Session G43: Synthesis and Properties of Magnetic Topological Materials

Mapping out the topological phase diagram of FeSn

Kagome metals exhibit a flat band and a Dirac point in their electronic band structure. The combination of these electronic states with long range magnetic order creates favourable conditions to search for strongly correlated and topological quantum phases of matter. Transition metal based kagome materials have recently emerged as a promising material platform whose electronic and magnetic properties can be controlled by the stoichiometry and elemental composition. Owing to the reduced coupling between the two-dimensional kagome planes, the inter metallic kagome series X₁Y₁ offers particularly attractive opportunities to investigate the interplay between strong electronic correlations, topology, and magnetism.

We have combined molecular beam epitaxy with electrical transport measurements to study the interplay of magnetism and band topology in thin films of the antiferromagnetic kagome metal FeSn. We will present results from a magnetic field and temperature dependent study of the anomalous Hall effect using transverse resistivity measurements. Combining these measurements with magnetic Monte-Carlo simulations and theoretical model calculations, we map out the topological phase diagram of FeSn over a large temperature range.   

Epitaxy growth of atomically smooth kagome metal film

    The two-dimensional kagome lattice is a well-known platform for exploring the interplay of topology, electron correlations, and magnetism. Significantly, the antiferromagnetic kagome metal FeSn, composed of alternating Fe₃Sn-Sn₂ layers, was found to host Dirac fermions and flat bands on the bulk crystal, owing to a comparably weak coupling between the Fe₃Sn kagome planes (1). However, the cleaved crystals often exhibit small Fe₃Sn and Sn₂ terminated terraces with a rather large defect density (2). This limits their suitability for the study with surface-sensitive probes

    Using molecular beam epitaxy (MBE), we have explored the growth process of the prototypical kagome metal FeSn on the STO(111) surface. Varying the growth process parameters, we have realized atomically smooth and defect-free FeSn films and islands. Our in-situ scanning tunneling microscopy and ex-situ electronic transport measurements demonstrate the high quality of the FeSn films. Making use of the precise growth control of MBE, we will further present our recent results on the stoichiometric doping and layer-by-layer growth of FeSn and its related compounds. Realizing atomically flat kagome metal films and islands down to the ultra-thin limit, our work provides avenues to study topological and correlated electronic states in kagome metals and heterostructures with the STM and electric transport measurements.

    

Molecular beam epitaxy of Sn-based Kagome antiferromagnets

Binary compounds T_mX_n (T = Mn, Fe, Co; X = Sn, Ge; m:n = 3:1, 3:2, 1:1) have been shown to exhibit a plethora of interesting magnetotransport phenomena such as strong anisotropic anomalous Hall effect. Furthermore, when coupled with an adjacent ferromagnet, phenomena like spin Hall effects give rise to novel spin-orbit-torques. So far, film growth has mostly been achieved using magnetron sputtering by incorporating nonmagnetic buffer layers. Here we report the synthesis of ferromagnet/T_mX_n bilayer films: Fe/Mn₃Sn, Co/Mn₃Sn, Fe/FeSn and Co/FeSn using molecular beam epitaxy on insulating oxide substrates. Rutherford backscattering spectroscopy was used to confirm the stoichiometric window, where these phases are stabilized, while transport and magnetometry measurements were conducted to explore metallicity and magnetic ordering in the films. Structural characterization using high-resolution X-ray diffraction, reflection high-energy electron diffraction, and electron microscopy reveals the Mn₃Sn films grow as crystalline three-dimensional islands whereas FeSn films are flat and continuous, paving the way to integrate these materials into devices.    

