Controlling Exchange Interactions and Emergent Magnetic Phenomena in Layered 3d‐Orbital Ferromagnets

Layered 3d‐orbital ferromagnet is an ideal research platform to experimentally achieve intrinsic 2D ferromagnetism and theoretically study the quantum nature of magnetic exchange interactions therein. A variety of magnetic phases can emerge from the strongly correlated feature of 3d‐orbital electrons, in which their exchange interactions can be effectively modulated by various kinds of external stimuli. Therefore, controlling the emergent magnetic phenomena of layered 3d‐orbital ferromagnets is significant in both fundamental science and practical applications. Considering the roles of magnetic exchange interactions, this review summarizes recent progress in controlling the emergent magnetic properties of layered 3d‐orbital ferromagnets by systematically introducing modulation methods, underlying mechanisms, and device applications. The existing challenges and future prospects for this research field are also outlined, shedding light on finding optimized magnetic materials, exploring powerful modulation techniques, and designing multifunctional new concept devices.


Introduction
The quantum nature of 3d-orbital electrons and their exchange interactions play key roles in generating emergent correlated phenomena in magnetic materials. Since the magnetic fluctuations increase and the exchange interactions change with reducing dimensions, [1][2][3] achieving the long-range ordering of magnetic moments (magnetic ordering) in the 2D limit is difficult: the Mermin-Wagner theorem concludes that ferromagnetic (FM) or anti-ferromagnetic (AFM) ordering can hardly exist at a finite temperature for the 2D case. [4] Surprisingly, with the rise of 2D materials in the last decade, van der Waals (vdW) layered 3dorbital magnetic materials that can realize magnetic anisotropy DOI: 10.1002/apxr.202200106 down to the atomically-thin limit sparked extensive theoretical and experimental research.  Abundant quantum phenomena in 2D vdW ferromagnets with 3d-orbitals have been observed, such as quantum anomalous Hall effect, [21] giant/colossal magnetoresistance effect, [27] moiré ferromagnetism, [28][29][30] and valley magnetization effect. [31] Layered 3dorbital ferromagnets also offer a great opportunity to fabricate 2D spintronic devices with high interface quality and new operating means, such as spin valve, [32] spin filter [33] and spin transistor [34] devices, which have all been successfully designed and demonstrated. Therefore, investigating and controlling the emergent magnetic phenomena of layered 3dorbital ferromagnets are significant in both fundamental science and practical applications. Normally, the larger Hund's coupling and the weaker spinorbital coupling (SOC) in a 3d-orbital magnetic electronic system, compared to those of its 4d-and 5d-orbital counterparts, [35][36][37][38] provide researchers with a great opportunity to generate emergent correlated phenomena and control magnetic properties in such materials for the following reasons: due to the smaller atomic number and larger Hund's coupling of 3d transition metal elements, [36] the L-S coupling scheme is appropriate for describing the total angular momentum J of 3d-orbital magnetic materials. Moreover, with stronger Coulomb interactions between electrons U (generally in the range of 5 − 7 eV), the total angular momentum J can have larger values of net Bohr magnetic moments μ B . Due to the smaller angular and magnetic quantum numbers (l and m) of 3d-orbitals, the coupling between their orbital angular momentum and spin angular momentum is also weaker, and the SOC effect of 3d-orbital electrons is generally on the order of 0.01 − 0.1 eV. [38] This indicates that the SOC effect in 3d-orbital electronic systems can be considered as a perturbation term so that theoretical models can provide a fairly accurate picture of the experimental results. Therefore, 3dorbital magnetic materials can serve as an excellent platform for understanding the physical properties of strongly correlated electronic systems.
Based on the Hubbard model describing the quantum nature of 3d-orbital electrons, two competitive parameters of band width (from the hopping effects of electrons between neighboring atomic sites) and Coulomb interactions U (the repulsive effect between electrons while located at the same atomic sites) are always used to determine the ground state of 3d-orbital magnetic Ising model for various magnetic materials can all be formally written asĤ ex = ∑ i,j J ij (S i ⋅ S j ) (a schematic illustration of J and S is shown in the middle right corner). Correspondingly, the microscopic mechanism of tuning the magnetic properties can be divided into tuning the exchange interaction term J (upper right corner) or the value and orientation of S (lower right corner). Reproduced with permission. [40] Copyright 2019, American Physics Society.
materials. For itinerant 3d-orbital FM metals such as -Fe, Co, and Ni, the magnetism mainly originates from the splitting of the spin-degenerated bands of the itinerant electrons, which makes the difference in the density of states (DOS) of the spin-up and spin-down electrons at the Fermi energy E F . This causes an increase in the kinetic energy of itinerant electrons on the order of 1/DOS, while lowering the Coulomb repulsive energy on the order of U, thus leading to the Stoner criterion of the appearance of ferromagnetism when U > 1/DOS. Correspondingly, the Stoner-Wohlfarth model [39] was presented to describe the magnetism therein.
When the localized magnetic moments of the 3d-orbital magnets dominate the magnetic properties of materials (mainly magnetic semiconductors), the magnetism mainly originates from exchange interactions between them. This exchange interaction can be formally written asĤ ex = ∑ i,j J ij (S i ⋅ S j ), where J ij is the exchange coupling constant between two effective microscopic magnetic moments S at different atomic sites i and j. Several exchange mechanisms have been proposed and extensively studied, including the direct Heisenberg exchange interactions between 3d-orbital electrons, and the double-exchange interaction, as well as Anderson super-exchange interactions via electrons from neighboring coordinated ions. The magnitude of J, together with the magnitude and dimension of the effective magnetic moment S, varies in different specific material systems. In particular, the positivity (negativity) of the exchange integral J results in the situation in which the local magnetic moments prefer the antiparallel (parallel) alignment, thus leading to macroscopic antiferromagnetism (ferromagnetism). Note that in real materials, there are more or less interactions between the itinerant electrons and the local magnetic moments. Therefore, the interplay between them leads to the complicated and various magnetic states therein. Thus, the tight-bonding model is always adopted to describe the magnetic crystal, and the details of the exchange interaction between the effective magnetic moment at different atomic sites are the focus, for exploring the exotic quantum magnetic phenomena therein.
In this review, we summarize the recent research progress on controlling the emergent magnetic properties of layered 3d-orbital ferromagnets, systematically introducing modulation methods, relevant applications, and their underlying mechanisms in the framework of exchange interactions. We conclude this review with an outlook on the remaining puzzling subjects, including the search for tunable layered ferromagnets with beyond-room-temperature ferromagnetism, the development of advanced characterization tools and more-effective regulation techniques, and the exploration of possible material integration to achieve the practicality of ferromagnet-based spintronic and electronic devices.

The Framework of Magnetic Tuning in Layered 3d-Orbital Ferromagnets
Regarding the roles of the microscopic magnetic moments S in 3d-orbital magnetic materials, three types of statistical lattice spin models [1][2][3]40] are frequently discussed as the microscopic mechanism, by considering the dimensions of the effective magnetic moments: Heisenberg, XY and Ising models (shown in the left panel of Figure 1). 1) In the Heisenberg model, the effective magnetic moments S of the atomic spins in the crystal lattice can orient with a spin dimensionality of n = 3, in which it is generally isotropic for the spin orientation; [15] 2) In the XY model, the effective magnetic moments S of the atomic spins can orient with a spin dimensionality of n = 2, and the spin system is anisotropic because the spins lie within a plane; [41][42][43] 3) In the Ising model, the orientation of the effective magnetic moments S points only up or down (i.e., the spin dimensionality of n = 1), and the material described by this model has strong uniaxial anisotropy of the spin orientation. [44] www.advancedsciencenews.com

