2D Magnetic Semiconductor Fe₃GeTe₂ with Few and Single Layers with a Greatly Enhanced Intrinsic Exchange Bias by Liquid-Phase Exfoliation

Abstract

A 2D van der Waals (vdW) magnet can get rid of the constraints of lattice matching and compatibility and then create a variety of vdW heterostructures, which provides a opportunity for spintronic devices. However, the ability to reliably exfoliate large, high-quality vdW ferromagnetic Fe 3 GeTe 2 (FGT) nanoflakes in scaled-up production is severely limited. Herein, an efficient and stable three-stage sonication-assisted liquid-phase exfoliation was developed for mass preparation of high-structural-integrity fewand single-layer FGT nanoflakes with a greatly enhanced intrinsic exchange bias. The three stages include slicing crystals, weakening interlayer vdW forces, and using ultrasonic cavitation. The highest yield of FGT nanoflakes is 22.3 wt % with single layers accounting for 6%. The size is controllable, and several micrometers, tens of micrometers, and a maximum of 103 μm are available. The 200 mg level output has overcome the limitations of mechanical exfoliation and molecular beam epitaxy in economically amplificated production. An intrinsic exchange bias is observed in the restacked nanoflakes due to the magnetic proximity on the interface of the FGT/natural surface oxide layer. The material reaches 578 Oe (2 K) and 2300 Oe after further oxidation, at least 250% higher than other precisely tailored vdW magnetic heterostructures. In addition, the unusual semiconductivity of the liquid-phase exfoliated FGT nanoflakes is reported. This work skillfully utilizes oxidation to enhance the potential of FGT for large-scale spintronics, optoelectronics, efficient data storage, and various extended applications, and it is beneficial for exfoliating other promising magnetic vdW materials.

Giant and Nonvolatile Control of Exchange Bias in Fe3GeTe2/Irradiated Fe3GeTe2/MgO Heterostructure Through Ultralow Voltage

“…Owing to the high Curie temperature and controllable magnetism of FGT, [27,32,33] and the unique interfaces of FM/oxides, we deposited a MgO thin film, one of the most efficient tunnel barriers in spin valves, on FGT nanoflakes to study the voltagecontrolled magnetic behavior of FGT/MgO heterostructures. We showed that an ionic-irradiated phase is formed in the FGT during MgO sputtering, and an exchange bias effect is obtained due to the coupling between the FGT and the irradiated phase.…”

Recent advances in two-dimensional intrinsic ferromagnetic materials Fe3X(X=Ge and Ga)Te2 and their heterostructures for spintronics

“…120 It has been demonstrated that the H EB in Fe 3 GeTe 2 /FePS 3 vdWHs can increase from 291 to 1112 Oe when the interlayer spacing varies from 6.6 to 4.2 Å at 5 K. 123 Notably, a remarkable H EB of 2300 Oe was reported in naturally oxidized few-layer Fe 3 GeTe 2 fabricated by sonication-assisted liquid-phase exfoliation in 2022. 81 It should be emphasized that the nature of the EB effect is interfacial magnetic coupling, in which the spins of the FM layer are efficiently pinned by the AFM layer at the interface. The FM and AFM layers with the collinear spin alignment are more likely to induce the EB effect.…”

“…The Al 2 O 3 -assisted mechanical exfoliation method was adopted by Deng et al 53 to obtain high-quality monolayer Fe 3 GeTe 2 owing to the weak interlayer interaction. In 2022, Ma et al 81 utilized a three-stage sonication-assisted liquid-phase exfoliation to prepare few-layer Fe 3 GeTe 2 with a size of ∼1–37 μm, in which ball milling was used to reduce the thickness and size, stirring to expand the layer spacing and reduce the interlayer vdW force, and ultrasonic cavitation to promote exfoliation.…”

“…120 It has been demonstrated that the H EB in Fe 3 GeTe 2 /FePS 3 vdWHs can increase from 291 to 1112 Oe when the interlayer spacing varies from 6.6 to 4.2 Å at 5 K. 123 Notably, a remarkable H EB of 2300 Oe was reported in naturally oxidized few-layer Fe 3 GeTe 2 fabricated by sonication-assisted liquid-phase exfoliation in 2022. 81…”

Non-Exchange Bias in Binary Nanoparticle Systems

“…Other related effects, like coercivity enhancement, high order anisotropies or asymmetric reversal are typically observed in these systems [1][2][3] . Such effects have been extensively studied in thin films due to the intriguing fundamental properties and their key role in many spintronic devices 2,[4][5][6][7][8][9][10][11] . Similarly, exchange bias in exchange-coupled core/shell nanoparticles is attracting a great deal of interest 2,12,13 due both to its novel basic properties and to the broad range of potential applications (e.g., magnetic recording, permanent magnets, or magnetic hyperthermia, among others) 2,[11][12][13][14][15][16][17][18] .…”