Session N05: Magnetic Topological Thin Films and Heterostructures II

Unidirectional Spin Hall Magnetoresistance in Antiferromagnetic Heterostructures

Unidirectional spin Hall magnetoresistance (USMR) has been widely reported in heavy metal/ ferromagnet bilayer systems. We observe the USMR in Pt/α-Fe2O3 bilayers where the α-Fe2O3 is an antiferromagnetic (AFM) insulator. Systematic field and temperature-dependent measurements confirm the magnonic origin of the USMR. The appearance of AFM-USMR is driven by the imbalance of creation and annihilation of AFM magnons by spin-orbit torque due to the thermal random field. However, unlike its ferromagnetic counterpart, theoretical modeling reveals that the USMR in Pt/α-Fe2O3 is determined by the antiferromagnetic magnon number with a non-monotonic field dependence. Our findings extend the generality of the USMR which paves the way for the highly sensitive detection of AFM spin state.   

Measurements of Non-Local Resistances in the Quantum Anomalous Hall Regime

The quantum anomalous Hall effect (QAHE) is promising for revolutionizing the dissemination of quantum electrical standards. An improved understanding of the limitations of the QAHE are needed to implement such next generation standards. We report experimental results of conventional resistance and non-local measurements in the QAH regime of a Hall bar geometry device made from a high-quality 6 quintuple layer modulation-doped sample of the magnetic topological insulator, Cr-doped (BiSb)₂Te₃. At dilution refrigerator temperatures (≈10 mK) this sample is well-quantized to within a few ppm of h/e². We observe non-local resistances at temperatures above the well-quantized QAHE regime.  They are measured as a function of temperature from the QAHE regime to 100 K which is well above the Curie temperature. The behavior is also characterized as a function of gate bias and current amplitude. We will discuss possible interpretations for these striking results.   

Giant odd-parity magnetoresistance in a topological material / ferromagnet bilayer heterostructure

Magnetoresistance (MR) is usually an even function of magnetic field (B) as described by Onsager reciprocal relation. However, it was recently found that MR can be an odd function of B, called odd-parity MR (OMR), in some materials with broken time reversal symmetry (TRS). However, OMR magnitudes reported thus far were too small (< 1%) for realistic applications. In this study, we have discovered giant OMR as large as 2000% in a bilayer heterostructure consisting of topological Dirac semimetal α-Sn (~ 3 nm) / ferromagnetic semiconductor (In_1-x,Fe_x)Sb (x = 14%, T_C = 300 K) grown by molecular beam epitaxy. The OMR reaches maximum when B is applied parallel to the current, excluding the contribution of Hall effects as a possible origin. When rotating the direction of B, angular dependence of the OMR is well explained by a model that takes into account a tilt term in the Weyl Hamiltonian induced by the TRS breaking [1]. This model is also consistent with our other findings such as TRS breaking indicated by the hysteresis in magnetoresistance, presence of linear-dispersion band components by Shubnikov-de Haas oscillations, and band structure predicted by our first-principles calculation. Our result will be promising for realistic applications of OMR to unidirectional magnetic sensors.

[1] A. Kundu, et al., New J. Phys. 22, 083081 (2020).   

Topological phase transition induced by the magnetic proximity effect

The discovery of magnetic topological materials such as magnetic Weyl semimetals (MWSMs) represents an important milestone in spintronics. MWSMs attract significant attention, and there is a demand for experimental evidence of the Weyl phase. The experimental reports are either looking for naturally occurring MWSM candidates or breaking the symmetries of topological phases like a Dirac semimetal. Another route to engineer MWSMs by inducing band inversion through Zeeman splitting from trivial states was theoretically proposed [1], but there has been no experimental report. In this work, we show the evidence for topological phase transitions induced by the magnetic proximity effect. We investigate the topological band structure in bilayers consisting of 1 monolayer-FeAs/α-Sn thin film, using quantum transport measurements and first-principles calculations. The Shubnikov-de Haas oscillations show that there is a linear band with high mobility in our bilayers. Furthermore, first-principles calculation reveals that band inversion occurs in this structure, suggesting that the observed linear band is a surface state within this inversion gap. Our work paves a new way for the realization of magnetic topological materials.