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Note that the exchange interaction J in the three kinds of microscopic lattice spin models for magnetism is usually studied in a 2D lattice where only the intralayer exchange interactions are considered. However, a more realistic model for describing the origin of magnetism in layered materials should also properly take account of the interlayer exchange interactions. For example, the magnitude of J in a specific material, together with the magnitude and dimension of the effective magnetic moment S therein, might be associated with the sample thickness. This is because the dimensions of the spin lattices play important roles in determining the ground states of magnetic systems, in which the quantum fluctuations compete with the long-range magnetic orderings with scaling down the sample thickness, and the Curie temperature of layered FM materials tends to decrease significantly. Correspondingly, the Néel temperature of layered AFM materials displays a similar tendency with decreasing thickness. Notably, even the same material should be described by different models in different situations. For example, some layered materials undergo transitions from in-plane spontaneous magnetization (described by the XY model) to out-of-plane spontaneous magnetization (described by the Ising model) as the thickness decreases. Nevertheless, in the low spin dimension cases (spin dimensionality n = 1 or 2), the induction of magnetic anisotropy is applicable for long-range magnetism by breaking the rotation symmetry of spin.
Importantly, the value of exchange interaction J in layered 3dorbital magnetic materials can display strong anisotropy due to the huge differences between the magnitudes of the intralayer and interlayer exchange couplings, originating from the strong in-plane/out-of-plane anisotropy of the layered magnetic materials. Typically, the values of intralayer interaction J are significantly larger than the interlayer J values. Interestingly, some layered magnetic materials have negative intralayer J values but positive interlayer J values, known as the second type of AFM material. This is why some of the magnetic insulators display specific magnetic structures with intralayer FM coupling while with the interlayer AFM coupling. Because of the opposite orientations of the magnetic moments of the two adjacent layers, a bilayer magnetic material composed of two FM single-layers will instead show AFM behavior. This is typically represented by the well-studied layered 3d-orbital magnet CrI 3 , which shows intralayer FM coupling (confirmed in monolayer CrI 3 ) while displaying interlayer AFM coupling when the layer number is <20. [10,[45][46][47] Therefore, the interlayer couplings in layered 3d-orbital ferromagnets are weak and easy to regulate, thus facilitating the effective modulation of their magnetic properties by external perturbations on the atomic layer and interlayer interactions. [48] Thus, the magnetic properties of layered 3d-orbital magnetic materials can be efficiently modulated by tuning the values of J, as well as the values and orientations of S, as shown in Figure 1. 1) For J, its magnitude and anisotropy can both be regulated. For the former, J < 0 (J > 0) corresponds to ferromagnetism (anti-ferromagnetism), and the magnitude of J directly determines the value of the Curie temperature T C (Néel temperature T N ) or coercive field H c . Therefore, regulating the sign of J can lead to the mutual transition between ferromagnetism and anti-ferromagnetism, while regulating the magnitude of J can elevate T C (T N ) and the H c values. For the latter, the anisotropy of J can be adjusted by tuning the interlayer distance or induc- Figure 2. Various techniques to modulate the emergent magnetic phenomena of layered 3d-orbital ferromagnets. These techniques are divided into three series, including electronic, mechanical and interfacial engineering methods. Counterclockwise from upper left to upper right: 1) Electronic engineering: Electrostatic field engineering. Reproduced with permission. [49] Copyright 2018, Springer Nature. Electrical current engineering. Reproduced with permission. [50] Copyright 2020, Springer Nature. Electrochemical intercalation. Reproduced with permission. [51] Copyright 2022, Springer Nature. 2) Interfacial engineering: Proximity coupling. Reproduced with permission. [52] Copyright 2020, Wiley-VCH. Spin THz generation. Reproduced with permission. [53] Copyright 2022, Wiley-VCH. Magnetic tunnel junctions. Reproduced with permission. [54] Copyright 2021, Wiley-VCH. 3) Mechanical engineering: Strain engineering. Reproduced with permission. [55] Copyright 2020, Wiley-VCH. High-pressure engineering. Reproduced with permission. [56] Copyright 2022, Wiley-VCH.
ing intercalation between layers. Specifically, the value of J can be changed by adjusting lattice parameters such as bond lengths and bond angles between neighboring magnetic ions for localized ferromagnetic materials or by tuning the DOS of itinerant electrons at E F for itinerant ferromagnetic materials. 2) For S, its magnitude can be changed by electrically adjusting the number of valence electrons of magnetic ions; meanwhile, its real-space orientation can be altered from in-plane and 2D (described by the XY model) to out-of-plane and 1D (described by the Ising model), by tuning the vdW coupling via electrochemical intercalation. Such a modulation directly changes the magnitude of the magnetic anisotropy energy (MAE) or the orientation of the magnetic easy axis in materials at the macroscopic level.
In particular, focusing on the J and S parameters in the magnetic exchange interactions, the magnetism of materials can be modulated by utilizing the following three main categories, as shown in Figure 2: 1) Electronic engineering: applying electrostatic fields can regulate the carrier concentration and Fermi level, employing current can achieve the injection of spin and reversal of magnetic domains, and using electrochemical intercalation can change the carrier concentration and interlayer coupling strength; 2) Mechanical engineering: the application of www.advancedsciencenews.com www.advphysicsres.com in-plane tensile or compressive stress, and hydrostatic high pressure will change bond lengths and bond angles between ions and the lattice constants, thus reconstructing the magnetic structure of the materials; 3) Interfacial engineering: stacking magnetic materials with other functional materials to form heterostructures can achieve magnetism regulation based on the proximity coupling effect or realize spintronics and magnetic memory devices. Therefore, those examples of magnetic tuning in layered 3d-orbital ferromagnets for emergent magnetic phenomena are significant in both fundamental science and practical applications.

Electronic Engineering of Magnetism in Layered 3d-Orbital Ferromagnets
Based on tuning the exchange interaction in FM electronic systems, electronic engineering has been extensively used as a convenient and efficient way to introduce charged particles (electrons, holes, and ions) in layered 3d-orbital materials. [57] For instance, the electrostatic field can directly change the DOS of itinerant electrons at the Fermi level, thus effectively affecting the itinerant ferromagnetism by altering the J value (see Section 3.1). Electrical current can switch the orientation of the magnetic domain by affecting the dynamic process of the rotation of S, while the crystal structure remains unchanged (see Section 3.2). For the case of electrochemical intercalation (see Section 3.3), the intercalated ions located in the vdW gap can change the interlayer distance and interlayer exchange coupling strength J of ferromagnets; meanwhile, the injection of ions will also change the electron filling configuration of 3d-orbitals and thus tune the properties of effective magnetic moments S. The tuning of the magnetic exchange interactions in materials and controlling the magnetic states therein via electronic engineering provide the opportunity for the application of magnetic devices.

Electrostatic Field Tuning of DOS of Itinerant Electrons for Enhancing Magnetism
By changing the DOS of itinerant electrons at the Fermi level and the corresponding J value, electrostatic field tuning based on field-effect transistors can effectively affect the itinerant ferromagnetism of layered 3d-orbital magnetic materials. [57] Typically, two types of dielectric layers are commonly used: 1) conventional dielectrics, such as high-quality h-BN films and oxides such as SiO 2 , Al 2 O 3 or HfO 2 , and 2) electrolytes, including ionic liquids and solid-state electrolytes, such as LaF 3 . [58,59] As a result, the carrier type and the carrier density of 3d-orbital magnetic nanoflakes can be tuned by the applied gate voltage, thus modulating the electronic properties of the materials. [57] Examples of tuning the magnetic properties via solid-state dielectric materials have been demonstrated in the study of fewlayer CrI 3 (Figure 3a-d), where insulating h-BN layers were introduced to both sides of the CrI 3 nanoflakes as dielectric, forming back and top gates simultaneously. [49,60] The special design of the dual-gate device enables independent control over the layerdependent electrostatic doping and the electric field-induced interlayer bias effect. As shown in Figure 3b, the coercive field value of monolayer CrI 3 can be effectively modulated by the applied gate voltage. The H c value decreases under the positive gate voltage (corresponding to electron doping), while it increases under the negative gate voltage (hole doping), where the doping level can reach an order of magnitude of 10 13 cm −2 . Interestingly, for the case of bilayer CrI 3 , an AFM-to-FM magnetic phase transition can be obtained by controlling the top and back gate voltages together, as shown in Figure 3d. Such results indicated that the interlayer exchange constant J can be effectively tuned from positive (layered AFM coupling) to negative (layered FM coupling) by changing the carrier density therein. This mechanism sharply contrasts with the AFM-to-FM transition in bilayer CrI 3 driven by applying the external displacement field, [61] since the latter is caused by the linear magnetoelectric effect where the occupation configuration of electrons of magnetic Cr atoms and the corresponding magnitude of the effective magnetic moments in top and bottom layers are changed, thus leading to the appearance of pure magnetization in AFM layered systems. [61] In addition, theoretical calculations also suggested that the MAE of ultrathin Fe 3 GeTe 2 flakes can be efficiently tuned by an electrostatic field (the same configuration shown in Figure 3a,c), due to the modulation of occupation and splitting of the Te(p z )-Fe(d z 2 ) bonds. [62] For liquid electrolyte gating, modulation of the MAE is achieved in Cr 2 Ge 2 Te 6 . [63] The microscopic origin of magnetism in Cr 2 Ge 2 Te 6 is the super-exchange coupling between Cr 3dorbitals via in-plane 5p-orbitals of Te atoms through Cr-Te-Cr  Figure 3f) and carrier density with increasing gating voltage were also observed. According to density functional theory (DFT) calculations, heavy electron doping can promote a double-exchange mechanism mediated by electric field-induced free carriers, dominating over the original super-exchange mechanism in the insulating state of pristine Cr 2 Ge 2 Te 6 . Importantly, the MAE can be continuously tuned from positive to negative as the carrier density increases, thus causing a spin-flip transition of the magnetic easy axis from an out-of-plane to an in-plane orientation. It should be addressed that the positive gate voltage will only cause carrier accumulation within a few topmost atomic layers and form 2D electron gas therein, while the crystal structure remains unchanged unless the electrochemical reaction happens, which is intrinsically different from the electrochemical intercalation effect (to be discussed in the next section). Note that liquid electrolyte gating based on the electric double layer transistor technique can also induce a PM-to-FM phase transition, as demonstrated in the study of (Ti,Co)O 2 . [64] According to Figure 3g, a hysteresis loop in the anomalous Hall conductivity AH can be observed during the gating process at 300 K, indicating the gate-induced roomtemperature FM state. The monotonically increasing AH with increasing carrier density ( Figure 3h) indicates that the ferromagnetism induced in this electronic system originates from a carrier-mediated mechanism.  [60] Copyright 2018, Springer Nature. c,d) Electrostatic modulation of bilayer CrI 3 flake. c) Schematic illustration of the testing geometry. d) Back and top gate voltage dependence of the reflective magnetic circular dichroism results, indicating a gate-induced layered AFM-to-FM phase transition when the applied magnetic field is 0.78 T. Reproduced with permission. [49] Copyright 2018, Springer Nature. e,f) Ionic liquid gating of Cr 2 Ge 2 Te 6 flake. e) Illustration of testing geometry (left panel) and distance-dependent carrier density (right panel). f) Temperature-dependent H u values for pristine flake and flake under different gate voltages, where H u = H ⟂ sat − H ∥ sat and H u = 0 indicate a spin-flip transition. Inset: Gate voltage-dependent T C values for two devices (solid and hollow circles). Reproduced with permission. [63] Copyright 2020, Springer Nature. g,h) Gate-induced room temperature ferromagnetism in (Ti,Co)O 2 . g) Anomalous Hall conductance AH under different gate voltages measured at 300 K. The inset shows the testing geometry. h) Carrier density dependence of AH measured at 300 K. Reproduced with permission. [64] Copyright 2011, American Association for the Advancement of Science.

