Near Degeneracy of Magnetic Phases in Two-Dimensional Chromium Telluride with Enhanced Perpendicular Magnetic Anisotropy

Controlled Growth of Submillimeter-Scale Cr5Te8 Nanosheets and the Domain Wall Nucleation Governed Magnetization Reversal Process

Two-dimensional (2D) ferromagnets have attracted widespread attention for promising applications in compact spintronic devices. However, the controlled synthesis of high-quality, large-sized, and ultrathin 2D magnets via facile, economical method remains challenging. Herein, we develop a hydrogen-tailored chemical vapor deposition approach to fabricating 2D Cr5Te8 ferromagnetic nanosheets. Interestingly, the time period of introducing hydrogen was found to be crucial for controlling the lateral size, and a Cr5Te8 single-crystalline nanosheet of lateral size up to ∼360 μm with single-unit-cell thickness has been obtained. These samples exhibit a leading role of domain wall nucleation in governing the magnetization reversal process, providing important references for optimizing the performances of associated devices. The nanosheets also show notable magnetotransport response, including nonmonotonous magnetic-field-dependent magnetoresistance and sizable anomalous Hall resistivity, demonstrating Cr5Te8 as a promising material for constructing high-performance magnetoelectronic devices. This study presents a breakthrough of large-sized CVD-grown 2D magnetic materials, which is indispensable for constructing 2D spintronic devices.

2D Ferroic Materials for Nonvolatile Memory Applications

The emerging nonvolatile memory technologies based on ferroic materials are promising for producing high-speed, low-power, and high-density memory in the field of integrated circuits. Long-range ferroic orders observed in 2D materials have triggered extensive research interest in 2D magnets, 2D ferroelectrics, 2D multiferroics, and their device applications. Devices based on 2D ferroic materials and heterostructures with an atomically smooth interface and ultrathin thickness have exhibited impressive properties and significant potential for developing advanced nonvolatile memory. In this context, a systematic review of emergent 2D ferroic materials is conducted here, emphasizing their recent research on nonvolatile memory applications, with a view to proposing brighter prospects for 2D magnetic materials, 2D ferroelectric materials, 2D multiferroic materials, and their relevant devices.

Strain-tunable Berry curvature in quasi-two-dimensional chromium telluride

Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr₂Te₃ thin films manifests a unique temperature-dependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr₂Te₃ epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr₂Te₃ near the coercive field during the magnetization switching process, owing to the presence of strain-modulated magnetic layers/domains. The versatile interface tunability of Berry curvature in Cr₂Te₃ thin films offers new opportunities for topological electronics.

Controllable CVD Growth of 2D Cr5Te8 Nanosheets with Thickness-Dependent Magnetic Domains

As a unique 2D magnetic material with self-intercalated structure, Cr₅Te₈ exhibits many intriguing magnetic properties. While its ferromagnetism of Cr₅Te₈ has been previously reported, the research on its magnetic domain remains unexplored. Herein, we have successfully fabricated 2D Cr₅Te₈ nanosheets with controlled thickness and lateral size by chemical vapor deposition (CVD). Then magnetic property measurement system revealed Cr₅Te₈ nanosheets exhibiting intense out-of-plane ferromagnetism with a Curie temperature (T_C) of 176 K. Significantly, we reported for the first time two magnetic domains: magnetic bubbles and thickness-dependent maze-like magnetic domains in our Cr₅Te₈ nanosheets by cryogenic magnetic force microscopy (MFM). The domain width of the maze-like magnetic domains increases rapidly with decreasing sample thickness; meanwhile, the domain contrast decreases. This indicates the dominant role of ferromagnetism shifts from dipolar interactions to magnetic anisotropy. Our research not only establishes a pathway for the controllable growth of 2D magnetic materials but also points toward novel avenues for regulating magnetic phases and methodically tuning domain characteristics.

Heteroepitaxy of 2D CuCr2 Te4 with Robust Room-temperature Ferromagnetism

Magnetic materials in 2D have attracted widespread attention for their intriguing magnetic properties. 2D magnetic heterostructures can provide unprecedented opportunities for exploring fundamental physics and novel spintronic devices. Here, the heteroepitaxial growth of ferromagnetic CuCr₂ Te₄ nanosheets is reported on Cr₂ Te₃ and mica by chemical vapor deposition. Magneto-optical Kerr effect measurements reveal the thickness-dependent ferromagnetism of CuCr₂ Te₄ nanosheets on mica, where a decrease of Curie temperature (T_C ) from 320 to 260 K and an enhancement of perpendicular magnetic anisotropy with reducing thickness are observed. Moreover, lattice-matched heteroepitaxial ultrathin CuCr₂ Te₄ on Cr₂ Te₃ exhibits an enhanced robust ferromagnetism with T_C up to 340 K due to the interfacial charge transfer. Stripe-type magnetic domains and single magnetic domain are discovered in this heterostructure with different thicknesses. The work provides a way to construct robust room-temperature 2D magnetic heterostructures for functional spintronic devices.

