermi level into the bulk band gap, we access the surface states and show a band gap of 50 meV at the Dirac point from quasiparticle interference measured by scanning tunneling microscopy (STM). Surface-dominant conduction is confirmed through transport spectroscopy measured by multiprobe STM below the Néel temperature. The surface band gap is robust against the out-of-plane magnetic field despite the promotion of field-induced ferromagnetism. Finally, the realization of bulk-insulating MTIs with the large exchange gap offers a promising platform for exploring emergent topological phenomena.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2021

Site Mixing for Engineering Magnetic Topological Insulators

The van der Waals compound, MnBi 2 Te 4 , is the first intrinsic magnetic topological insulator, providing a materials platform for exploring exotic quantum phenomena such as the axion insulator state and the quantum anomalous Hall effect. However, intrinsic structural imperfections lead to bulk conductivity, and the roles of magnetic defects are still unknown. With higher concentrations of the same types of magnetic defects, the isostructural compound MnSb 2 Te 4 is a better model system for a systematic investigation of the connections among magnetism, topology, and lattice defects. In this work, the impact of antisite defects on the magnetism and electronic structure is studied in MnSb 2 Te 4 . Mn-Sb site mixing leads to complex magnetic structures and tunes the interlayer magnetic coupling between antiferromagnetic and ferromagnetic. The detailed nonstoichiometry and site mixing of MnSb2Te4 crystals depend on the growth parameters, which can lead to ≈40% of Mn sites occupied by Sb and ≈15% of Sb sites by Mn in as-grown crystals. Single-crystal neutron diffraction and electron microscopy studies show nearly random distribution of the antisite defects. Band structure calculations suggest that the Mn-Sb site mixing favors a ferromagnetic interlayer coupling, consistent with experimental observation, but is detrimental to the band inversion required for a nontrivial topology. Overall, our results suggest a long-range magnetic order of Mn ions sitting on Bi sites in MnBi 2 Te 4 . The effects of site mixing should be considered in all layered heterostructures that consist of alternating magnetic and topological layers, including the entire family of MnTe(Bi 2 Te 3 )n, its Sb analogs, and their solid solution.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2024

Imaging the Breakdown and Restoration of Topological Protection in Magnetic Topological Insulator MnBi 2 Te 4

Quantum anomalous Hall (QAH) insulators transport charge without resistance along topologically protected chiral 1D edge states. Yet, in magnetic topological insulators to date, topological protection is far from robust, with zero-magnetic field QAH effect only realized at temperatures an order of magnitude below the Néel temperature T N , though small magnetic fields can stabilize QAH effect. Understanding why topological protection breaks down is therefore essential to realizing QAH effect at higher temperatures. Here a scanning tunneling microscope is used to directly map the size of exchange gap (E g,ex ) and its spatial fluctuation in the QAH insulator 5-layer MnBi 2 Te 4 . Long-range fluctuations of E g,ex are observed, with values ranging between 0 (gapless) and 70 meV, appearing to be uncorrelated to individual surface point defects. The breakdown of topological protection is directly imaged, showing that the gapless edge state, the hallmark signature of a QAH insulator, hybridizes with extended gapless regions in the bulk. Finally, it is unambiguously demonstrated that the gapless regions originate from magnetic disorder, by demonstrating that a small magnetic field restores E g,ex in these regions, explaining the recovery of topological protection in magnetic fields. The results indicate that overcoming magnetic disorder is the key to exploiting the unique properties of QAH insulators.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Twisted bilayers of thin film magnetic topological insulators