Electrical Current Controlling of the Dynamics of Spin Relaxation of Magnetism
The electrical current control of layered 3d-orbital magnetic materials can effectively affect the dynamics of the relaxation process of the effective magnetic moment S. This current-induced spin dynamics can be explained by the Landau-Lifshitz-Gilbert equation, [65] thus causing the current-induced domain wall motion. Generally, both the continuous current and current pulse can be applied for modulating the magnetic states of layered 3dorbital materials. Since the domain wall can be treated as digital memory bits (originating from the opposed orientations of S in the Ising model), it can provide distinct and stable "0" or "1" state with quick response to the applied current. Such an electrical current-induced domain wall motion has long been a feasible approach to achieving practical applications for modern magnetic information storage devices. [66] The study of electrical current control of Fe 3 GeTe 2 nanosheets from a hard magnetic state to a soft magnetic state serves as a paradigm for controlling the magnetic structure based on continuous current, [67] as shown in Figure 4. The shape of the hysteresis loop varies significantly, and the coercive field values decrease monotonically as the applied electrical current increases. These observations originate from the strong spin orbit torque effect in Fe 3 GeTe 2 , since the applied electric current can effectively reduce the barrier height between the local minima of the free energy. Such a current-tunable magnetic structure of Fe 3 GeTe 2 can be applicable to magnetic memory devices, whose geometry is shown in Figure 4a. Figure 4b demonstrates the working process of such a device. The H c value can be effectively reduced by the writing current I write , so that an alternation from "0" to "1" state can be achieved with a small external magnetic field. However, when I write ≈ 0, the H c value becomes extremely large, so the written "1" state can be stably maintained, providing a feasible and energy-efficient solution for magnetic memory devices. Furthermore, apart from the direct current injection, the application of electrical current on the neighboring material attached to the FM layer can also tune the magnetic behavior of the FM layer. For instance, the magnetization switching in Fe 3 GeTe 2 of the bilayer Fe 3 GeTe 2 /Pt structure [68] is shown in Figure 4c-e. The device structure contains Fe 3 GeTe 2 and Pt layers (Figure 4c), where the magnetization of few-layer Fe 3 GeTe 2 can be switched by the spin orbit torque from the current flowing in the heavy metal Pt layer. As shown in Figure 4d,e, the magnetization of Fe 3 GeTe 2 can be switched from one state to another during the sweeping of electrical current, and these two states can be well sustained when the applied current equals zero. Finally, examples of current pulse-induced domain wall motion have been demonstrated in both the Fe 3 GeTe 2 layer [69] and Co nanofilm. [70] The domain wall can move (driven by the current pulses) under a constant . Electrical current control of layered 3d-orbital ferromagnets. a,b) Current-induced hard-to-soft magnetic state transition in Fe 3 GeTe 2 and its application for magnetic memory devices. a) Illustration of the configuration of the current-controlled magnetic memory device. b) Procedure and writing (large current) and storage (ignorable current) of the Fe 3 GeTe 2 -based memory device. Reproduced with permission. [67] Copyright 2020, Wiley-VCH. ce) Current-induced magnetization switching in the Pt/Fe 3 GeTe 2 bilayer. c) Schematic illustration of device structure. d,e) Current-driven magnetization switching with in-plane magnetic fields of d) +50 mT and e) −50 mT at 100 K. Reproduced with permission. [68] Copyright 2019, American Association for the Advancement of Science. f,g) Current-driven magnetic domain-wall logic. f) Colored scanning electron microscope image (left panel) and illustration (right panel) of reconfigurable NAND/NOR logic gates. g) Logic-circuit diagram (left panel) and magnetic force microscope image of the full adder. Scale bar, 1 μm. Reproduced with permission. [50] Copyright 2020, Springer Nature. magnetic field. Interestingly, the velocities of domain wall motion along different crystal orientations and their magnetic field dependence behaviors vary significantly in the case of Fe 3 GeTe 2 , which is helpful for accelerating the domain wall velocity of layered 3d-orbital ferromagnets.
It should be further addressed that the current-driven magnetic domain wall motion effect has a profound influence on the design of logic circuits, and the current-driven magnetic domain-wall logic based on Co nanofilm has been successfully demonstrated. [50] The Boolean logical values "1" ("0") can be represented by the magnetization direction pointing perpendicularly down (up). Since the applied electrical current pulse can invert the orientation of the magnetization direction, a NOT gate can be achieved. Furthermore, reconfigurable NAND and NOR logic gates can be successfully achieved by switching the logic state of the bias, and the colored scanning electron microscope image and schematic illustration of the building block of the logic circuit are shown in Figure 4f. Finally, a full adder gate, shown in Figure 4g can be achieved by cascading such configurations. The size of magnetic domain-wall logic can be further reduced within the scale of a few nanometers since the coupling induced by the Dzyaloshinskii-Moriya interaction [71,72] can still be effective at the scale of magnetic moments. Such a demonstration provides low-power memory-in-logic applications.