Growth, structure, and morphology of van der Waals epitaxy Cr1+δTe2 films

The preparation of two-dimensional magnetic materials is a key process to their applications and the study of their structure and morphology plays an important role in the growth of high-quality thin films. Here, the growth, structure, and morphology of Cr_1+δTe₂ films grown by molecular beam epitaxy on mica with variations of Te/Cr flux ratio, growth temperature, and film thickness have been systematically investigated by scanning tunneling microscopy, reflection high-energy electron diffraction, scanning electron microscope, and X-ray photoelectron spectroscopy. We find that a structural change from multiple phases to a single phase occurs with the increase in growth temperature, irrespective of the Cr/Te flux ratios, which is attributed to the desorption difference of Te atoms at different temperatures, and that the surface morphology of the films grown at relatively high growth temperatures (≥ 300 °C) exhibits a quasi-hexagonal mesh-like structure, which consists of nano-islands with bending surface induced by the screw dislocations, as well as that the films would undergo a growth-mode change from 2D at the initial stage in a small film thickness (2 nm) to 3D at the later stage in thick thicknesses (12 nm and 24 nm). This work provides a general model for the study of pseudo-layered materials grown on flexible layered substrates.

A general thermodynamics-triggered competitive growth model to guide the synthesis of two-dimensional nonlayered materials

Two-dimensional (2D) nonlayered materials have recently provoked a surge of interest due to their abundant species and attractive properties with promising applications in catalysis, nanoelectronics, and spintronics. However, their 2D anisotropic growth still faces considerable challenges and lacks systematic theoretical guidance. Here, we propose a general thermodynamics-triggered competitive growth (TTCG) model providing a multivariate quantitative criterion to predict and guide 2D nonlayered materials growth. Based on this model, we design a universal hydrate-assisted chemical vapor deposition strategy for the controllable synthesis of various 2D nonlayered transition metal oxides. Four unique phases of iron oxides with distinct topological structures have also been selectively grown. More importantly, ultra-thin oxides display high-temperature magnetic ordering and large coercivity. Mn_xFe_yCo_3-x-yO₄ alloy is also demonstrated to be a promising room-temperature magnetic semiconductor. Our work sheds light on the synthesis of 2D nonlayered materials and promotes their application for room-temperature spintronic devices.

Room-Temperature Magnetic Skyrmions and Large Topological Hall Effect in Chromium Telluride Engineered by Self-Intercalation

Room-temperature magnetic skyrmion materials exhibiting robust topological Hall effect (THE) are crucial for novel nano-spintronic devices. However, such skyrmion-hosting materials are rare in nature. In this study, a self-intercalated transition metal dichalcogenide Cr_{1+ x} Te₂ with a layered crystal structure that hosts room-temperature skyrmions and exhibits large THE is reported. By tuning the self-intercalate concentration, a monotonic control of Curie temperature from 169 to 333 K and a magnetic anisotropy transition from out-of-plane to the in-plane configuration are achieved. Based on the intercalation engineering, room-temperature skyrmions are successfully created in Cr_1.53 Te₂ with a Curie temperature of 295 K and a relatively weak perpendicular magnetic anisotropy. Remarkably, a skyrmion-induced topological Hall resistivity as large as ≈106 nΩ cm is observed at 290 K. Moreover, a sign reversal of THE is also found at low temperatures, which can be ascribed to other topological spin textures having an opposite topological charge to that of the skyrmions. Therefore, chromium telluride can be a new paradigm of the skyrmion material family with promising prospects for future device applications.

Highly Tunable Beyond-Room-Temperature Intrinsic Ferromagnetism in Cr-Doped Topological Crystalline Insulator SnTe Crystals

The combination of topological phase and intrinsic beyond-room-temperature ferromagnetism is expected to realize the quantum anomalous Hall effect at a high temperature. However, no beyond-room-temperature intrinsic ferromagnetism has been reported in either topological insulator or topological crystalline insulator (TCI) so far. Here, we report Cr-doping in TCI-phase SnTe crystals which possess highly tunable beyond-room-temperature intrinsic ferromagnetism including T_c, magnetic moment, and coercivity by varying Cr contents and crystal thickness. With the increase of the Cr content, the T_c increases by 159 K from 221 to 380 K and the saturation magnetic moments increase by ∼23.6 times from 0.018 to 0.421 μ_B/f.u. This intrinsic beyond-room-temperature ferromagnetism is fully demonstrated by the anomalous Hall effect and magneto-optical Kerr effect in a single Cr_xSn_1-xTe nanosheet. Moreover, the room-temperature tunneling magnetoresistance effect has been realized by using a Cr_xSn_1-xTe flake, a Fe thin film, and a commercially compatible ultrathin AlO_x tunneling barrier. This work indicates a great potential of Cr_xSn_1-xTe crystals in room-temperature magnetoelectronic and spintronic devices.

