Morphology-Tunable Synthesis of Intrinsic Room-Temperature Ferromagnetic γ-Fe2O3 Nanoflakes

Spintronic Devices upon 2D Magnetic Materials and Heterojunctions

In spintronics, there has been increasing interest in two-dimensional (2D) magnetic materials. The well-defined layered crystalline structure, interface conditions, and van der Waals stacking of these materials offer advantages for the development of high-performance spintronic devices. Spin-orbit torque (SOT) devices and the tunneling magnetoresistance (TMR) effect based on these materials have emerged as prominent research areas. SOT devices utilizing 2D magnetic materials can efficiently achieve SOT-driven magnetization switching by modulating the interaction between spin and orbital degrees of freedom. Notably, crystal structure symmetry breaking in 2D magnetic heterojunctions leads to field-free perpendicular magnetization switching and an extremely low SOT-driven magnetization switching current density of down to 106 A/cm2. This review provides a comprehensive overview of the construction, measurement, and mechanisms of 2D SOT heterojunctions. The TMR effect observed in 2D materials also exhibits significant potential for various applications. Specifically, the spin-filter effect in layered A-type antiferromagnets has led to giant TMR ratios approaching 19,000%. Here, we review the physical mechanisms underlying the TMR effect, along with the design of high-performance devices such as magnetic tunnel junctions (MTJ) and spin valves. This review summarizes different structural types of 2D heterojunctions and key factors that enhance TMR values. These advanced devices show promising prospects in fields such as magnetic storage. We highlight significant advancements in the integration of 2D materials in SOT, MTJ, and spin valve devices, which offer advantages such as high-density storage capability, low-power computing, and fast data transmission rates for Magnetic Random Access Memory and logic integrated circuits. These advancements are expected to revolutionize future developments in information technology.

Spin Transport Modulation of 2D Fe3O4 Nanosheets Driven by Verwey Phase Transition

Realizing spin transport between heavy metal and two-dimensional (2D) magnetic materials at high Curie temperature (TC) is crucial to advanced spintronic information storage technology. Here, environmentally stable 2D nonlayered Fe3O4 nanosheets are successfully synthesized using a reproducible process and found that they exhibit vortex magnetic domains at room temperature. A Verwey phase transition temperature (TV) of ≈110 K is identified for ≈3 nm thick nanosheet through Raman characterization and spin Hall device measurement of the Pt/Fe3O4 bilayer. The anisotropic magnetoresistance ratio decreases near TV, while both the spin Hall magnetoresistance ratio and spin mixing conductance (Gr) increase at TV. As the temperature approaches 112 K, the anomalous Hall effect ratio tends to become zero. The maximum Gr reaches ≈5 × 1015 Ω-1m-2 due to the clean and flat interface between Pt and 2D nanosheet. The observed spin transport behavior in Pt/Fe3O4 spin Hall devices indicates that 2D Fe3O4 nanosheets possess potential for high-power micro spintronic storage devices applications.

Multi-Level Switching of Spin-Torque Ferromagnetic Resonance in 2D Magnetite

2D magnetic materials hold substantial promise in information storage and neuromorphic device applications. However, achieving a 2D material with high Curie temperature (TC), environmental stability, and multi-level magnetic states remains a challenge. This is particularly relevant for spintronic devices, which require multi-level resistance states to enhance memory density and fulfil low power consumption and multi-functionality. Here, the synthesis of 2D non-layered triangular and hexagonal magnetite (Fe3O4) nanosheets are proposed with high TC and environmental stability, and demonstrate that the ultrathin triangular nanosheets show broad antiphase boundaries (bAPBs) and sharp antiphase boundaries (sAPBs), which induce multiple spin precession modes and multi-level resistance. Conversely, the hexagonal nanosheets display slip bands with sAPBs associated with pinning effects, resulting in magnetic-field-driven spin texture reversal reminiscent of "0" and "1" switching signals. In support of the micromagnetic simulation, direct explanation is offer to the variation in multi-level resistance under a microwave field, which is ascribed to the multi-spin texture magnetization structure and the randomly distributed APBs within the material. These novel 2D magnetite nanosheets with unique spin textures and spin dynamics provide an exciting platform for constructing real multi-level storage devices catering to emerging information storage and neuromorphic computing requirements.

Highly Robust Room-Temperature Interfacial Ferromagnetism in Ultrathin Co2Si Nanoplates

The reduced dimensionality and interfacial effects in magnetic nanostructures open the feasibility to tailor magnetic ordering. Here, we report the synthesis of ultrathin metallic Co2Si nanoplates with a total thickness that is tunable to 2.2 nm. The interfacial magnetism coupled with the highly anisotropic nanoplate geometry leads to strong perpendicular magnetic anisotropy and robust hard ferromagnetism at room temperature, with a Curie temperature (TC) exceeding 950 K and a coercive field (HC) > 4.0 T at 3 K and 8750 Oe at 300 K. Theoretical calculations suggest that ferromagnetism originates from symmetry breaking and undercoordinated Co atoms at the Co2Si and SiO2 interface. With protection by the self-limiting intrinsic oxide, the interfacial ferromagnetism of the Co2Si nanoplates exhibits excellent environmental stability. The controllable growth of ambient stable Co2Si nanoplates as 2D hard ferromagnets could open exciting opportunities for fundamental studies and applications in Si-based spintronic devices.

Controlled Growth and Size-Dependent Magnetic Domain States of 2D γ-Fe2O3

Nonlayered two-dimensional (2D) magnets have attracted special attention, as many of them possess magnetic order above room temperature and enhanced chemical stability compared to most existing vdW magnets, which offers remarkable opportunities for developing compact spintronic devices. However, the growth of these materials is quite challenging due to the inherent three-dimensionally bonded nature, which hampers the study of their magnetism. Here, we demonstrate the controllable growth of air-stable pure γ-Fe2O3 nanoflakes by a confined-vdW epitaxial approach. The lateral size of the nanoflakes could be adjusted from hundreds of nanometers to tens of micrometers by precisely controlling the annealing time. Interestingly, a lateral-size-dependent magnetic domain configuration was observed. As the sizes continuously increase, the magnetic domain evolves from single domain to vortex and finally to multidomain. This work provides guidance for the controllable synthesis of 2D inverse spinel-type crystals and expands the range of magnetic vortex materials into magnetic semiconductors.

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.

Deep eutectic solvent electrolysis for preparing water-soluble magnetic iron oxide nanoparticles

Magnetic iron oxide nanoparticles have been proven to have versatile applications in biomedicine. Although numerous strategies have been developed to synthesize hydrophilic magnetic nanoparticles, there is still a challenge in the quantity and controllability of preparation of highly dispersible, stably water-dispersive magnetic nanoparticles. The current work presents a deep-eutectic solvent electrolysis to synthesize magnetic nanoparticles. In the electrolysis process, iron atoms at the anode electrode are oxidized to ferric ions, and then the ferric ions are combined with reactive oxygen species that derived from the decomposition of deep-eutectic solvents to form iron oxide nanocrystals. Concomitantly, hydrophilic radicals of amine groups produced by electrolyte decomposition are grafted on the particles. The monodisperse nanoparticle size ranged from 6 to 9 nm. The hydrophilic group loaded nanoparticles can be highly dispersed in water with neither surface post-modification nor organic stabilizers. The hydrodynamic particle diameter is between 20 and 30 nm. The transparent aqueous dispersions can be maintained for more than 600 days without precipitation.