    

Competition between Chern and Mott insulating phases in oxide superlattices

Transition metal oxides exhibit a strong interplay of lattice, charge, spin and orbital degrees of freedom which leads to a rich functionality including the tendency to break time reversal symmetry. This makes them attractive candidates to realize Chern insulating phases with some advantages over magnetic doping of Z₂ topological insulators. I will review recent activities in the theoretical prediction of Chern insulators (CI) in oxide superlattices based on density functional theory calculations including an on-site Coulomb term and spin-orbit coupling. A systematic study of superlattices that host a (buckled) honeycomb pattern, derived from the perovskite, (LaXO₃)₂/(LaAlO₃)₄(111) [1,2], or corundum structure, (X₂O₃)₁/(Al₂O₃)₅ (0001) [3,4], with X=3d, 4d or 5d element, reveals several promising candidates. Ferromagnetic coupling and the symmetry of the two honeycomb sublattices turns out to be an essential ingredient.  The caveat is that many of those are metastable w.r.t. spontaneous symmetry breaking (e.g. Jahn-Teller effect, charge and/or orbital ordering, antiferromagnetic coupling) and the ground state is often a trivial Mott insulating phase, albeit with magnetic and electronic reconstruction distinct from the reference bulk materials. Strain or non-equilibrium excitations may be auspicious to reach the CI state. Last but not least, taking as an example a rocksalt-derived strained (EuO)₁/(MgO)₃(001) superlattice, a link is established  between Chern insulating behavior and enhanced thermoelectric response [5].

[1] D. Doennig, S. Baidya, W.E. Pickett and R. Pentcheva, Phys. Rev. B  93, 165145 (2016).

[2] O. Köksal and R. Pentcheva, Sci. Rep. 9, 17306 (2019).

[3] O. Köksal, S. Baidya and R. Pentcheva, Phys. Rev. B 97, 035126 (2018).

[4] O. Köksal and R. Pentcheva, J. Phys. Chem. Solids 128, 301-309 (2019).

[5] O. Köksal and R. Pentcheva, Phys. Rev. B 103, 045135 (2021).

    

Exploring the behavior of EuCuP and EuCuAs

Materials where magnetism, the lattice and electronic topology are coupled provide opportunities to advance both fundamental science and future technologies. Topological semimetals possessing intrinsic magnetic order afford a particularly promising avenue of study. Under this motivation, we will examine the physical properties of EuCuP and EuCuAs, the former being a complex ferromagnet while the latter is an antiferromagnet. In some regards the nature of the ordered magnetism drives different responses, such as a strong metamagnetic transition in EuCuAs that is lacking in EuCuP. However, the two materials also possess many similarities, including particularly interesting electronic transport properties. The existing literature and our new findings will be discussed and compared to behaviors in other candidate topological semimetals.   

Creation of Chiral Interface Channels for Quantized Transport in Magnetic Topological Insulator Multilayer Heterostructures

One-dimensional (1D) topologically protected states are usually formed at the interface between two-dimensional (2D) materials with different topological invariants. Therefore, 1D chiral interface channels (CICs) can be created at the boundary of two quantum anomalous Hall (QAH) insulators with different Chern numbers. Such a QAH junction can function as a chiral edge current distributer at zero magnetic field, but its realization remains challenging. In this work, by employing an in-situ mechanical mask, we use molecular beam epitaxy (MBE) to synthesize QAH insulator junctions, in which two QAH insulators with different Chern numbers are connected along a 1D junction. For the junction between C = 1 and C = -1 QAH insulators, we observe quantized transport and demonstrate the appearance of the two parallel propagating CICs along the magnetic domain wall at zero magnetic field. Moreover, since the Chern number of the QAH insulators in magnetic topological insulator (TI)/TI multilayers can be tuned by altering magnetic TI/TI bilayer periods, the junction between two QAH insulators with arbitrary Chern numbers can be achieved by growing different periods of magnetic TI/TI on the two sides of the sample. For the junction between C = 1 and C = 2 QAH insulators, our quantized transport shows that a single CIC appears at the interface. Our work lays down the foundation for the development of QAH insulator-based electronic and spintronic devices, topological chiral networks, and topological quantum computations.