[1] D. Kurebayashi et al., J. Phys. Soc. Jpn. 83, 063709 (2014).   

Giant Hall Switching by Surface-State-Mediated Spin-Orbit Torque in a Hard Ferromagnetic Topological Insulator

Topological insulators (TI) and magnetic topological insulators (MTI) can apply highly efficient spin-orbit torque (SOT) and manipulate the magnetization with their unique topological surface states with ultra-high efficiency. Here, we demonstrate efficient SOT switching of a hard MTI, V-doped (Bi,Sb)₂Te₃ (VBST) with a large coercive field that can prevent the influence of an external magnetic field. A giant switched anomalous Hall resistance of 9.2 kΩ is realized, among the largest of all SOT systems. The SOT switching current density can be reduced to 2.8×10⁵ A/cm². Moreover, as the Fermi level is moved away from the Dirac point by both gate and composition tuning, VBST exhibits a transition from edge-state-mediated to surface-state-mediated transport, thus enhancing the SOT effective field to 1.56±0.12 T/ (10⁶ A/cm²) and the spin Hall angle to 23.2±1.8 at 5 K. The findings establish VBST as an extraordinary candidate for energy-efficient magnetic memory devices.   

Interlayer Coupling Induced Plateau Phase Transition in Quantum Anomalous Hall Insulators

Recently, the high Chern number quantum anomalous Hall (QAH) effect has been realized in magnetic topological insulator(TI)/TI multilayers. In this work, we employed molecular beam epitaxy (MBE) to fabricate a series of magnetic TI/TI penta-layer heterostructures, specifically 3 quintuple layers (QL) (Bi, Sb)_1.73Cr_0.27Te₃/4QL (Bi, Sb)₂Te₃/d QL (Bi, Sb)_1.73Cr_0.27Te₃/4 QL (Bi, Sb)₂Te₃ /3 QL (Bi, Sb)_1.73Cr_0.27Te₃, by systematically altering the thickness d of the middle magnetic TI layer. By performing electrical transport measurements, we observed a zero magnetic field quantum phase transition between C = 1 and C = 2 QAH states.  For d ≤ 1, the sample exhibits the well-quantized C = 1 QAH state. For d ≥ 2, the sample exhibits the well-quantized C = 2 QAH state. However, for 1 ≤  d ≤ 2, we observed a monotonic reduction in Hall resistance from h/e² to h/2e², with a concomitant peak in the longitudinal resistance. The quantum phase transition driven by changes in the Chern number is attributed to the interlayer coupling between the upper and lower undoped TI layers, where two C= 1 QAH states emerge.   

Smallest to date ΔRK/RK uncertainty with zero-field Quantum Anomalous Hall Effect in a V-doped (BiSb)2Te3 device

The quantum anomalous Hall effect (QAHE), the quantized version of the anomalous Hall effect, is realized in magnetically-doped topological insulator (TI) materials that present a ferromagnetic ground state. The Hall conductivity acquires then quantized values that are integer multiples of the (von Klitzing) conductance quantum e²/h. The integer, in turn, is equal to the Chern number arising out of topological properties of the material band structure, and the QAHE is, hence a topologically protected state. Since its first realization in V and Cr-doped (BiSb)₂Te₃, this phenomenon has attracted enormous interest in the condensed matter physics, magnetism, and metrology communities, for it provides a route to tune desired quantum functionality paving the way to a primary electrical resistance standard, without the complications of a magnetic field applied with He-cooled superconducting coils. To this end, we focus on a V-doped (BiSb)₂Te₃ Hall-bar device, measured using a precision resistance bridge based on a state-of-the-art 14-bit cryogenic current comparator (CCC). We combine higher-resolution electronics, improved Hall bar contacts, and advanced film growth techniques to accomplish the QAHE in zero applied magnetic field with the lowest uncertainties in the ΔR_K/R_K = 10nΩ/Ω range. These measurements were carried out in the temperature T = 30-700 mK range, with relatively high electrical currents of 40-300 nA, which make this device an excellent addition to the quantum metrology toolbox.   

