PCSI2024 Session PCSI-MoE: Topological Materials & Interfaces II

Topological van der Waals (vdW) ferromagnets has emerged as a promising material platform for investigating novel magentotransport responses and spintronic functionalities. Their unique topological electronic structures in combination of magnetism, spin-orbit interaction, and orbital-driven topological band degeneracy gives rise to large magnetotransport responses and magnetic tunability. In addition, their unique vdW structure allows for the isolation of atomically thin layers as well as the creation of atomically sharp and clean interfaces in heterostructures. In this talk, I will discuss our recent demonstrations of these magnetotransport and spintronic properties on various topological vdW ferromagnets and their heterostructures, highlighting large anomalous Hall effect [1], large angular magnetoresistance [2], highly-tunable spin-valve operations [3], and highly efficient magnetic switching [4]. These findings demonstrate that topological vdW ferromagnets have great potential for realizing novel spin-dependent electronic functionalities, which may be suitable for all-vdW-material-based spintronic applications.

[1] K.Kim, et al. Nat. Mater. 17. 794 (2018)

[2] J. Seo et al. Nature 599, 576–581 (2021).

[3] K.-H. Min, et al. Nat. Mater. 21, 1144 (2022)

[4] G. S. Choi et al. submitted

⁺Author for correspondence: js.kim@postech.ac.kr

Metals consisting of kagome lattices have interesting band structures consisting of topological flat bands and Dirac cones. Systems with flat bands are ideal for studying strongly correlated electronic states and related phenomena due to the smaller bandwidth W compared to the Coulomb repulsion U. Kagome metals such as CoSn have been recognized as promising candidates due to the proximity between the flat bands and the Fermi level. A key next step will be to realize epitaxial kagome thin films with flat bands to enable tuning of the flat bands across the Fermi level via electrostatic gating or strain. Here we report the band structures of epitaxial CoSn thin films grown directly on insulating substrates [1]. Flat bands are observed using synchrotron-based angle-resolved photoemission spectroscopy (ARPES). The band structure is consistent with density functional theory (DFT) calculations, and the transport properties are quantitatively explained by the band structure and semiclassical transport theory. We are also developing kagome metals that have the Dirac cones near the Fermi level, which are interesting for investigating the intrinsic anomalous Hall effect and to potentially realize the quantum anomalous Hall effect at elevated temperatures.

[1] Cheng et al., Nano Letters, 23(15), 7107-7113 (2023).

Antiperovskite materials are intermetallic compounds with perovskite crystal structure (space group Pm3m) but with anion and cation positions interchanged in the unit cell [1]. Similar to oxide-perovskite structure, antiperovskite materials have a variety of physical properties including antiferromagnetism, superconductivity and giant magnetoresistance [2]. There have been very few studies of antiperovskite structure Mn₃GaN in general although it was seen in molecular beam epitaxial growth as a second-phase precipitate when growing MnGaN [3]. Here we discuss the molecular beam epitaxial growth and surface study of Mn₃GaN. In our work, Mn₃GaN is deposited at 250 ± 10 °C onto magnesium oxide (001) substrates with a Mn: Ga: N flux ratio of 3:1:1. The sample surface is continuously monitored throughout the growth using reflection high energy electron diffraction. During the growth, the RHEED pattern was observed to be highly streaky, indicating an atomically smooth surface. The calculated in-plane lattice constant based on RHEED is 3.89 ± 0.06 Å. This value is close to the theoretical lattice constant a of Mn₃GaN (3.898 Å) [3]. X-ray diffraction confirms the majority 002 peak, and the value calculated is 3.84 ± 0.06 Å which also agrees well with the theoretical value (3.898 Å) [3] and with the experimental reported c value (3.881 Å) [2]. Since we did not observe significant second-phase peaks, the phase purity of the sample is quite high. Furthermore, cross-sectional STEM was done to understand the interface and the surface of the film. The plan is to also present in-situ scanning tunneling microscopy results for the surfaces of these MBE-grown Mn₃GaN layers.

This work is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317.

[1] S. V. Krivovichev, Minerals with antiperovskite structure: A review. Z. Kristallogr. 223, 109–113 (2008).

