Zhijie Chen

Chiral materials and multiferroics offer symmetry-controlled electronic and magnetic functionalities, yet their integration in two-dimensional systems remains challenging due to the difficulty of simultaneously sustaining chirality, ferroelectricity, and ferromagnetism at practical temperatures. Here we introduce a chiral molecular intercalation strategy to construct chiral 2D multiferroics by inserting enantiomeric molecules into layered ferroelectric CuInP₂S₆ and ferromagnetic Fe₃GaTe₂. Molecular insertion reshapes the interfacial electrostatic environment, induces charge redistribution, and expands the interlayer spacing, resulting in enhanced ferroic order, including a 5-fold increase in magnetic anisotropy energy (0.35→1.6 meV/Fe) and strengthened ferroelectric polarization. The resulting chiral CIPS–FGT heterostructures exhibit robust room-temperature magnetoelectric coupling (∼4.8% magnetization modulation) and enable helicity-dependent control of ferroic states under circularly polarized light, producing a 54.4% resistance modulation. This work establishes molecular intercalation as a general strategy for engineering light-responsive 2D multiferroics for optically tunable magnetoelectric and spintronic devices.

Quantum oscillations in electrical transport have long served as a powerful probe of fundamental Fermi surface physics. To date, however, such studies have been restricted to linear electrical transport. This letter presents the experimental observation of quantum oscillations in nonlinear electrical transport and, more importantly, demonstrates their ability to uncover key features of the Fermi surface that lie beyond the reach of linear-transport quantum-oscillation techniques. Using α-Sn, a topological Dirac semimetal known to support both linear and nonlinear transport, this study shows that quantum oscillations of nonlinear resistance are highly sensitive to both the geometry and spin texture of the Fermi contour-features to which linear counterparts are insensitive. Further, nonlinear-transport oscillations exhibit markedly distinct dependencies on magnetic field and temperature compared to their linear analogs. These findings establish nonlinear-transport quantum oscillations as a transformative tool for exploring Fermi surface physics, opening new avenues for revealing hidden Fermi surface properties in known materials and discovering new exotic electronic states and phases.

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).

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 CMOS-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 the exchange bias effect by 23% and 0.1 T (16%) at 5 K, respectively. These findings highlight the characteristics 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.

Three-dimensional (3D) nanomagnetic systems offer promise toward implementing neuromorphic computing due to their intricate spin textures, magnetization dynamics, and nontrivial topology. However, the investigation of 3D nanomagnetic systems is often constrained by demanding fabrication and characterization requirements. Here, we present interconnected networks of self-assembled magnetic nanowires (NW) as a novel 3D platform with attractive characteristics for neuromorphic computing. The networks contain multiple unique transport pathways, each hosting discrete magnetization states. These pathways can be selectively addressed, and the magnetic state within them can be electrically controlled by applying current pulses. Consequently, the pathways can serve as synaptic weights, allowing for diverse programming by switching specific sections of the network using current pulses of varying magnitudes and durations. Additionally, unique features such as history-dependent magnetic state switching and interconnected transport paths are observed in these networks. These capabilities are leveraged to illustrate the potential of interconnected magnetic NW networks as reservoir layers in a neural network architecture, highlighting their promise as an efficient platform for neuromorphic computing.

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

Ferrimagnets have received renewed attention as a promising platform for spintronic applications. Of particular interest is the Mn4N from the ε-phase of the manganese nitride as an emergent rare-earth-free spintronic material due to its perpendicular magnetic anisotropy, small saturation magnetization, high thermal stability, and large domain wall velocity. We have achieved high-quality (001)-ordered Mn4N thin film by sputtering Mn onto η-phase Mn3N2 seed layers on Si substrates. As the deposited Mn thickness varies, nitrogen ion migration across the Mn3N2/Mn layers leads to a continuous evolution of the layers to Mn3N2/Mn2N/Mn4N, Mn2N/Mn4N, and eventually Mn4N alone. The ferrimagnetic Mn4N, indeed, exhibits perpendicular magnetic anisotropy and forms via a nucleation-and-growth mechanism. The nitrogen ion migration is also manifested in a significant exchange bias, up to 0.3 T at 5 K, due to the interactions between ferrimagnetic Mn4N and antiferromagnetic Mn3N2 and Mn2N. These results demonstrate a promising all-nitride magneto-ionic platform with remarkable tunability for device applications.

Two-dimensional (2D) magnetic van der Waals materials provide a powerful platform for studying the fundamental physics of low-dimensional magnetism, engineering novel magnetic phases, and enabling thin and highly tunable spintronic devices. To realize high-quality and practical devices for such applications, there is a critical need for robust 2D magnets with ordering temperatures above room temperature that can be created via exfoliation. Here, the study of exfoliated flakes of cobalt-substituted Fe₅GeTe₂ (CFGT) exhibiting magnetism above room temperature is reported. Via quantum magnetic imaging with nitrogen-vacancy centers in diamond, ferromagnetism at room temperature was observed in CFGT flakes as thin as 16 nm corresponding to 16 layers. This result expands the portfolio of thin room-temperature 2D magnet flakes exfoliated from robust single crystals that reach a thickness regime relevant to practical spintronic applications. The Curie temperature T_c of CFGT ranges from 310 K in the thinnest flake studied to 328 K in the bulk. To investigate the prospect of high-temperature monolayer ferromagnetism, Monte Carlo calculations were performed, which predicted a high value of T_c of ∼270 K in CFGT monolayers. Pathways toward further enhancing monolayer T_c are discussed. These results support CFGT as a promising platform for realizing high-quality room-temperature 2D magnet devices.

Interconnected magnetic nanowire (NW) networks offer a promising platform for three-dimensional (3D) information storage and integrated neuromorphic computing. Here we report discrete propagation of magnetic states in interconnected Co nanowire networks driven by magnetic field and current, manifested in distinct magnetoresistance (MR) features. In these networks, when only a few interconnected NWs were measured, multiple MR kinks and local minima were observed, including a significant minimum at a positive field during the descending field sweep. Micromagnetic simulations showed that this unusual feature was due to domain wall (DW) pinning at the NW intersections, which was confirmed by off-axis electron holography imaging. In a complex network with many intersections, sequential switching of nanowire sections separated by interconnects was observed, along with stochastic characteristics. The pinning/depinning of the DWs can be further controlled by the driving current density. These results illustrate the promise of such interconnected networks as integrated multistate memristors.