Twisted bilayer graphene (TBG) near "magic angles" has emerged as a rich platform for strongly correlated states of two-dimensional (2D) Dirac semimetals. Here we show that twisted bilayers of thin film magnetic topological insulators (MTIs) with large in-plane magnetization can realize flat bands near 2D Dirac nodes. Using a simple model for thin films of MTIs, we derive a continuum model for two such MTIs, twisted by a small angle with respect to each other. When the magnetization is in plane, we show that interlayer tunneling terms act as effective SU (2) vector potentials, which are known to lead to flat bands in TBG. We show that by changing the in-plane magnetization, it is possible to tune the twisted bilayer MTI band dispersion to quadratic band touching or to flat bands, similar to the TBG. If realized, this system can be a highly tunable platform for strongly correlated phases of two-dimensional Dirac semimetals.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Linear magneto-resistance in Bi{sub 2}SeTe{sub 2} topological insulator

Magnetic field and temperature dependent electronic transport measurements have been carried out on Bi{sub 2}SeTe{sub 2} topological insulator single crystals. The measurements reveal an insulating behavior and the carriers were found to be electrons (n-type) from Hall measurement. Magneto-resistance (MR) measurements in the field range (B) of 15 T to -15 T carried out at 4.2 K showed a cusp like weak anti-localization behavior for lower fields (-5 T 5 T. Upon increasing temperature, MR transforms to linear dependence of B at 40, 50 and 100 K. On further increasing temperatures (> 200 K), a parabolic MR is observed. Temperature dependent Hall data also showed a transition from a nonlinear to linear behavior upon increasing temperatures. Disorder induced changes in the electronic transport characteristics of bulk and surface electrons are believed to cause such changes in the magneto-transport behavior of this system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2017

Rearrangement of van der Waals stacking and formation of a singlet state at T = 90 K in a cluster magnet

Insulating Nb3Cl8 is a layered chloride consisting of two-dimensional triangular layers of Seff = 1/2 Nb3Cl13 clusters at room temperature. Magnetic susceptibility measurement show a sharp, hysteretic drop to a temperature independent value below T = 90 K. Specific heat measurements show that the transition is first order, with ΔS ≈ 5 J K-1 mol-1 f.u.-1, and a low temperature T-linear contribution originating from defect spins. Neutron and X-ray diffraction show a lowering of symmetry from trigonal P[3 with combining macron]m1 to monoclinic C2/m symmetry, with a change in layer stacking from –AB–AB– to –AB'–BC'–CA'– and no observed magnetic order. This lowering of symmetry and rearrangement of successive layers evades geometric magnetic frustration to form a singlet ground state. It is the lowest temperature at which a change in stacking sequence is known to occur in a van der Waals solid, occurs in the absence of orbital degeneracies, and suggests that designer 2-D heterostructures may be able to undergo similar phase transitions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2017

Tuning the antiferromagnetic helical pitch length and nanoscale domain size in Fe 3 PO 4 O 3 by magnetic dilution

The insulating magnetic material Fe 3 PO 4 O 3 features a noncentrosymmetric lattice composed of Fe 3 + triangular units. Frustration, due to competing near-neighbor (J 1 ) and next-nearest-neighbor (J 2 ) antiferromagnetic interactions, was recently suggested to be the origin of an antiferromagnetic helical ground state with unusual needlelike nanoscale magnetic domains in Fe 3 PO 4 O 3 . Magnetic dilution is shown here to tune the ratio of these magnetic interactions, thus providing deeper insight into this unconventional antiferromagnet. Dilution of the Fe 3+ lattice in Fe 3 PO 4 O 3 was accomplished by substituting nonmagnetic Ga 3+ to form the solid solution series Fe 3-x Ga x PO 4 O 3 with x=0.012, 0.06, 0.25, 0.5, 1.0, 1.5. Magnetic susceptibility and neutron powder diffraction data from this series are presented. A continuous decrease of both the helical pitch length and the domain size is observed with increasing dilution up to at least x=0.25, while for x≥0.5, the compounds lack long-range magnetic order entirely. The decrease in the helical pitch length with increasing x can be qualitatively understood by reduction of the ratio of J 2 /J 1 in the Heisenberg model, consistent with mean-field considerations. Intriguingly, the magnetic correlation length in the ab plane remains nearly equal to the pitch length for each value of x≤0.25, showing that the two quantities are intrinsically connected in this unusual antiferromagnet.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Plated lamination structures for integrated magnetic devices