Magneto-thermopower of Cr-doped Sb2Te3 films

Thin films of topological insulators (TI) doped with 3d transition metals have received considerable research interests in recent years as the broken time reversal symmetry in these systems may lead to quantum anomalous Hall effect (QAHE) and magnetically ordered phases with novel spin textures.  The sesqui-chalcogenide Sb₂Te₃ is a well-known topological material. Doping of Sb₂Te₃ with elements such as Cr, Mn, Fe and Co has the potential to lead to low carrier density ordered magnetic phases of significant fundamental and technological interest. We deposit multilayers of the type [(n=1-5 nm)CrTe₂/(m=5 nm)Sb₂Te₃]_p using pulsed laser ablation on c-plane sapphire. X-ray diffraction and high-resolution electron microscopy studies revealed a single-phase Cr-doped Sb₂Te₃ for n= 1-2 nm.  These samples are ferromagnetic with out-of-plane magnetic anisotropy (PMA) as revealed by the measurements of anomalous Hall effect (AHE) and dc magnetization. We have also performed the measurements of thermopower and anomalous Nernst effect in these samples. The existence of PMA, AHE and Nernst effect in these Cr-doped low carrier density Sb₂Te₃ films throws some interesting questions related to the ordering of Cr moments in this TI, which will be discussed during the presentation.   

Topology stabilized finite-temperature ferromagnetism in twisted transition metal dichalcogenides

The role topology plays in forming and stabilizing ferromagnetism has been studied for decades. The most renowned example is the quantum Hall ferromagnetism, in which the nontrivial topology enhances the exchange interaction by forcing the electron wavefunctions to be more spread out. In this talk, I will discuss another crucial effect topology can play in stabilizing two-dimensional ferromagnetism at finite temperature. In two dimensions, the Mermin–Wagner theorem forbids any translation symmetry breaking at finite temperature, and therefore constrains the Curie temperature of most two-dimensional materials with (approximate) spin SU(2) symmetry. We show that the spin SU(2) symmetry is strongly broken in spin-orbit coupled systems with valley Chern bands, e.g. twisted transition metal dichalcogenides. In twisted homobilayer MoTe2 and WSe2, in particular, the Curie temperature can reach tens of Kelvin, as obtained from a numerical soft mode calculation based on the self-consistent Hartree-Fock ground state. The underlying mechanism can be understood as the winding energy cost of the inter-valley coherent state compared to the valley polarized state, which corresponds to the anisotropy between the in-plane ferromagnet and the out-of-plane ferromagnet. These results are consistent with the relatively high temperature quantum anomalous state observed in twisted MoTe2.   

Magnetism at Topological Insulator-Ferromagnetic Insulator Interfaces

Recent experiments have shown a remarkable enhancement of the Curie temperature Tc of ferromagnetic insulators (FMI) proximitized with topological insulators (TI); examples include EuS/Bi₂Se₃ [1] and monolayer Cr₂Te₃/(Bi,Sb)₂Te₃ [2]. Motivated by these observations, we develop a theory of how the Dirac surface states of the TI influence magnetism near the interface, building on earlier work done in the context of the quantum anomalous Hall effect. We show that the Dirac electrons mediated exchange between magnetic ions have two contributions: an oscillatory Ruderman-Kittel-Kasuya-Yosida (RKKY) piece and a non-oscillatory Bloembergen-Rowland (BR) interaction. We find that the ferromagnetic BR interaction dominates in the regime of low carrier concentration and leads to an effective easy-axis magnetic anisotropy. It thus stabilizes out-of-plane ferromagnetic order in the FMI layer with an enhanced Curie temperature. We show that our theory provides a detailed microscopic understanding of the experiments [2] on monolayer Cr₂Te₃ films proximitized with (Bi,Sb)₂Te₃.   

