Chen Liu

Multiferroics host simultaneous and coupled ferroic orders, allowing disparate external stimuli to induce abrupt transformations in their structures and properties. Creating multiferroicity in two-dimensional (2D) van der Waals (vdW) platforms would add the elements of strong quantum confinement, enhanced quasiparticle excitations, and wide tunability to the capabilities of these systems. In this work, we constructed vdW heterostructures of ferromagnetic triiron gallium ditelluride (Fe ₃ GaTe ₂ ) and ferroelectric copper indium thiophosphate (CuInP ₂ S ₆ ) to integrate their respective orders. We observed strong interferroic coupling at room temperature by demonstrating ferroelectrically reconfigurable magnetic anisotropy of 2D Fe ₃ GaTe ₂ . The interferroic magnetoelectricity diminished with increasing Fe ₃ GaTe ₂ thickness, revealing the interfacial nature of heterostructure multiferroicity. Our discovery of all-vdW heterostructure multiferroicity opens the door to the artificial assembly of vdW layers for designer 2D multiferroics.

The rapid advancement of generative artificial intelligence has significantly increased the demand for both energy and data storage. Magneto-ionics, which utilizes ionic motion to control magnetism, often driven by an electric field in heterostructures, has gained significant attention for its potential to enable energy-efficient modulation of magnetic properties with large effects. This study proposes a unique COMS-compatible solid-state magneto-ionic system composed of all Mn nitrides and demonstrates that nitrogen ionic motion can induce reversible phase transitions between ferrimagnetic and antiferromagnetic Mn nitrides. This magnetic phase transition is manifested in dramatic changes in the resultant exchange bias effect which can be increased by over an order of magnitude when more nitrogen is introduced into the nitrides during deposition and subsequently reduced by over 70% when nitrogen is taken out of the nitrides through post-annealing. Additionally, voltage-induced nitrogen ionic motion can lead to reversible changes in saturation magnetization and exchange bias effect by 23% and 0.1 T (16%) at 5 K, respectively. These findings highlight the potential of this all-Mn-nitride system as an industrially viable and environmentally sustainable platform, offering tunable magnetic properties and energy-efficient operation, and potential for magnetic field immunity. This work has been supported in part by the NSF (DMR-2005108, ECCS-2151809, ECCS-2132098) and KAUST (OSR-2019-CRG8-4081).

Reversible electrodeposition of metals is a crucial route to developing high-energy and rechargeable batteries. However, uncontrolled and nonplanar morphological evolution and parasitic reactions at the metal anodes are fundamental barriers to realizing full reversibility. Here, using aqueous electrochemistry as a probe, we develop multiscale characterization tools that can precisely determine the root cause of these morphological instabilities and parasitic reactions. Our analysis indicates that these issues are fundamentally from the free water molecules in aqueous electrolytes, leading to low reversibility of metal anodes. We therefore demonstrate a straightforward and effective strategy, based on modulating the solute anions in aqueous electrolytes, to suppress free water molecule concentration in conventional aqueous electrolytes. A proof of concept is demonstrated using a Zn metal anode, which shows unprecedented reversibility and stability in conventional aqueous electrolytes with structure-making anions under a harsh condition of 10 milliampere hours per square centimeter. This work unlocks an alternative angle to develop sustainable electrolytes for cost-efficient, practical battery chemistries.

The synthesis of p-type two-dimensional (2D) semiconductor 2H-MoTe2 films on various dielectric substrates is essential for realizing multilayer architectures for electronic and optoelectronic devices. However, achieving high-efficiency and complete 2H phase fabrication on diverse substrates is challenging. In this study, a multipoint nucleation approach was proposed for achieving controlled phase transitions of MoTe2 films via space confinement in the CVD growth process. This interstitial environment limited Te evaporation and considerably enhanced the reactant collision probability, leading to a high nucleation density of 2H-MoTe2 films. Using the proposed approach, large-area 2H-MoTe2 films were fabricated in ∼30 min. This high-efficiency phase-transition approach has promising implications for the industrial fabrication of large-scale 2D semiconductor films and other potential applications in electronic and optoelectronic devices.

The prominent properties of van der Waals (vdW) magnets make them a promising platform for prospective spintronic applications. Fe3GaTe2 is a newly discovered vdW material that exhibits room-temperature ferromagnetism and topological spin textures. In this paper, we report the observation of an unconventional topological Hall effect (THE) up to room temperature in Fe3GaTe2. When the current flows along the ab plane, a conventional anomalous Hall effect is observed under an out-of-plane magnetic field, which can be depicted by the intrinsic Karplus–Luttinger mechanism. Intriguingly, by rotating the magnetic field to align with the current direction, unexpected Hall resistivity cusps emerge. This Hall resistivity anomaly can be explained by the in-plane THE, which may originate from the noncoplanar spin structures in Fe3GaTe2. These spin structures have a nonzero scalar spin chirality and act as a fictitious magnetic field that produces a real-space Berry curvature. By extracting the topological Hall resistivity at different temperatures, a THE phase diagram is successfully constructed, showing a maximum value of 1.48 μΩ cm at 250 K. The room-temperature THE reveals the great potential of Fe3GaTe2 for spintronic applications.