Emergent Topological Hall Effect from Exchange Coupling in Ferromagnetic Cr2Te3/Noncoplanar Antiferromagnetic Cr2Se3 Bilayers

The topological Hall effect has been observed in magnetic materials of complex spin structures or bilayers of trivial magnets and strong spin-orbit-coupled systems. In view of current attention on dissipationless topological electronics, the occurrence of the topological Hall effect in new systems or by an unexpected mechanism is fascinating. Here, we report a robust topological Hall effect generated in bilayers of a ferromagnet and a noncoplanar antiferromagnet, from the interfacial Dzyaloshinskii-Moriya interaction due to the exchange coupling of magnetic layers. Molecular beam epitaxy has been utilized to fabricate heterostructures of a ferromagnetic metal Cr₂Te₃ and a noncoplanar antiferromagnet Cr₂Se₃. A significant topological Hall effect at low temperature implies the development of nontrivial spin chirality, and density functional theory calculations explain the correlation of the Dzyaloshinskii-Moriya interaction increase and inversion symmetry breaking at the interface. The presence of noncoplanar ordering in the antiferromagnet plays a pivotal role in producing the topological Hall effect. Our results suggest that the exchange coupling in ferromagnet/noncoplanar antiferromagnet bilayers could be an alternative mechanism toward topologically protected magnetic structures.

Engineered neutrophil-derived exosome-like vesicles for targeted cancer therapy

Neutrophils are the most abundant innate immune cells in human circulation; however, their derived exosomes have been rarely studied for tumor treatment. Here, we reported that exosomes from neutrophils (N-Ex) induce tumor cell apoptosis by delivering cytotoxic proteins and activating caspase signaling pathway. In addition, we decorated N-Ex with superparamagnetic iron oxide nanoparticles (SPIONs) to achieve higher tumor-targeting therapeutic effect. We further fabricated exosome-like nanovesicles from neutrophils (NNVs) at high yield. Compared with liposome-loaded doxorubicin (DOX) and natural NNVs, DOX-loaded NNVs show an improved inhibition of tumor cell proliferation. Moreover, DOX-loaded, SPION-decorated NNVs selectively accumulate at the tumor sites under an external magnetic field, effectively restraining tumor growth and extensively prolonging the survival rate in mice. Overall, a simple and effective method to engineer N-Ex and NNVs at clinical applicable scale was developed, which enables the efficient and safe drug delivery for targeted and combined tumor therapy.

Van der Waals Superstructure and Twisting in Self-Intercalated Magnet with Near Room-Temperature Perpendicular Ferromagnetism

The emergence of van der Waals (vdW) magnets has created unprecedented opportunities to manipulate magnetism for advanced spintronics based upon all-vdW heterostructures. Among various vdW magnets, Cr_1+δTe₂ possesses high temperature ferromagnetism along with possible topological spin textures. As this system can support self-intercalation in the vdW gap, it is crucial to precisely pinpoint the exact intercalation to understand the intrinsic magnetism of the system. Here, we developed an iterative method to determine the self-intercalated structures and show evidence of vdW "superstructures" in individual Cr_1+δTe₂ nanoplates exhibiting magnetic behaviors distinct from bulk chromium tellurides. Among 26,332 possible configurations, we unambiguously identified the Cr-intercalated structure as 3-fold symmetry broken Cr_1.5Te₂ segmented by vdW gaps. Moreover, a twisted Cr-intercalated layered structure is observed. The spontaneous formation of twisted vdW "superstructures" not only provides insight into the diverse magnetic properties of intercalated vdW magnets but may also add complementary building blocks to vdW-based spintronics.

Observation of Exchange Bias in Antiferromagnetic Cr0.79Se Due to the Coexistence of Itinerant Weak Ferromagnetism at Low Temperatures

We report on the structural, electrical transport, and magnetic properties of antiferromagnetic transition-metal monochalcogenide Cr0.79Se. Different from the existing off-stoichiometric compositions, Cr0.79Se is found to be synthesized into the same NiAs-type hexagonal crystal structure as that of CrSe. Resistivity data suggest Cr0.79Se to be a Fermi-liquid-type metal at low temperatures, while at intermediate temperatures, the resistivity depends sublinearly on the temperature. Eventually, at elevated temperatures, the rate of change of resistivity rapidly decreases with increasing temperature. Magnetic measurements suggest a transition from the paramagnetic phase to an antiferromagnetic phase at a Néel temperature of 225 K. Further reduction of the sample temperature results in the coexistence of weak ferromagnetism along with the antiferromagnetic phase below 100 K. As a result, below 100 K, we identify a significant exchange bias due to the interaction between the ferro- and antiferromagnetic phases. In addition, from temperature-dependent X-ray diffraction measurements, we observe that the NiAs-type structure is stable up to as high as 600 °C.