    

Comparative studies of MBE-grown MnBi2Te4 on Si(111) and epitaxial graphene substrates

Discerning the topological surface state is critical for understanding exotic quantum phenomena including the Quantum Anomalous Hall effect and Axion insulator states in intrinsic magnetic topological insulators (MTI). In order to achieve an intrinsic surface state of MnBi₂Te₄, the sample thickness must be controlled down to a very thin regime where the complexity added by underlying anti-site defects is minimized. Using molecular beam epitaxy (MBE), we gain control of high-quality MnBi₂Te₄ and MnBi₂Te₄/Bi₂Te₃ super-lattice growths to exploit their rich topological quantum phase diagram. By combining in-situ electron diffraction and ex-situ x-ray diffraction techniques we report differences in the preferred alignment of MnBi₂Te₄ depending on sample interactions with different substrates (e.g. Si(111), epitaxial graphene) during the growth. We further investigate characteristic electronic structures at the surface of weakly bonded MnBi₂Te₄ flakes formed on top of van der Waals substrates (Gr/SiC, HOPG) compared to Si(111), using in-situ STM and ARPES.

    

Molecular beam epitaxy growth and nanoscale characterization of antiferromagnetic topological insulator MnBi2Te4 thin films

MnBi2Te4 has recently attracted a lot of interest as the first intrinsic antiferromagnetic topological insulator. The majority of experiments thus far have focused on the synthesis and exploration of bulk single crystals and exfoliated flakes of MnBi2Te4. However, growing and studying high-quality thin films of MnBi2Te4 down to a single unit cell thickness remains a challenge. In this talk, I will discuss our experiments exploring the growth of MnBi2Te4 thin films of various thicknesses using molecular beam epitaxy (MBE). We use atomic force microscopy, X-ray diffraction and in-situ reflection high-energy electron diffraction to analyze the structural properties of MnBi2Te4 films. We perform magnetization measurements to probe its magnetic transition. We further investigate the electronic structure by using scanning tunneling microscopy and spectroscopy. Our work explores the possibility of fabricating ultra-thin films of magnetic topological insulators, which should be of high interest for future applications.

    

Very High Curie temperature (MnSb2Te4)x(Sb2Te3)1-x Magnetic Topological Insulator Structures Grown by Molecular Beam Epitaxy

Previously, it was shown that we can control the composition (x) of the magnetic topological materials (MnSb2Te4)x(Sb2Te3)1-x by adjusting the Mn flux ratio during MBE growth.1 We learned that Mn incorporates as a structural element to form MnSb2Te4 septuple layers (SL) or as anti-site defects. Samples with 70-80% SLs (x = 0.7-0.8) exhibited Curie temperatures (Tc) as high as 80K. Here we report further enhancement of the Tc by modifying two MBE growth parameters: Mn beam equivalent pressure (BEP) ratios and growth rates (GR). We observed that, for samples with 70 – 80 % SLs, gradually increasing the Mn BEP ratio from 0.04 to 0.09 led to higher Tc values (from 20K to 80K). Furthermore, at very low BEP ratios there was evidence for the coexistence of ferromagnetic and antiferromagnetic phases. This suggests the approach to stoichiometric MnSb2Te4 growth and low Tc values at low Mn contents, while excess substitutional Mn in the structures promoted the higher Tc values. Samples grown at a constant Mn BEP ratio of 0.09 with a reduced GR of 0.5-0.6 nm/min (compared to 0.9-1.0 nm/min used previously) resulted in structures with ~80% SLs that exhibit even higher Tc values of 100K and above. These Tc values are higher than any values reported to date for these materials. EDX analysis suggests significantly increased Mn(Te) and Mn(Sb) substitutions for samples with reduced GR. We will present these results as they relate to the (MnSb2Te4)x(Sb2Te3)1-x structural properties, as well as the details of the proposed growth mechanism.   

    

Magnetic anisotropy of the layered van der Waals antiferromagnet MnSb2Te4

Recent discovery of van der Waals (vdW) layered antiferromagnets (AFM) of the Mn(Bi,Sb)₂Te₄ class provides unprecedented opportunities for exploring a richness of magnetic states with non-trivial topology. MnBi₂Te₄ (MBT) is a confirmed topological insulator, known to host quantum anomalous Hall (QAH) state [1]. MnSb₂Te₄ (MST) has the same tetradymite-type crystal structure with the R3m space group as MBT, but by contrast to MBT can have a significant population of Sb_Mn antisite defects and has been surmised to be a type II Weyl semimetal. Here we report on magnetic anisotropy deduced from the detailed angular field-dependence of magnetotransport in AFM MST. Negative magnetoresistance (n-MR) is observed under arbitrary field orientation below and above the Néel temperature T_N ~ 18.5 K, indicating strong spin scattering in both the AFM and paramagnetic states. However, the low-field dependence of n-MR is strikingly different for H||c and H||ab, rapidly decreasing above the characteristic AFM field H₂ ~1.5 T for the former but strictly field-linear for the latter, with the parabolic (∝ B²) positive MR recovered above ~ 6 T. The comparison of magnetic anisotropy obtained from n-MR in MST and MBT and investigated by the DFT calculations will be presented, and the relation to the putative chiral anomaly in MST expected in a Weyl semimetal will be discussed.