Electrochemical Intercalation Tuning of Exchange Interaction of Magnetism
The intensity of interlayer exchange coupling J of layered 3dorbital ferromagnets, being extremely sensitive to the interlayer distance, can be efficiently modulated by inserting foreign charged particles into the vdW gaps. That is why electrochemical intercalation serves as an effective method for tuning the magnetism of layered 3d-orbital materials to meet the Stoner criterion for enhancing the Curie temperature T C . Since the interlayer interaction between adjacent layers is typically weak in these layered ferromagnets, even a small change in the interlayer distance can drastically change the interlayer coupling effect. The relatively large interlayer distance due to the existence of the vdW gap makes it possible for the intercalation of small atoms, ions or even molecules. [57] More importantly, the net charge induced by the intercalated ions can cause the electronic doping effect due to the condition of electrical neutrality. Distinct from the selfintercalation technique [73][74][75] reported recently, electrochemical intercalation serves as another easy-approach method for the fine tuning of layered 3d-orbital ferromagnets. According to experimental and calculation works, the T C values, magnetic anisotropy and magnetic easy axis, can all be effectively adjusted based on electrochemical intercalation. [5,[76][77][78]  The maximum T C can reach 310 K. Reproduced with permission. [5] Copyright 2018, Springer Nature. c-e) Proton intercalation-induced interlayer AFM coupling in Fe 3 GeTe 2 . c) Schematic illustration of experimental setup. d) Schematic illustration of differing AFM-FM interface. e) Gate-tuned hysteresis loops in Fe 3 GeTe 2 . The gate voltages from negative to positive are marked near each experimental curve. Reproduced with permission. [76] Copyright 2020, American Physics Society. f-h) Li + -intercalation-induced spin-flop transition in Fe 5 GeTe 2 . f) Schematic illustration of the spin-flop transition in view of free energy. Inset shows the experimental setup, where "PEO" represents polyethylene oxide. g) Hysteresis loop of anomalous Hall resistance at 2 K with varying gate voltage. h) Polar chart of gate-dependent spin-flop transition. The light blue, light pink and light yellow areas represent the in-plane, out-of-plane and canted spin orientations, respectively. The red dots and yellow arrow indicate a gate-modulated spin-flop transition. Reproduced with permission. [51] Copyright 2022, Springer Nature.
Cations, including most alkali ions (especially Li + ) and protons (H + ), are commonly-used intercalated guest-species for electrochemical control of layered 3d-orbital ferromagnets. Technically, the reaction rate of electrochemical intercalation is strongly limited by the ionic conductivity because of the slow movements of the ions, thus requiring a low bias scanning rate and a long time for relaxation. As an example of intercalation-induced T C enhancement, an experimental study of tuning few-layer Fe 3 GeTe 2 samples based on Li + -intercalation has been demonstrated. [5] According to the layer number dependence of the phase diagram (shown in Figure 5a), the T C values decrease monotonically from bulk (≈180 K) to single layer (≈20 K). However, ionic gating can dramatically increase the T C values of few-layer Fe 3 GeTe 2 samples, and can even hold ferromagnetism above room temperature (310 K), as shown in Figure 5b. Such an observed phenomenon can be explained by the Stoner model, since the electron-dopingcaused DOS increase at the Fermi level can promote the formation of FM ordering in itinerant magnetic systems. In the case of Fe 3 GeTe 2 , the enhancement of ferromagnetism originates from the gate-induced electron filling of sub-bands formed by the 3d-orbitals of Fe atoms. Similar results on Na + intercalated Fe 3 GeTe 2 and the enhancement of T C were also reported. [78] Apart from the Li + -intercalation method for J modulation introduced above, proton intercalation in Fe 3 GeTe 2 and the corresponding exchange bias effect were also studied, [76] as shown in (Figure 5c-e). The experimental results (Figure 5d,e) and DFT calculations indicated that the interlayer interaction of Fe 3 GeTe 2 favors FM coupling if the vdW effect between layers is ignorable, while AFM coupling is favored if the vdW effect increases, and the same goes for protonated Fe 3 GeTe 2 . For the case of intrinsic Fe 3 GeTe 2 , the vdW effect is indeed weak, thus the interlayer FM coupling is preferred; however, the H + -intercalation enhances the vdW coupling effect in protonated Fe 3 GeTe 2 , so that the interlayer AFM coupling can be more energetically favorable therein. Therefore, the proportion of interlayer AFM coupling increases as the proton concentration increases. As a result, the value of the coercive field in Figure 5e decreases monotonically with increasing gate voltage (proton concentration). Such a tunable interlayer coupling strength in the protonated Fe 3 GeTe 2 nanoflakes makes it more appropriate for applicable 2D spintronic devices. Interestingly, unlike small-sized cations, organic-anions can also serve as electrochemical dopants for enhancing the Curie temperature of layered 3d-orbital ferromagnets. For instance, tetrabutyl ammonium doped Cr 2 Ge 2 Te 6 was synthesized via an electrochemical reaction, and the magnetic properties of such an organic molecule-intercalated hybrid superlattice were reported. [77] The Curie temperature increases significantly from 67 K (pristine Cr 2 Ge 2 Te 6 flake) to 208 K (tetrabutyl ammonium-intercalated Cr 2 Ge 2 Te 6 ), originating from the mechanism that the electron doping effect changes the exchange interaction from weak super-exchange to metallic double-exchange FM ordering caused by the electron doping effect.
Note that electrochemical intercalation can also modulate the MAE of layered ferromagnets, thus being even able to induce continuous variation in the magnetic easy axis. For instance, the spin-flop transition is found in an Fe 5 GeTe 2 thin film based on Li + -intercalation. [51] Figure 5f presents a schematic illustration of the spin-flop transition in view of local free-energy minima, where the magnetic easy axis rotates continuously from outof-plane to in-plane orientation. The shape of the gate voltagedependent hysteresis loop in the anomalous Hall effect (Figure 5g), together with the angle-rotating experiment clearly depicted such a feature, indicating that the magnetic easy axis can be tuned by increasing the concentration of Li + , serving as a paradigm for tuning the dimensionality of S. Note that such a modulation of MAE based on an intercalation approach is intrinsically different from the electrostatic field tuning mentioned above (see Section 3.1 and Figure 3e,f), since the transmission electron microscope image of the cross-sectional Li-intercalated Fe 5 GeTe 2 sample indicated that the Li + ions were distributed evenly in the sample with tens of nanometers in thickness, [51] while the electrostatic gating based on ionic liquid can only affect a few layers (typically several nanometers) near the surface of Cr 2 Ge 2 Te 6 . [79]

Mechanical Engineering of Magnetism in Layered 3d-Orbital Ferromagnets
Simply because the lattice parameters, bonding lengths and bonding angles play important roles in determining the electronic properties of layered materials, the mechanical engineering can alter the exchange interactions between neighboring 3d-orbital transition metal ions, and such variations of chemical bonding can directly affect the strength of exchange interaction J, thereby affecting the magnetic properties of layered ferromagnets. Taking the itinerant FM system as an example, mechanical engineering, including the application of uniaxial tensile/compressive strain (see Section 4.1) or hydrostatic pressure (see Section 4.2), can affect the electronic band structure and the DOS at the Fermi level, thus effectively tuning the T C and magnetic structure, or even inducing the electronic phase transition of layered 3d-orbital ferromagnets.

Strain Engineering for Enhancing Exchange Interactions of Magnetism
Since even small changes of bonding angle and length can be efficiently tuned by strain engineering technique, which can lead to a dramatic change in the band structure and electronic correlation interactions, [80][81][82][83][84] the strain and mechanical deformation in layered 3d-orbital materials can have a profound effect on the magnetic exchange interactions J between neighboring local magnets and the corresponding magnetic ground state therein. Technically, strain can be applied to the targeted nanoflake by a variety of means, including substrate surface topography modification, thermal expansion mismatch, strain relaxation and other methods. As a result of strain engineering, the enhancement of ferromagnetism (increased T C and H c ), [55,85] strain-controlled domain wall propagation, [86,87] or the phase transition from nonmagnetic or anti-ferromagnetic to ferromagnetic states [88,89] can generally be achieved.
As an example of the strain-enhanced FM state confirmed by increasing the T C and H c values of the Fe 3 GeTe 2 nanoflake, the effect of strain engineering on the magnitude of the exchange interaction J based on the electronic transport studies has been demonstrated, [55] as shown in Figure 6. A home-made uniaxial strain setup with a three-point bending geometry (shown in Figure 6a) was adopted to apply controllable strain on the sample. As shown in Figure 6b, the magnetic field-dependent Hall resistance with increasing strain level at a fixed temperature (1.5 K) was explored, and the value of the coercive field obtained from the hysteresis loop first increased and then saturated as the strain increased. Correspondingly, the anomalous Hall effect was obtained with an increasing strain value at 180 K (presented in Figure 6c). Furthermore, by investigating the temperaturedependent electronic transport properties under different strain values, a temperature-strain phase diagram was obtained (see Figure 6d). One can see from the phase diagram that both the transition temperatures from the single-domain ferromagnet (labeled as "FM1") to the labyrinthine-domain ferromagnet (labeled as "FM2"), and from the labyrinthine domain ferromagnet to the paramagnetic (labeled as "PM") state increase monotonically as the effective strain increases. In addition, theoretical calculations of monolayer Cr 2 Ge 2 Te 6 indicated that ferromagnetism might be enhanced by strain engineering, [90] and similar results were obtained for VS 2 and VSe 2 . [91] Interestingly, a theoretical work based on quantum Monte Carlo simulation indicated that the tension strain could maintain the Ising FM ordering in monolayer CrCl 3 , while the compression strain can induce the transition from Ising to XY FM ordering, and both cases can enhance the T C value. [92] Similar calculation results for other CrX 3 materials (X denotes halogen group elements Cl, Br and I) were also reported. [93] Meanwhile, experimental works on ferromagnetic Cr 2 Te 3 indicated that the T C value could be positively tuned by tensile (varied value of ≈ +40 K) and negatively tuned by compressive strain (varied value of ≈ −90 K). [85] Correspondingly, a recent application of strain engineering for the strain-induced AFM-to-FM transition has been reported in CrSBr. [88] Unlike the irreversible AFM-to-FM transition in bilayer CrI 3 based on the high pressure tuning method (to be discussed later), the application of strain engineering can serve as a method to achieve a reversible AFM-to-FM transition in CrSBr. [88] An in situ uniaxial tensile strain was applied on the sample flake by a cryogenic strain apparatus (Figure 6e,f), and the photoluminescence spectra before and after the strain were studied (see Figure 6g-i). The intrinsic (unstrained) CrSBr sample shows interlayer AFM coupling, [94][95][96][97] and can be tuned to the FM state by Magnetic field-dependent Hall resistance R xy with increasing applied strain at 1.5 K. c) Offset Hall resistance R xy with increasing applied strain at 180 K. d) A temperature-strain phase diagram summarizing this work. "PM", "FM1" and "FM2" represent paramagnetic, ferromagnetic with a single domain, and ferromagnetic with a labyrinthine domain, respectively. Reproduced with permission. [55] Copyright 2020, Wiley-VCH. e-i) Strain-induced AFM-to-FM phase transition in CrSBr. e) Schematic illustration of the orientation of applied strain and f) the cryogenic strain apparatus used in this work. g) Photoluminescence spectra of CrSBr with 0.9% (black curve) and 1.4% (blue curve) strain. h) Photoluminescence spectra of unstrained CrSBr sample with zero applied magnetic field (black curve) and magnetic field of 0.4 T (blue curve). i) Reversible photoluminescence spectra with increasing and decreasing strain. The strain values are marked beside each curve. Reproduced with permission. [88] Copyright 2022, Springer Nature.
the external magnetic field of 0.4 T along the magnetic easy axis, marked by a redshift of 11 meV of a cusp photoluminescence peak (blue curve in Figure 6h). Similarly, such a redshift of the cusp photoluminescence peak (12 meV) can be observed when the applied strain is 1.4%, as shown in Figure 6g. Thus, it can be concluded that a strain-induced interlayer AFM-to-FM transition is obtained in CrSBr. The reversibility of such an electronic phase transition was proven by the almost identical photoluminescence spectra before and after the strain was applied (see Figure 6i). Calculation works also suggested that a similar AFM-to-FM transition can be achieved in perovskite RMnO 3 films (R denotes lanthanides from La to Lu) based on strain engineering. [98] Meanwhile, forming wrinkles in low-dimensional nanoflakes can also induce FM ordering in non-FM materials. Several works on calculations also claimed that applying strain can cause the ferromagnetic transition of non-magnetic materials, including superconductors (NbS 2 and NbSe 2 [89] ). Interestingly, in contrast to the strain-induced AFM-to-FM transitions, calculation works indicated that the FM-to-AFM phase transition could also be achieved in ferromagnetic FeI 3 by strain engineering. [99]

