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

   https://doi.org/10.1002/adma.202406772  

   Tai, Lixuan; He, Haoran; Chong, Su Kong; Zhang, Huairuo; Huang, Hanshen; Qiu, Gang; Ren, Yuxing; Li, Yaochen; Yang, Hung‐Yu; Yang, Ting‐Hsun; et al (November 2024, Advanced Materials)

   Abstract 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 (TSS) with ultrahigh efficiency. Here, 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, is demonstrated. A giant switched anomalous Hall resistance of 9.2 kΩ is realized, among the largest of all SOT systems, which makes the Hall channel a good readout and eliminates the need to fabricate complicated magnetic tunnel junction (MTJ) structures. The SOT switching current density can be reduced to 2.8 × 10⁵ A cm⁻², indicating its high efficiency. 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) × 10⁻⁶ T A⁻¹ cm²and the interfacial charge‐to‐spin conversion efficiency to 3.9 ± 0.3 nm⁻¹. The findings establish VBST as an extraordinary candidate for energy‐efficient magnetic memory devices. 

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2. Drastic enhancement of magnetic critical temperature and amorphization in topological magnet EuSn2P2 under pressure

   https://doi.org/10.1038/s41535-022-00451-9  

   Bi, Wenli; Culverhouse, Trenton; Nix, Zachary; Xie, Weiwei; Tien, Hung-Ju; Chang, Tay-Rong; Dutta, Utpal; Zhao, Jiyong; Lavina, Barbara; Alp, Esen E.; et al (December 2022, npj Quantum Materials)

   Abstract High pressure is an effective tool to induce exotic quantum phenomena in magnetic topological insulators by controlling the interplay of magnetic order and topological state. This work presents a comprehensive high-pressure study of the crystal structure and magnetic ground state up to 62 GPa in an intrinsic topological magnet EuSn 2 P 2 . With a combination of high resolution X-ray diffraction, 151 Eu synchrotron Mössbauer spectroscopy, X-ray absorption spectroscopy, molecular orbital calculations, and electronic band structure calculations, it has been revealed that pressure drives EuSn 2 P 2 from a rhombohedral crystal to an amorphous phase at 36 GPa accompanied by a fourfold enhancement of magnetic ordering temperature. In the pressure-induced amorphous phase, Eu ions take an intermediate valence state. The drastic enhancement of magnetic ordering temperature from 30 K at ambient pressure to 130 K at 41.2 GPa resulting from Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions likely attributes to the stronger Eu–Sn interaction at high pressure. These rich results demonstrate that EuSn 2 P 2 is an ideal platform to study the correlation of the enhanced RKKY interactions, disordered lattice, intermediate valence, and topological state. 

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3. Anomalous Landau quantization in intrinsic magnetic topological insulators

   https://doi.org/10.1038/s41467-023-40383-x  

   Chong, Su Kong; Lei, Chao; Lee, Seng Huat; Jaroszynski, Jan; Mao, Zhiqiang; MacDonald, Allan H.; Wang, Kang L. (December 2023, Nature Communications)

   Abstract The intrinsic magnetic topological insulator, Mn(Bi_1−xSb_x)₂Te₄, has been identified as a Weyl semimetal with a single pair of Weyl nodes in its spin-aligned strong-field configuration. A direct consequence of the Weyl state is the layer dependent Chern number,$$C$$ $C$. Previous reports in MnBi₂Te₄thin films have shown higher$$C$$ $C$states either by increasing the film thickness or controlling the chemical potential. A clear picture of the higher Chern states is still lacking as data interpretation is further complicated by the emergence of surface-band Landau levels under magnetic fields. Here, we report a tunable layer-dependent$$C$$ $C$ = 1 state with Sb substitution by performing a detailed analysis of the quantization states in Mn(Bi_1−xSb_x)₂Te₄dual-gated devices—consistent with calculations of the bulk Weyl point separation in the doped thin films. The observed Hall quantization plateaus for our thicker Mn(Bi_1−xSb_x)₂Te₄films under strong magnetic fields can be interpreted by a theory of surface and bulk spin-polarised Landau level spectra in thin film magnetic topological insulators. 

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4. Distinguishing surface and bulk electromagnetism via their dynamics in an intrinsic magnetic topological insulator

   https://doi.org/10.1126/sciadv.adn5696  

   Nguyen, Khanh Duy; Lee, Woojoo; Dang, Jianchen; Wu, Tongyao; Berruto, Gabriele; Yan, Chenhui; Ip, Chi_Ian Jess; Lin, Haoran; Gao, Qiang; Lee, Seng Huat; et al (August 2024, Science Advances)

   The indirect exchange interaction between local magnetic moments via surface electrons has been long predicted to bolster the surface ferromagnetism in magnetic topological insulators (MTIs), which facilitates the quantum anomalous Hall effect. This unconventional effect is critical to determining the operating temperatures of future topotronic devices. However, the experimental confirmation of this mechanism remains elusive, especially in intrinsic MTIs. Here, we combine time-resolved photoemission spectroscopy with time-resolved magneto-optical Kerr effect measurements to elucidate the unique electromagnetism at the surface of an intrinsic MTI MnBi₂Te₄. Theoretical modeling based on 2D Ruderman-Kittel-Kasuya-Yosida interactions captures the initial quenching of a surface-rooted exchange gap within a factor of two but overestimates the bulk demagnetization by one order of magnitude. This mechanism directly explains the sizable gap in the quasi-2D electronic state and the nonzero residual magnetization in even-layer MnBi₂Te₄. Furthermore, it leads to efficient light-induced demagnetization comparable to state-of-the-art magnetophotonic crystals, promising an effective manipulation of magnetism and topological orders for future topotronics. 

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5. Families of asymmetrically functionalized germanene films as promising quantum spin Hall insulators

   https://doi.org/10.1039/D0CP06231F  

   Shi, Lawrence; Li, Qiliang (February 2021, Physical Chemistry Chemical Physics)

   null (Ed.)

   Topological insulators (TIs), exhibiting the quantum spin Hall (QSH) effect, are promising for developing dissipationless transport devices that can be realized under a wide range of temperatures. The search for new two-dimensional (2D) TIs is essential for TIs to be utilized at room-temperature, with applications in optoelectronics, spintronics, and magnetic sensors. In this work, we used first-principles calculations to investigate the geometric, electronic, and topological properties of GeX and GeMX (M = C, N, P, As; X = H, F, Cl, Br, I, O, S, Se, Te). In 26 of these materials, the QSH effect is demonstrated by a spin–orbit coupling (SOC) induced large band gap and a band inversion at the Γ point, similar to the case of an HgTe quantum well. In addition, engineering the intra-layer strain of certain GeMX species can transform them from a regular insulator into a 2D TI. This work demonstrates that asymmetrical chemical functionalization is a promising method to induce the QSH effect in 2D hexagonal materials, paving the way for practical application of TIs in electronics. 

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