Phonon and defect mediated quantum anomalous Hall insulator to metal transition in magnetically doped topological insulators

Quantum Anomalous Hall (QAH) state in six quintuple layer Cr_0.1(Bi_0.2Sb_0.8)_1.9Te₃ thin films were studied through scanning tunneling spectroscopy (STS) and electrical transport measurements. While the surface state is gapless above the Curie temperature (T_C ≈ 30 K), scanning tunneling spectroscopy (STS) of the sample reveals a topologically non-trivial gap with an average value of ≈ 13.5 meV at 4.2 K below the ferromagnetic transition. Nonetheless, areal STS scans exhibit energy modulations on the order of several meV's in the surface bands which result in the valence band maximum in some regions becoming higher than the energy of the conduction band minimum of some other regions that are spatially separated by no more than 3 nm. First principle calculations demonstrate that the origin of the observed inhomogeneous energy band alignment is an outcome of many-body interactions, namely electron-defect interactions and electron-phonon interactions. Defects play the role of locally modifying the energy landscape of surface bands while electron-phonon interactions renormalize the surface bands such that the surface gap becomes reduced by more than 1 meV as temperature is raised from 0 to 4.2 K. These many-body interactions at a finite temperature result in substantial increase of electron tunneling across the spatially separated conduction band pockets even for finite temperatures well below T_C, thus compromising the chiral edge channels of the QAH insulator and transforming it into a metallic phase at a relatively low temperature.   

Magnetic Phase Transitions and transport properties of Heterostructures (MnBi2Te4)/(Bi2Te3)n using First Principles

The interplay between magnetism and topology led to the realization of quantum anomalous Hall effect in heterostructures, exhibiting the axion insulator MnBi₂Te₄ and well-known topological insulators like the Bi₂Te₃ family [1]. The current study aims to explore the magneto-transport properties like anomalous Hall conductivity of heterostructures such as Bi₂Te₃/MnBi₂Te_4, which is an extension of previous study on Bi₂Se₃/Bi₂MnSe₄ reporting non-trivial topological surface states [2]. MnBi2Te4 is found to exhibit antiferromagnetic ordering, substantiated by ground state energy eigenvalues analysis. The Hubbard U value of 1.1 eV [3] is included to commensurate the electron correlation effect for Mn - ‘d’ orbitals. The amount of non-magnetic Bi₂Te₃ layers sandwiched between two antiferromagnetic MnBi₂Te₄ layers suppresses the interlayer exchange coupling between the two antiferromagnetic layers. Such is beneficial for quantized anomalous Hall conductivity at low intrinsic/extrinsic magnetic fields [4]. This inspires the examination of magnetic properties of the heterostructure forms, MnBi₂Te₄/(Bi₂Te₃)_n/MnBi₂Te₄ ,with n = 1 to n = 4 to minimize the interlayer exchange coupling between two MnBi2Te4 layers, leading to exotic magneto-transport properties like anomalous Hall conductivity.

[1] Haiming Deng et al., High-temperature quantum anomalous Hall regime in a MnBi₂Te₄/Bi₂Te₃ superlattice, Nature Physics, 17 (2021) 36–42

[2] Joseph A Hagmann et al., Molecular beam epitaxy growth and structure of self-assembled Bi₂Se₃/Bi₂MnSe₄ multilayer heterostructures, New J. Phys. 19 (2017) 085002

[3] Aki Pulkkinen et al., Coulomb correlation in noncollinear anti-ferromagnetic α-Mn, Phys. Rev. B. 101 (2020) 075115.

[4] M. Z. Shi et al., Magnetic and transport properties in the magnetic topological insulators MnBi₂Te₄(Bi₂Te₃)n (n = 1, 2), Phys. Rev. B. 100, (2019) 155144