High Pressure Engineering for Tuning Exchange Interaction of Magnetism
To mechanically control the magnitude and anisotropy of J, another engineering method is high pressure engineering (based on diamond anvil cells or other techniques), which is a versatile and efficient in situ technique leading to the transition of magnetic states of layered 3d-orbital materials. [100,101] Generally, applying high pressure can tune the J value between neighboring local magnets or neighboring atomic layers (similar to the strain engineering introduced in Section 4.1), and affect the electronic properties that originate from the band structure near the Fermi energy (corresponding to the case of itinerant ferromagnetic materials). Typically, the lattice parameters change continuously with increasing pressure, and such a parameter variation causes the continuous variation of the energy band in the k-space, thus possibly resulting in band closing or the appearance of a new Fermi surface sheet accommodating the longrange ordering. Note that the type of crystal structure and the symmetry (space group) of crystal structure might also change at a specific critical pressure, leading to a drastic modulation of the electronic band structure in the k-space. Therefore, the DOS at E F can be modulated, so that the J value can be tuned for itinerant FM materials. Typically, experimental characterization techniques, such as Raman spectroscopy, X-ray diffraction, and DFT calculations are adopted to determine whether a structural transition occurs during the process of applying pressure. One can see that both approaches mentioned above (tuning the magnetic properties with/without structural transition) can be obtained during the high-pressure engineering of layered 3d-orbital ferromagnets. [56,102,103] An interlayer AFM-to-FM magnetic structure transition in bilayer CrI 3 was confirmed by magnetic circular dichroism and electron tunneling measurements, [102,103] as shown in Figure 7a-f. The polarized Raman spectroscopy study under high pressure (presented in Figure 7b,c) indicates that such an  [103] Copyright 2019, Springer Nature. g) Pressureinduced crystalline-to-amorphous structural transition of Cr 2 Ge 2 Te 6 . Reproduce with permission. [106] Copyright 2019, American Chemistry Society. h,i) Pressure-induced anomalous lattice softening FM state disappearance in Fe 3 GeTe 2 . h) Pressure-dependent T C value of Fe 3 GeTe 2 . i) Pressure-dependent Raman shift (upper panel) and full width at half maximum (FWHM) of Raman peaks (lower panel). Reproduced with permission. [107] Copyright 2020, Chinese Physics Society.
AFM-to-FM transition was associated with a monoclinic-torhombohedral stacking order change (see Figure 7d) caused by high pressure. The reflective magnetic circular dichroism result of bilayer CrI 3 after the removal of ≈2. 45 GPa high pressure shows the FM state (Figure 7e), while that of pristine bilayer CrI 3 shows interlayer AFM coupling behavior, indicating that such a pressure-induced magnetic structure transition is irreversible. Furthermore, the electronic tunneling measurement of trilayer CrI 3 indicated that high pressure can also create coexisting domains of one FM and two AFM phases. [103] The studies of pressurized Cr 2 Ge 2 Te 6 show more complicated behavior: a decrease in T C with increasing pressure from ambient pressure to 4.5 GPa, [104,105] causing the electronic structure transition from semiconductor to metal (marked by a huge drop in electronic resistance). Interestingly, with increasing pressure up to 9 GPa, the T C value increases, and a nearly room-temperature FM state can be obtained. [105] Finally, an irreversible structural transition from the crystalline to amorphous phase at ≈18 GPa can be found (shown in Figure 7g). [106] Meanwhile, the study of Fe 3 GeTe 2 under high pressure [107] also indicated that pressure-induced anomalous lattice softening (confirmed by a high-pressure Raman study) at ≈10 GPa effectively suppressed the FM ordering therein (Figure 7h,i). Different from the pressure-induced disorder state introduced above, some of the pressure-induced structural transitions can also result in the formation of a new longrange ordering. For instance, the experimental and calculation results of pressurized CrSiTe 3 indicated that a structural transition occurs at a pressure of 8 GPa, accompanied by an FMto-superconducting electronic phase transition. [108] Interestingly, the experimental study of VI 3 indicated that the pressure-induced new structure can still show ferromagnetism, while the T C value can further increase. [109] In contrast to the pressure-induced stacking order change and the resulting FM transition that occurred in bilayer CrI 3 , [102,103]  Arrott plot study of Hall signal. The T C value of Fe 5 GeTe 2 can be enhanced to >360 K by applying 14 GPa high pressure. c) Normalized Hall resistance with different pressures at 320 K. An apparent anomalous Hall effect can be observed at 9.5 GPa, indicating the appearance of an FM state. Reproduced with permission. [56] Copyright 2022, Wiley-VCH. d-g) High pressure-enhanced FM state in CrSiTe 3 . (d) Structure of the CrSiTe 3 crystal. e) Schematic illustration of the in situ high pressure magnetic circular dichroism (labeled as "MCD" in the figure) experimental setup. f) Pressure-dependent magnetic circular dichroism signal at 16 K. An apparent hysteresis loop can be observed at 4.6 GPa, indicating a soft-to-hard FM transition. g) The Curie temperatures obtained from magnetic circular dichroism measurements, showing an increasing tendency with applied pressure. Reproduced with permission. [110] Copyright 2021, American Chemistry Society.
magnetic property control has been demonstrated to significantly enhance the T C value of pressurized Fe 5 GeTe 2 [56] (Figure 8a-c). As shown in Figure 8c, the appearance of the anomalous Hall effect in the R xy signal with increasing pressure at 320 K indicates the pressure-tunable room temperature ferromagnetism. Interestingly, the T C value goes beyond 400 K after the pressure loading-deloading process based on Arrott plot analysis (Figure 8b), which is in sharp contrast to pressurized Fe 3 GeTe 2 where ferromagnetism is suppressed under pressure. [107] It should be noted that no apparent structural transition in Fe 5 GeTe 2 can be found (confirmed by a high-pressure X-ray diffraction study [56] ). Thus, such behavior might be associated with pressure-induced metastable FM ordering with stronger exchange interaction J in the electronic structure in Fe 5 GeTe 2 . Similar results go for the study of layered CrSiTe 3 ferromagnet under pressure [110] (Figure 8d-g). Increases in T C and H c , together with a soft-to-hard ferromagnetic transition, were observed with increasing pressure, as shown in Figure 8f,g. The DFT calculations indicate that the stacking order and the corresponding space group remain unchanged up to 8 GPa, and that the enhancement of ferromagnetism might be related to the change in intralayer nearestneighbor exchange interactions and the strong many-body effects therein. [110] Applying high pressure can also modulate the transi-tion from FM semiconductor to FM metal via pressure-induced band closing by compressing the vdW gap but maintaining the space group of the crystal. For instance, experimental results and spin-resolved band structures of VS 2 nanosheets indicated that high pressure induces the semiconductor-to-metal transition, while ferromagnetism remains. [111]

