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1. Giant chiral magnetoelectric oscillations in a van der Waals multiferroic

   https://doi.org/10.1038/s41586-024-07678-5  

   Gao, Frank Y; Peng, Xinyue; Cheng, Xinle; Viñas_Boström, Emil; Kim, Dong Seob; Jain, Ravish K; Vishnu, Deepak; Raju, Kalaivanan; Sankar, Raman; Lee, Shang-Fan; et al (August 2024, Nature)

   Abstract Helical spin structures are expressions of magnetically induced chirality, entangling the dipolar and magnetic orders in materials^1–4. The recent discovery of helical van der Waals multiferroics down to the ultrathin limit raises prospects of large chiral magnetoelectric correlations in two dimensions^5,6. However, the exact nature and magnitude of these couplings have remained unknown so far. Here we perform a precision measurement of the dynamical magnetoelectric coupling for an enantiopure domain in an exfoliated van der Waals multiferroic. We evaluate this interaction in resonance with a collective electromagnon mode, capturing the impact of its oscillations on the dipolar and magnetic orders of the material with a suite of ultrafast optical probes. Our data show a giant natural optical activity at terahertz frequencies, characterized by quadrature modulations between the electric polarization and magnetization components. First-principles calculations further show that these chiral couplings originate from the synergy between the non-collinear spin texture and relativistic spin–orbit interactions, resulting in substantial enhancements over lattice-mediated effects. Our findings highlight the potential for intertwined orders to enable unique functionalities in the two-dimensional limit and pave the way for the development of van der Waals magnetoelectric devices operating at terahertz speeds. 

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2. Composite Spin Hall Conductivity from Non‐Collinear Antiferromagnetic Order

   https://doi.org/10.1002/adma.202209866  

   Novakov, Steve; Meisenheimer, Peter_B; Pan, Grace_A; Kezer, Patrick; Vu, Nguyen_M; Grutter, Alexander_J; Need, Ryan_F; Mundy, Julia_A; Heron, John_T (June 2023, Advanced Materials)

   Abstract Non‐collinear antiferromagnets (AFMs) are an exciting new platform for studying intrinsic spin Hall effects (SHEs), phenomena that arise from the materials’ band structure, Berry phase curvature, and linear response to an external electric field. In contrast to conventional SHE materials, symmetry analysis of non‐collinear antiferromagnets does not forbid non‐zero longitudinal and out‐of‐plane spin currents with polarization and predicts an anisotropy with current orientation to the magnetic lattice. Here, multi‐component out‐of‐plane spin Hall conductivities are reported in L1₂‐ordered antiferromagnetic PtMn₃thin films that are uniquely generated in the non‐collinear state. The maximum spin torque efficiencies (ξ  =J_S /J_e ≈ 0.3) are significantly larger than in Pt (ξ  ≈  0.1). Additionally, the spin Hall conductivities in the non‐collinear state exhibit the predicted orientation‐dependent anisotropy, opening the possibility for new devices with selectable spin polarization. This work demonstrates symmetry control through the magnetic lattice as a pathway to tailored functionality in magnetoelectronic systems. 

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3. Twisted Type‐II Rashba Homobilayers: A Platform for Tunable Topological Moiré Flat Bands

   https://doi.org/10.1002/adfm.202425454  

   Xu, Xilong; Wang, Haonan; Yang, Li (May 2025, Advanced Functional Materials)

   Abstract The recent discovery of topological flat bands in twisted transition metal dichalcogenide homobilayers and multilayer graphene has sparked significant research interest. Here, a new platform for realizing tunable topological moiré flat bands: twisted type‐II Rashba homobilayers, is proposed. By maintaining centrosymmetry, the interplay between Rashba spin‐orbit coupling and interlayer interactions generates an effective pseudo‐antiferromagnetic field, opening a gap within the Dirac cone with non‐zero Berry curvature. Using twisted BiTeI bilayers as an example, it is predicted that the emergence of flat topological bands with a remarkably narrow bandwidth (below 20 meV). Notably, the system undergoes a transition from a valley Hall insulator to a quantum spin Hall insulator as the twisting angle increases. This transition arises from a competition between the twisting‐driven effective spin‐orbit coupling and sublattice onsite energies presented in type‐II Rashba moiré structures. The high tunability of Rashba materials in terms of the spin‐orbit coupling strength, interlayer interaction, and twisting angle expands the range of materials suitable for functionalizing and manipulating correlated topological properties. 

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4. Coherent helicity-dependent spin-phonon oscillations in the ferromagnetic van der Waals crystal CrI3

   https://doi.org/10.1038/s41467-022-31786-3  

   Padmanabhan, P.; Buessen, F. L.; Tutchton, R.; Kwock, K. W. C.; Gilinsky, S.; Lee, M. C.; McGuire, M. A.; Singamaneni, S. R.; Yarotski, D. A.; Paramekanti, A.; et al (August 2022, Nature Communications)

   Abstract The discovery of two-dimensional systems hosting intrinsic magnetic order represents a seminal addition to the rich landscape of van der Waals materials. CrI₃is an archetypal example, where the interdependence of structure and magnetism, along with strong light-matter interactions, provides a new platform to explore the optical control of magnetic and vibrational degrees of freedom at the nanoscale. However, the nature of magneto-structural coupling on its intrinsic ultrafast timescale remains a crucial open question. Here, we probe magnetic and vibrational dynamics in bulk CrI₃using ultrafast optical spectroscopy, revealing spin-flip scattering-driven demagnetization and strong transient exchange-mediated interactions between lattice vibrations and spin oscillations. The latter yields a coherent spin-coupled phonon mode that is highly sensitive to the driving pulse’s helicity in the magnetically ordered phase. Our results elucidate the nature of ultrafast spin-lattice coupling in CrI₃and highlight its potential for applications requiring high-speed control of magnetism at the nanoscale. 

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5. Uncovering spin-orbit coupling-independent hidden spin polarization of energy bands in antiferromagnets

   https://doi.org/10.1038/s41467-023-40877-8  

   Yuan, Lin-Ding; Zhang, Xiuwen; Acosta, Carlos Mera; Zunger, Alex (August 2023, Nature Communications)

   Abstract Many textbook physical effects in crystals are enabled by some specific symmetries. In contrast to such ‘apparent effects’, ‘hidden effect X’ refers to the general condition where the nominal global system symmetry would disallow the effect X, whereas the symmetry of local sectors within the crystal would enable effect X. Known examples include the hidden Rashba and/or hidden Dresselhaus spin polarization that require spin-orbit coupling, but unlike their apparent counterparts are demonstrated to exist in non-magnetic systems even in inversion-symmetric crystals. Here, we discuss hidden spin polarization effect in collinear antiferromagnets without the requirement for spin-orbit coupling (SOC). Symmetry analysis suggests that antiferromagnets hosting such effect can be classified into six types depending on the global vs local symmetry. We identify which of the possible collinear antiferromagnetic compounds will harbor such hidden polarization and validate these symmetry enabling predictions with first-principles density functional calculations for several representative compounds. This will boost the theoretical and experimental efforts in finding new spin-polarized materials. 

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