    

Effects of anti-site defect-induced disorder in compensated topological magnet MnBi1.36Sb0.64Te4

The gapped Dirac-like surface states of magnetic topological insulator MnBi_2-xSb_xTe₄ (MBST) are a promising host for exotic quantum phenomena such as the quantum anomalous Hall effect and axion insulating states. However, it has become clear that atomic defects undermine the stabilization of such quantum phases as they lead to spatial variations in the surface state gap and doping levels. The large number of possible defect configurations in MBST make studying the influence of individual defects virtually impossible. Here, we present a statistical analysis of the nanoscale effect of defects in MBST by scanning tunneling microscopy/spectroscopy (STM/S). We identify (Bi,Sb)_Mn anti-site defects to be the main source of doping fluctuations, resulting in the formation of nanoscale charge puddles, while Mn_(Bi/Sb) anti-site defects are the main source of fluctuations in the surface state gap. Our findings can guide further optimization of this material system via defect engineering.   

Distinct magnetic gaps between antiferromagnetic and ferromagnetic orders driven by surface defects in the topological magnet MnBi2Te4

Many experiments observed a metallic behavior at zero magnetic fields (antiferromagnetic phase, AFM) in MnBi₂Te₄ thin film transport, which coincides with gapless surface states observed by angle-resolved photoemission spectroscopy, while it can become a Chern insulator at a field larger than 6 T (ferromagnetic phase, FM). Thus, the zero-field surface magnetism was once speculated to be different from the bulk AFM phase. However, recent magnetic force microscopy refutes this assumption by detecting persistent AFM order on the surface. In this work, we propose a mechanism related to surface defects that can rationalize these contradicting observations in different experiments. We find that co-antisites (exchanging Mn and Bi atoms in the surface van der Waals layer) can strongly suppress the magnetic gap down to several meV in the AFM phase without violating the magnetic order but preserve the magnetic gap in the FM phase. The different gap sizes between AFM and FM phases are caused by the exchange interaction cancellation/collaboration of top two van der Waals layers manifested by defect-induced surface charge redistribution among the top two van der Waals layers. This theory can be validated by the position- and field-dependent gap in future surface spectroscopy measurements. Our work suggests suppressing related defects in samples to realize the quantum anomalous Hall insulator or axion insulator at zero fields.   

Magnetooptical Investigation of Intrisinc Magnetic Topological Insulator MnBi2Te4

MnBi₂Te₄ brings profound attention because of its rich nature by incorporating topology and magnetism. However, the properties of its surface states in the magnetic phase remain poorly understood. Here we report recent magnetooptical infrared spectroscopy measurements on the intrinsic magnetic topological insulator MnBi₂Te₄. We carry out infrared absorption measurements under magnetic fields up to 34 T and observe an optical absorption in the mid-infrared that shifts to higher energy as a function of increasing magnetic field. The energy of the transition versus magnetic field reflects the behavior of the anomalous Hall effect up to 70T. The transition energy increases through the canted magnetic state of MnBi₂Te₄ and then saturates at 8 T when the system enters a ferromagnetic state. Then, the transition energy further increases when the magnetic field is above 25 T, where the magnetization of Mn antisite defects likely start to flip. In addition, this absorption shows a strong magnetic circular dichroism which likely originates from the bands splitting under the effect of magnetic exchange. Our studies not only reveal the infrared response of MnBi₂Te_a but also provide a picture of how the electronic structure changes through the phase diagram of this material, consistently following the behavior of the magnetization.