Interfacial Engineering of Magnetism in Layered 3d-Orbital Ferromagnets
Interfacial engineering mainly affects the J value rather than the dimension of S, promoting scientific understanding and spintronic applications in low-dimensional materials. [112,113] Examples of moiré exciton, [114,115] symmetry-breakinginduced spontaneous photovoltaic effect, [116,117] and clean 2D superconductivity [118,119] have been demonstrated by stacking and assembling the layered materials to form heterojunctions or homojunctions. In particular, the stacking of layered FM materials and other types of low-dimensional materials can induce proximity coupling and quasiparticle interactions in the system (see Section 5.1). Technically, interfacial engineering can achieve multifunctional applications of layered materials by assembling them with different functions similar to the Temperature-dependent Kerr rotation for pristine Fe 3 GeTe 2 flake and FePS 3 /Fe 3 GeTe 2 /FePS 3 heterojunction. Reproduced with permission. [52] Copyright 2020, Wiley-VCH. c-f) Double-switching behavior and T C enhancement in molecular beam epitaxy grown FM/AFM heterojunction. c) Illustration of the Fe 3 GeTe 2 /CrSb superlattice. d) Anomalous Hall resistance of the Fe 3 GeTe 2 /CrSb superlattice at 2.5 K. Both "major" and "minor" loops can be observed during scanning. e) TEM image of an (Fe 3 GeTe 2 /CrSb) unit, formed by stacking four layers of Fe 3 GeTe 2 on top of an approximately 1.6 nm CrSb layer. f) (Fe 3 GeTe 2 /CrSb) n stacking period dependence of T C values, showing an increasing tendency. CrSb is labeled as "CS" in the inset figure. Reproduced with permission. [121] Copyright 2019, Oxford University Press. g,h) T C enhancement of the FM/TI heterojunction consists of Fe 3 GeTe 2 and Bi 2 Te 3 (denoted as "Bi 2 Te 3 (8)|FGT(4)" in the figure). The Arrott plot results indicate an enhanced T C values of ≈400 K. h) Layer thickness-dependent T C values of pristine Fe 3 GeTe 2 flake and Fe 3 GeTe 2 /Bi 2 Te 3 heterojunctions. Reproduced with permission. [128] Copyright 2020, American Chemistry Society.
Lego bricks. [120] Specifically, layered 3d-orbital ferromagnets are suitable for fabricating terahertz (THz) devices and magnetic tunnel junctions, and the corresponding works are introduced in Sections 5.2 and 5.3, respectively.

The Proximity Coupling Effect Controlling of Magnetism
At the interface of mono-and few-layer 3d-orbital ferromagnets, the atomic orbitals of two materials are hybridized due to the dispersive nature of wavefunctions. This proximity coupling effect causes the hybridization of physical properties, so the magnitude and anisotropy of J values of these materials will be significantly affected by proximate SOC from surrounding environments, which is an effective way to strongly promote the FM performance. Therefore, the magnetic properties of targeted ferromagnets can be effectively tuned by forming superlattices or stacking them with other types of layered materials. [48] Theoretical rationale is that the T C values of the ferromagnets can be increased dramatically by forming an FM/AFM heterostructure. [52,121,122] In addition, an inverse spin-galvanic effect and an increase in T C can be achieved based on FM/TI (TI stands for "topological insulator") heterostructures, [123][124][125][126][127][128] and Skyrmion-vortex pair, magnon-fluxon interaction can be demonstrated in FM/superconductor heterostructures. [129][130][131][132][133] More importantly, theoretical calculations also suggested that quantum anomalous Hall effect can be observed in ferro-magnetic NiI 2 /graphene heterojunction, [134] and that proximity magnetization reversal and emergence of an antiferromagnetic Dirac dispersion exist in twisted Cr 2 Ge 2 Te 6 /graphene heterojunctions. [135] Typically, the above-mentioned heterojunction can be fabricated via both "top-down" (for example, drytransfer or pick-up technique) and "bottom up" (for instance, growth with molecular beam epitaxy or chemical vapor deposition techniques) approaches.
Forming an FM/AFM heterostructure can effectively modulate the T C and H c values of the FM material. For instance, vdW heterojunctions were fabricated via a "bottom-up" technique based on Fe 3 GeTe 2 and anti-ferromagnetic FePS 3 nanoflakes (shown in Figure 9a,b), and the magnetic properties were studied based on magneto-optical Kerr effect (MOKE) spectroscopy. [52] One can see that the hysteresis loop of such an FM/AFM junction is apparently different from that of the pure Fe 3 GeTe 2 flake: the values of H c and Kerr rotation angle both increase after forming the heterostructure. More importantly, the exchange-bias phenomenon can be observed both in the FePS 3 /Fe 3 GeTe 2 heterojunction and FePS 3 /Fe 3 GeTe 2 /FePS 3 sandwich structure. Finally, the temperature-dependent MOKE experiments indicate dramatic enhancements of T C values in both FePS 3 /Fe 3 GeTe 2 (≈30 K) and FePS 3 /Fe 3 GeTe 2 /FePS 3 (≈35 K), as shown in Figure 9b. Such phenomena originate from the strong exchange interaction due to the proximity coupling between the FM/AFM interface, and the same results were found in similarly structured FePS 3 /Fe 5 GeTe 2 heterojunction. [122] In addition, the FM/AFM www.advancedsciencenews.com www.advphysicsres.com superlattice obtained from the "bottom-up" technique also shows similar behavior. The FM/AFM superlattice can be formed by an alternating arrangement of Fe 3 GeTe 2 and CrSb layers based on the molecular bean epitaxy technique, [121] as shown in Figure 9cf. A unique "double-switching" phenomenon can be observed in the anomalous Hall resistance (Figure 9d): apart from the major loop (green curve), two minor loops (red and orange curves) can be observed. This phenomenon can be explained by the interlayer exchange effect and the corresponding parallel FM coupling between Fe 3 GeTe 2 and CrSb. The researchers also studied the T C values of the Fe 3 GeTe 2 /CrSb superlattice with different stacking periods n. Although the magnetic easy axis remains pointing along the out-of-plane direction (same as pristine Fe 3 GeTe 2 ), the T C values show an increasing trend with increasing n values for n ⩽ 5, and then saturates at ≈230 K (see Figure 9f). Similarly, the thickness dependence of the exchange bias field and H c was studied in Fe 3 GeTe 2 /Co-phthalocyanine heterojunction, and an exchange bias field as large as −840 Oe was obtained at 10 K when the thickness of Fe 3 GeTe 2 was 20 nm. [136] Apart from the FM/AFM heterojunction, the FM/TI junction can also enhance the T C value, as shown in Figure 9g,h. In this work, a clean FM/TI interface consisting of Fe 3 GeTe 2 and Bi 2 Te 3 was obtained based on the molecular beam epitaxy method, the electronic transport properties of the heterojunction were studied, and these properties were compared with those of pristine Fe 3 GeTe 2 nanoflakes. [128] According to the Arrott plot results, the T C value of 8/4 nm Bi 2 Te 3 /Fe 3 GeTe 2 heterojunction (Figure 9g) can reach up to 400 K, while the T C of a thick pristine Fe 3 GeTe 2 flake is only ≈230 K. Furthermore, by depositing Fe 3 GeTe 2 layer with different thicknesses on top of the 8 nm-thick Bi 2 Te 3 layer and investigating their Curie temperatures, a layer thicknessdependent Curie temperature for such FM/TI heterojunctions was obtained, as shown in Figure 9h. One can see that the optimized T C can be obtained for 4 nm-thick Fe 3 GeTe 2 deposition, and then decreases monotonically with increasing Fe 3 GeTe 2 thickness. Such an observation can be theoretically explained by the nontrivial topologically protected surface state of Bi 2 Te 3 , and the intralayer spin interaction of the FM material can be dramatically enhanced via interfacial exchange coupling with the TI at the interface.