Solid-state magneto-ionic (MI) effects have shown promise for energy-efficient nanoelectronics, where ionic migration may be used to achieve atomic-scale control of interfaces in magnetic nanostructures. To date, magneto-ionics have been mostly explored in oxygen-based systems [1-4], while there is a surge of interest in alternative ionic systems due to their different ionic migration mechanisms and characteristics [5-7]. We have recently demonstrated effective MI control of magnetic functionalities using a variety of ionic species, particularly nitrogen. In nitride-based Ta/CoFe/MnN/Ta films, the chemically induced MI effect is combined with the electric field driving of nitrogen to electrically manipulate exchange bias [8]. Upon field-cooling the heterostructure, ionic diffusion of nitrogen from MnN into the Ta layers occurs. A significant exchange bias is observed, which can be further enhanced by ~ 20% after voltage conditioning (Figs. 1a-1e). This enhancement can be reversed by voltage conditioning with an opposite polarity. Nitrogen migration within the MnN layer and into the Ta capping layer causes the enhancement in exchange bias, which is observed in polarized neutron reflectometry studies. We have also achieved all-nitride-based magneto-ionic systems [9]. Thin films of (001)-ordered Mn₄N are grown by sputtering Mn onto Mn₃N₂ seed layer on Si (100) substrate. Nitrogen ion migration across the Mn₃N₂ /Mn layers leads to a continuous evolution of the layers to Mn₃N₂ /Mn₄N, Mn₂N/Mn₄N, and eventually Mn₄N alone (Figs. 1f-1g). Furthermore, we have demonstrated MI control of the exchange bias effect in an all-nitride Mn₄N/MnN_x system, where the field-trained exchange field can be varied up to ten times by introducing or extracting nitrogen from the nitride system. This is achieved by adjusting the nitrogen gas partial pressure during deposition or varying annealing temperature after deposition. These effects demonstrate contrasts with oxygen-based MI effects in terms of operating principles, switching speed, and reversibility. Such MI systems are valuable platforms to gain quantitative understanding at buried interfaces. They also offer potentials for device applications based on electric modulation of magnetic functionalities. This work has been supported in part by the NSF (ECCS-2151809, DMR-2005108, DMR-1828420), SRC/NIST SMART Center, and KAUST. [1] U. Bauer et al., Nat. Mater. 14, 174 (2015). [2] C. Bi et al., Phys. Rev. Lett. 113, 267202 (2014). [3] D. A. Gilbert et al., Nat. Commun. 7, 11050 (2016). [4] G. Chen et al., Sci. Adv. 6, eaba4924 (2020). [5] A. J. Tan et al., Nat. Mater. 18, 35 (2019). [6] G. Chen et al., Phys. Rev. X 11, 021015 (2021). [7] J. de Rojas et al., Nat. Commun. 11, 5871 (2020). [8] C. J. Jensen et al., ACS Nano 17, 6745 (2023). [9] Z. J. Chen et al., Appl. Phys. Lett. 123, 082403 (2023). Figure 1

Magnetization switching driven by magnons is a promising technology capable of substantially decreasing energy dissipation and potential damage to spintronic devices. In this study, we investigated the temperature-dependent magnon torque effect in SrIrO3/NiO/ferromagnetic multilayers. It is found that the magnon-mediated damping-like spin–orbit torque (SOT) efficiency decreases with increasing temperature. Enhanced magnon transmission was observed in the vicinity of the blocking temperature of the NiO layer, which can be ascribed to the amplification of damping-like SOT efficiency by the spin fluctuations. More importantly, we have demonstrated that the magnon-mediated SOT is an effective method to manipulate a perpendicular magnetization, particularly using a critical switching current density that is pretty low, as evidenced by ∼ 4 × 105 A/cm2 for SrRuO3/NiO/SrIrO3 trilayers in this study. These findings suggest a promising avenue for the development of highly efficient spintronic devices operable through magnon currents.

2D van der Waals (vdW) ferromagnetic crystals are a promising platform for innovative spintronic devices based on magnetic skyrmions, thanks to their high flexibility and atomic thickness stability. However, room‐temperature skyrmion‐hosting vdW materials are scarce, which poses a challenge for practical applications. In this study, a chemical vapor transport (CVT) approach is employed to synthesize Fe₃GaTe₂ crystals and room‐temperature Néel skyrmions are observed in Fe₃GaTe₂ nanoflakes above 58 nm in thickness through in situ Lorentz transmission electron microscopy (L‐TEM). Upon an optimized field cooling procedure, zero‐field hexagonal skyrmion lattices are successfully generated in nanoflakes with an extended thickness range (30–180 nm). Significantly, these skyrmion lattices remain stable up to 355 K, setting a new record for the highest temperature at which skyrmions can be hosted. The research establishes Fe₃GaTe₂ as an emerging above‐room‐temperature skyrmion‐hosting vdW material, holding great promise for future spintronics.

Spin‐orbit torque resulting from non‐magnetic materials with strong spin‐orbit coupling enables electrically controlled magnetization switching, offering potential applications in ultralow‐power memory and logic devices. However, such switching of perpendicular magnetization usually requires an in‐plane magnetic field along the applied current direction, which limits its use. To address this challenge, an all‐oxide superlattice is designed and fabricated that show both the perpendicular magneto‐crystalline anisotropy and in‐plane magnetic anisotropies induced by interfacial engineering. The results demonstrate that the coexistence of perpendicular and in plane magnetic anisotropy breaks the symmetry and thus enables the pure electrical switching of perpendicular magnetization.