[2] KH. Kim, KJ. Lee, HS. Kang, FC. Yu, JA. Kim, DJ. Kim, KH. Baik, SH. Yoo, CG. Kim, YS. Kim, “Molecular beam epitaxial growth of GaN and GaMnN using a single precursor,” Physica Status Solidi (b) 241(7), 1458 (2004).

[3] E. F. Bertaut, D. Fruchart, J. P. Bouchaud, and R. Fruchart, (1968). Diffraction Neutronique de Mn3GaN. Solid State Commun. 6, 251–256 (1968).

Recently, Chen et al. studied the all-antiferromagnetic tunnel junction consisting of Mn₃Sn / MgO/ Mn₃Sn (0111), where they observed a tunnel magnetoresistance (TMR) effect at a ratio of 2% at room temperature. ¹ Furthermore, Bangar et al. reported the epitaxial growth of c-plane Mn₃Sn on the Al₂O₃ substrate using a Ru seed layer. They demonstrated a technique of engineering intrinsic spin Hall conductivity in Mn₃Sn by adjusting the Mn composition slightly for functional spintronic devices.² These works indicate great potential for kagome antiferromagnetic material, and it is essential to investigate the growth of Mn₃Sn on various substrates. In our previous work, we demonstrated the deposition of Mn₃Sn (0001) on Al₂O₃ (0001) at 524 ± 5°C, which resulted in a 3D island growth. We observed dome-like structures, which may be related to the significant lattice mismatch with sapphire (19%).³ Subsequently, we began to explore new substrates, and recently, we tried the growth on the MBE-grown N-polar GaN (0001). The growth was monitored in-situusing reflection high energy electron diffraction and measured ex-situ using X-ray diffraction, Rutherford backscattering, and atomic force microscopy. The sample grew at 524 ± 5°C for 71 mins, resulting in an epitaxially smooth growth of Mn₃Sn on GaN (0001). The in-plane lattice constants indicate a strain of -2.13 %, while the XRD indicates a 0001 orientation with a strain of -0.53% and an 1120 orientation with a strain of + 2.73%. Furthermore, the effect of varying growth temperature and Mn: Sn flux ratio on film orientation and crystallinity will be discussed in detail. We are also planning to begin scanning tunneling microscope experiments.

The authors acknowledge support from the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-FG02-06ER46317. We acknowledge the financial support of the Nanoscale & Quantum Phenomena Institute.

[1] X. Chen et al., "Octupole–driven magnetoresistance in an antiferromagnetic tunnel junction.” Nature 613, 490 (2023).

[2] H. Bangar et al., “Large Spin Hall Conductivity in Epitaxial thin films of Kagome Antiferromagnet Mn₃Sn at room temperature”, Adv. Quantum Technol. 6, 2200115 (2023).

[3]S. Upadhyay et al., “Exploring the interfacial structure and Crystallinity for Direct Growth of Mn₃Sn (0001) on Sapphire (0001) by Molecular Beam Epitaxy”, Surfaces and Interfaces (accepted).

⁺Author for correspondence: smitha2@ohio.edu

Heterostructures of two-dimensional (2D) van der Waals (vdW) magnets and topological insulators (TI) are of substantial interest as candidate materials for efficient spin-torque switching, quantum anomalous Hall effect, and chiral spin textures. However, since many of the vdW magnets have Curie temperatures below room temperature, we want to understand how materials can be modified to stabilize their magnetic ordering to higher temperatures. In this work, we utilize molecular beam epitaxy to systematically tune the Curie temperature (T_C) in thin film Fe₃GeTe₂/Bi₂Te₃ from bulk-like values (∼220 K) to above room temperature by increasing the growth temperature from 300 °C to 375 °C (Figure 1). For samples grown at 375 °C, cross-sectional scanning transmission electron microscopy (STEM) reveals the spontaneous formation of different Fe_mGe_nTe₂ compositions (e.g. Fe₅Ge₂Te₂ and Fe₇Ge₆Te₂) as well as intercalation in the vdW gaps, which are possible origins of the enhanced Curie temperature. This observation paves the way for developing various Fe_mGe_nTe₂/TI heterostructures with novel properties.