Utilizing SOC in Magnetic Heterojunctions for Terahertz Generation
At the heterointerface, the SOC can cause spin-up and spin-down electrons to be deflected in opposite directions, thus emitting THz waves. Normally, the optical spin injection rate, spin-tocharge conversion ratio, and the sensitivity during the detection of ultrafast spin currents play key roles in determining the quality of THz emitters and detectors. The ordinary strategy for THz emitters is based on FM/heavy metal heterojunctions because of the large spin Hall angles and the resulting spin-to-charge conversion of heavy metals, [137][138][139] while other layered materials that can replace heavy metals are still under exploration. The formation of the FM/AFM and FM/TI heterojunctions mentioned above can not only improve the properties of the attached FM materials, but also be applied to THz emitters. [140,141] The use of ultrafast (femto-and atto-second) light pulses is the fastest way to excite and manipulate the magnetization dynamics of matter, which has been an active area of research for the past two decades. For conventional ferromagnetic-nonmagnetic metal heterostructures, a longitudinal ultrafast spin current can be produced upon illumination with femtosecond laser pulses. As introduced in Section 5.1, the T C values of FM materials increase by forming FM/TI or FM/AFM heterojunctions, making it possible to realize optical spin injection at a higher temperature. More importantly, the spin-momentum locking surface state of topological insulators can provide a high spin-to-charge conversion ratio, and the large spin Hall angles of AFM materials make it possible to achieve THz emission without the application of external magnetic fields. Therefore, fabricating FM/TI or FM/AFM heterojunctions via interfacial engineering serves as an efficient tool to achieve high-performance THz devices.
For the case of forming THz devices based on the FM/TI heterojunction, the generation and control of THz spin currents were demonstrated based on the Fe 3 GeTe 2 /Bi 2 Te 3 heterojunction, [53] as shown in Figure 10a-e. The elevated T C of Fe 3 GeTe 2 to room temperature in the Fe 3 GeTe 2 /Bi 2 Te 3 heterojunction allows the spin-to-charge conversion dynamics in Fe 3 GeTe 2 to be monitored via THz emission spectroscopy. Moreover, the efficient spin-to-charge conversion provided by TI material facilitates the THz spin currents generation driven by femtosecond laser pulses. Based on these advancements, Fe 3 GeTe 2 /Bi 2 Te 3 heterojunction was successfully applied for THz emitters. It has been demonstrated that the inverse Edelstein effect contributing to spin-to-charge conversion is the mechanism for THz generation. Additionally, their nontrivial THz transient measurements also indicated that topologyenhanced interlayer exchange coupling contributes to the increased T C of Fe 3 GeTe 2 . Another example for replacing the heavy metal layer is to use the light metal alloy NiCu due to the large SOC and the corresponding large spin Hall angle (comparable to that of Pt) therein. [142] In addition, layered semiconductors with a large SOC effect can also be applied for THz generation. For instance, the Co/MoS 2 heterojunction can generate giant spin current and thus suitable for THz emission. [143] However, note that these kinds of spintronic THz emitters based on the inverse spin Hall effect (requiring an external magnetic field), while the ferromagnetic layer cannot reach the saturated magnetization state in the absence of an external magnetic field, which affects the THz radiation efficiency. Therefore, the device requires external magnetic fields to serve as the driving force to initiate the spinto-charge conversion, which greatly limits practical applications.
To address this problem, a unique AFM/FM (IrMn 3 /Co 20 Fe 60 B 20 ) heterostructure was further designed [144] to realize efficient spin-THz radiation under zero external magnetic field, as shown in Figure 10f-h. The underlying field-free emission mechanism is assigned to the exchange bias or interfacial exchange coupling effect and enhanced anisotropy at the interface between FM and AFM materials. Additionally, experimental works also indicated that the IrMn 3 /Co heterojunction can induce apparent THz outputs. [145] Interestingly, recent calculation works have also indicated that heterojunctions based on AFM/heavy metal layers (Fe 50 Mn 50 /Pt) can also serve as efficient THz emitters. [146] On the one hand, contactless and field-free THz emission spectroscopy technique is an effective means to characterize ultrafast spin currents in the THz band compared  (8 nm). An obviously enhanced THz yield is obtained from Fe 3 GeTe 2 (4 nm)/TI(8 nm). d) Radiated THz electric field signals with opposite magnetization, when the pump pulse was incident on the Fe 3 GeTe 2 side (defined as n + ) or the substrate side (n − ). e) THz electric field peak signal as a function of the applied magnetic field direction M . The symmetric distribution in panel (e) and the emission-dependent behavior of the pump incidence and magnetization reversal in panel (d) unambiguously corroborates the spin-to-charge conversion mechanism dominating the THz emission from Fe 3 GeTe 2 /TI heterojunction. Reproduced with permission. [53] Copyright 2022, Wiley-VCH. f-h) THz generator based on FM/AFM heterojunction. f) Schematic diagram of the magnetic properties of FM, AFM, and exchange bias or coupling effects in FM/AFM heterostructures. g) THz temporal waveforms emitted from IrMn 3 (2.0 nm)/Co 20 Fe 60 B 20 (2.0 nm) heterostructures. h) Corresponding Fourier transform spectra of panel (g). Reproduced with permission. [144] Copyright 2022, Wiley-VCH.
with traditional spintronic materials that require electrodes for devices; on the other hand, spintronics-based THz sources have advantages of ultra-broad band, low cost, easy integration, and tunable polarization, which have broad prospects in applications. The interaction between femtosecond laser pulses and spin allows for spin manipulation on the sub-picosecond time scale, which holds great promise for spintronic applications.

The Spin Valve Effect in Magnetic Tunnel Junction
One prominent type of applicable spintronic device is the magnetic tunnel junction based on the spin valve effect. Different from conventional Si-based metal-oxide-semiconductor devices where the "charge" degree of freedom of electrons is manipulated, spintronic devices demonstrate a brand-new approach to manipulating the "spin" degree of freedom of electrons. [147] A typical structure of an FM-based magnetic tunnel junction device with the spin valve effect is shown in Figure 11a, where two FM nanoflakes are separated by the insulating spacer and form the sandwich structure. The magnetoresistance of the ferromagnetic tunnel junction will be smaller when the magnetization orientations of the two FM layers are parallel, and larger when the orientations are antiparallel. Such a two-state feature of magnetoresistance can have a one-to-one correspondence to the Boolean operators "0" and "1" state in information technology, thus being suitable for next-generation electronic devices. Two key param-eters determining the quality of the magnetic tunnel junction devices are the magnetoresistance ratio and the switching field. The magnetoresistance ratio is defined as (R AP − R P )∕R P × 100%, where R AP and R P represent the magnetoresistance when the magnetization orientations of the two FM layers are anti-parallel and parallel, respectively. Thus, a larger magnetoresistance ratio indicates a more distinguishable difference between the two states. In particular, the layered 3d-orbital ferromagnetic materials maintain the ferromagnetism down to thicknesses within several atomic layers, thus offering an opportunity to fabricate 2D spintronic devices with high-quality interfaces, and serve as promising candidates for building blocks for constructing magnetic tunnel junctions. In addition, the regulation of the magnetic properties of the FM layer affects the multiple scattering process of tunneling electrons through the FM layers, thus playing a key role in the performance of magnetic tunnel junction.
A schematic illustration of a standard FM-based magnetic tunnel junction device with the spin valve effect is shown in Figure 11a, where the Fe 3 GeTe 2 flakes are separated by the semiconducting InSe layer. [54] Typically, the magnetoresistance ratio of the magnetic tunnel junction device can be modulated by optimizing the component and thickness of the spacer layer. Here, a large magnetoresistance ratio over 40% can be achieved (Figure 11b) based on the intentionally chosen InSe spacer because of the theoretically-predicted giant tunneling magnetoresistance and the close lattice match with Fe 3 GeTe 2 . In addition, by forming a graphene/CrI 3 /graphene sandwich structure, Figure 11. Spintronic devices based on layered 3d-orbital ferromagnets by interfacial engineering. a,b) Fe 3 GeTe 2 /InSe/Fe 3 GeTe 2 -based magnetic tunnel junction device. a) Schematic illustration of the device structure. b) Tunneling magnetoresistance and the corresponding magnetoresistance ratio (labeled as "MR(%)" in the figure) measured at 10 K. Reproduced with permission. [54] Copyright 2021, Wiley-VCH. c-f) Tunable magnetic tunnel junction device. c) Illustration of a tunable magnetic tunnel junction based on Fermiology engineering. d) Optical image and testing geometry of the Fe 3 GeTe 2based magnetic tunnel junction. e) Schematic of Li + intercalation in a tunable magnetic tunnel junction device. f) Tunneling magnetoresistance and magnetoresistance ratio of the magnetic tunnel junction measured at 2 K with increasing gate voltage. Reproduced with permission. [148] Copyright 2022, The Authors, published by UESTC and Wiley-VCH. g,h) magnetic tunnel junction based on the Fe 3 GeTe 2 homojunction without an insulating spacer. g) Tunneling magnetoresistance and magnetoresistance ratio of the two-state magnetic tunnel junction measured at 10 K. h) Tunneling magnetoresistance and magnetoresistance ratio of the three-state magnetic tunnel junction measured at 10 K. Insets of both figures indicate the stacking sequences and testing geometry. Reproduced with permission. [149] Copyright 2020, Elsevier.
the multiple-spin-filter magnetic tunnel junction device was fabricated, and the magnetoresistance ratio with increasing CrI 3 layer numbers was studied. [33] The experimental results indicated that the magnetoresistance ratio could be drastically enhanced as the thickness of CrI 3 increases. Strikingly, a magnetoresistance ratio up to 19,000% can be achieved for a 4layer CrI 3 -based magnetic tunnel junction device under an outof-plane magnetic field, highlighting the potential of 2D ferromagnets in spintronic device applications. Apart from such theoretically-predicted suitable materials for out-of-plane and inplane magnetic tunnel junction devices, [150,151] recent theoretical works claimed that the 3d-orbital ferromagnet Cr 2 Ge 2 Te 6 /WS 2 junction, [152] 1T-VSe 2 [153] and CrPS 4 [154] are also possible candidates for magnetic tunnel junction devices due to their giant magnetoresistance values. In addition, recent experimental work demonstrated a room-temperature magnetic tunnel junction device based on the Fe 3 GaTe 2 /WSe 2 heterojunction with a magnetoresistance ratio up to 85% at 300 K, [155] opening a promising route for spintronic applications.
It should be addressed that new types of 2D ferromagnet-based magnetic tunnel junctions have been demonstrated to broaden their applications. On the one hand, the concept of a tunable magnetic tunnel junction device by combining the magnetic tunnel junction device with the electrochemical intercalation technique was proposed (introduced in Section 3.3). [148] A schematic illustration of the mechanism of the tunable magnetic tunnel junction is shown in Figure 11c. Li + -intercalation can effectively adjust the Fermi energy of the Fe 3 GeTe 2 layer while not affecting the electronic behavior of the insulating h-BN layer, thus making it possible to control the switching field and magnetoresistance ratio for magnetic tunnel junction devices. The insulating layer (h-BN or MoS 2 ) was sandwiched by two exfoliated Fe 3 GeTe 2 flakes (shown in Figure 11d) and then covered by LiClO 4 /polyethylene oxide electrolyte (Figure 11e). The corresponding magnetoresistance results shown in Figure 11f indicate that the switching field first increases and then decreases as the gating voltage increases, and that the magnetoresistance ratio can be enhanced up to 65% at 2 K when the applied gate voltage is 2.8 V, serving as a modification of the ordinary Fe 3 GeTe 2 /h-BN/Fe 3 GeTe 2 magnetic tunnel junction device. [32] The enhanced spin polarization can be attributed to the carrier doping induced by Li + intercalation, which has been confirmed to enhance the ferromagnetism of Fe 3 GeTe 2 [5] by J modulation according to the Stoner criterion. A different strategy by applying electrical bias (introduced in Section 3.1) can also tune the magnetoresistance ratio of such Fe 3 GeTe 2 -based magnetic tunnel junction device. [156] Such results pave routes to the modulation of spin transport properties in spin valve devices. Interestingly, a third electronic engineering method for tuning the properties of FM materials, applying electrical current (introduced in Section 3.2), can also serve as an efficient approach to modulating the magnetoresistance ratio of the magnetic tunnel junction device, as demonstrated in the Cr 1−x Te/Al 2 O 3 /Cr 1−x Te heterojunction. [157] On the other hand, a magnetic tunnel junction setup based on an Fe 3 GeTe 2 homojunction without the insulating spacer layer was demonstrated, [149] as shown in Figure 11g,h. The decoupling effect of vdW interfaces between adjacent layers allows the removal of the insulating spacer, thus greatly reducing the difficulty of processing. Figure 11g demonstrates the tunneling magnetoresistance resistance and the corresponding magnetoresistance ratio of Fe 3 GeTe 2 /Fe 3 GeTe 2 homojunction, and one can see that the magnetoresistance ratio is much smaller compared with those of Fe 3 GeTe 2 /InSe/Fe 3 GeTe 2 ( Figure 11b) and Fe 3 GeTe 2 /h-BN/Fe 3 GeTe 2 (Figure 11f) because of the absence of spacer layer. Interestingly, by stacking three Fe 3 GeTe 2 nanoflakes with different coercive fields, a three-state spin valve device can also be achieved, shown in Figure 11h. Such results allow the realization of multi-state, non-volatile applications of 3d-orbital ferromagnet-based spintronic devices.