Semiconductor integrated magnetic devices such as inductors, transformers, etc., having laminated magnetic-insulator stack structures are provided, wherein the laminated magnetic-insulator stack structures are formed using electroplating techniques. For example, an integrated laminated magnetic device includes a multilayer stack structure having alternating magnetic and insulating layers formed on a substrate, wherein each magnetic layer in the multilayer stack structure is separated from another magnetic layer in the multilayer stack structure by an insulating layer, and a local shorting structure to electrically connect each magnetic layer in the multilayer stack structure to an underlying magnetic layer in the multilayer stack structure to facilitate electroplating of the magnetic layers using an underlying conductive layer (magnetic or seed layer) in the stack as an electrical cathode/anode for each electroplated magnetic layer in the stack structure.

42 ENGINEERING↗
DOE OSTI2020

Topological properties of SnSe/EuS and SnTe/CaTe interfaces

Herein, we use density functional theory calculations to study the electronic structure of epitaxial (111) interfaces of the topological crystalline insulators SnSe and SnTe with the magnetic insulator EuS and the non-magnetic insulator CaTe, respectively. We further consider both interface slab models with a vacuum region and periodic heterostructures without vacuum. We find that gaps of 21 meV at the Γ point and 9 meV at the M point arise in the topological state at the SnSe/EuS interface, due to the magnetic proximity effect, which breaks the time reversal symmetry. The surface state at Γ is shifted below the Fermi level by 88 meV and the surface state at M is shifted above the Fermi level by 47 meV, owing to band bending at the interface. By comparison, the topological state at the interface of SnTe/CaTe is unperturbed by the presence of non-magnetic CaTe.

36 MATERIALS SCIENCE↗
DOE OSTI2024

Giant spin transport anisotropy in magnetic topological insulators

Here, we report on exceptionally long spin transport and giant spin lifetime anisotropy in the gapped surface states of three-dimensional (3D) magnetic topological insulators (MTIs). We examine the properties of these states using the three-dimensional Fu-Kane-Mele Hamiltonian model in the presence of a magnetic exchange field. The corresponding spin textures of surface states, which are well reproduced by an effective two-band model, hint at a considerable enhancement of the lifetime of out-of-plane spins compared to in-plane spins. This is confirmed by large-scale spin transport simulations for 3D MTIs with disorder. The energy dependence of the spin lifetime anisotropy arises directly from the nontrivial spin texture of the surface states, and is correlated with the onset of the quantum anomalous Hall phase. Our findings suggest different spin filtering capabilities of the gapped surface MTI states, which could be explored by nonlocal spin valve measurements.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2019

Unidirectional Magneto-Resistance in Modulation-Doped Magnetic Topological Insulators

Nonlinear unidirectional spin Hall magnetoresistance (USMR) has been reported in heavy metal/ferromagnet bilayers, which could be employed as an effective method in detecting the magnetization orientation in spintronic devices with 2-terminal geometry. Recently, another unidirectional magnetoresistance (UMR) was reported in magnetic topological insulator (TI)-based heterostructures at cryogenic temperature, whose amplitude is orders of magnitude larger than the USMR measured in heavy metal-based magnetic heterostructures at room temperature. Here, we report the UMR effect in the modulation-doped magnetic TI structures. This UMR arises due to the interplay between the magnetic dopant’s magnetization and the current-induced surface spin polarization, when they are parallel or antiparallel to each other in the TI material. By varying the dopant’s position in the structure, we reveal that the UMR is mainly originating from the interaction between the magnetization and the surface spin-polarized carriers (not bulk carriers). Furthermore, from the magnetic field-, the angular rotation-, and the temperature-dependence, we highlight the correlation between the UMR effect and the magnetism in the structures. The large UMR versus current ratio in TI-based magnetic bilayers promises the easy readout in TI-based spintronic devices with 2-terminal geometry.

42 ENGINEERING↗