Summary and Outlook
In summary, controlling the exchange interactions of magnetism in layered 3d-orbital ferromagnets, especially the J and S parameters, can drastically improve their emergent physical properties and display strong adaptability. Theoretically, although the magnetic properties of a 3d-orbital magnetic system can be generally described by considering only the spin-exchange interactions, the modulation of the charge (electronic state), spin (magnetic) and orbital degrees of freedom of electrons in such materials is also important for strongly correlated electronic systems. Methods for describing the couplings among spin, orbit, and lattice degrees of freedom, developed from early experimental and theoretical work on 3d-orbital magnets, have been successfully extended to other materials. For example, the famous "Goodenough-Kanamori-Anderson rule" [158] has been widely used to explain the phenomena and phase diagrams in 3d-orbital transition metal oxides. In addition, the investigation of various techniques to modulate the J and S parameters for magnetism in materials can provide further insight into fundamental research and make them more suitable for practical applications. For scientific research, the modulating techniques introduced in this review are helpful for understanding the behaviors of magnons, [159,160] skyrmions, [161,162] and their interactions with other quasiparticles. [130] Layered magnetic materials with stronger SOC effect, including specially-designed 3d-orbital ferromagnets and layered 4d-or 5d-orbital ferromagnets, can also be used to study emergent quantum phenomena such as the quantum spin liquid and Kitaev model. [163][164][165][166] For practical applications, these modulating approaches can achieve logic operations on the microscale, thus achieving high-efficiency, lowdelay and low-energy consumption spintronic devices for information technology, showing great potential for next-generation electronics. [167] It should be addressed that there are still several challenges and prospects in manipulating the performance of layered 3dorbital ferromagnets that require further investigation, summarized as follows: 1) Layered ferromagnetic materials with large MAE (enhancing the anisotropy of J to break the continuous rotation symmetry in the system and go beyond the renowned Mermin-Wagner theory [4] ) and achieving higher T C values are still in high demand, since applicable electronic and spintronic devices require stable performance at room temperature. Although some methods, such as applying high pressure in Fe 5 GeTe 2 [56] or intercalating Li + ions in Fe 3 GeTe 2 , [5] can significantly enhance the T C (approach or beyond the room temperature), these tuning methods can hardly be used for massive production. Also, too much disorder introduced during these modulation processes might affect the intrinsic properties of these materials. A recent study on Ni-doped Fe 5 GeTe 2 [168] achieved a record high T C value of 478 K (over 200°C), and a similar enhancement of T C was reported in Co-doped Fe 5 GeTe 2 , [169] providing an idea for solving such a problem.
2) The modulation approaches of tuning the J parameter for magnetism can be optimized from two aspects. On the one hand, it is possible to find more quantitative regulation methods to tune the carrier concentration, layer spacing, stacking mode, strain, etc. in materials, to better investigate how these variations affect exchange interactions and magnetic anisotropy. For example, it is worth mentioning that precise tuning of the interlayer stacking angle of 2D heterojunctions has demonstrated novel electronic states and superconductivity in magic-angle graphene. [118,170] This key parameter for generating novel phenomena in the moiré system can also be a powerful tool for tuning 2D magnetic materials, which has not yet been studied much. On the other hand, combinations of multiple modulation means (for example, gate-tunable magnetic tunnel junctions that combine the interfacial and electronic engineering methods [148] ) are potentially more powerful modulation means. Coupling spin with other degrees of freedom (e.g., superconductivity, ferroelectricity, topology, and thermoelectricity, etc.) can greatly expand the research boundary of layered 3d-orbital ferromagnetic materials.
3) The development of more advanced multidimensional probing techniques will undoubtedly boost the microscopic study of exchange interactions for magnetism in low-dimensional magnetic materials. For example, four-probe scanning tunneling microscopy combined with in situ electrical and optical modulation modules can perform in situ electron transport and photoelectric response measurements in the nanoscale region, revealing the evolution of magnetic fields under applied electric/current/optical fields at the atomic scale; spinpolarized scanning tunneling microscopy can probe the sample current-induced magnetization information at the atomic scale. These techniques facilitate a deep understanding of the microscopic magnetic mechanisms and their correlation with macroscopic magnetic properties and novel physical phenomena. 4) As a new member of the 2D materials family, the construction of heterostructures with 2D magnetic materials is an attractive and important research direction for tuning the interlayer exchange interactions, exploring novel quantum effects and developing multifunctional new concept devices. In these previous studies on the correlated electronic states in twisted and stacked vdW heterostructures, most attention has been given to materials with similar lattice symmetry of triangular/hexagonal lattices, and to the strongly correlated flat-band energy band structures associated with the moiré period. Recently, a "lattice symmetry engineering" concept, i.e., the use of materials with different lattice symmetries to construct 2D heterostructures and to perturb and modify the energy band www.advancedsciencenews.com www.advphysicsres.com at the interface, has been demonstrated, [116] which should inspire a multitude of promising new studies on 2D magnetic materials.