Recently discovered ferromagnetism of the layered van der Waals material VI$_3$ attracts much research attention. Despite substantial progress,in the following important aspects no consensus has been reached: (i) a possible deviation of the easy axis from the normal to the VI$_3$ layers, (ii) a possible inequivalence of the V atoms, (iii) the value of the V magnetic moments. The theoretical works differ in the conclusions on the conduction nature of the system,the value and the role of the V orbital moments. To the best of our knowledge there is no theoretical works addressing issues (i) and (ii) and only one work dealing with the reduced value of the V moment. By combining the symmetry arguments with density functional theory (DFT) and DFT+$U$ calculations we have shown that the antidimerization distortion of the crystal structure reported in Phys. Rev. B {\bf 99}, 041402(R) (2019) must lead to the deviation of the easy axis from the normal to the VI$_3$ layers in close correlation with the experimental results. The antidimerization accompanied by the breaking the inversion symmetry leads to the inequivalence of the V atoms. Our DFT+U calculations result in large value 0.8μ_B$ of the V orbital moments of the V atoms leading to reduced total V moment in agreement with a number of experimental results and with the physical picture suggested in Phys. Rev. B bf 101, 100402(R) (2020). We obtained large intraatomic noncollinearity of the V spin and orbital moments revealing strong competition between effects coursed by the on-site electron correlation, spin-orbit coupling, and interatomic hybridization since pure intraatomic effects lead to collinear spin and orbital moments. Our calculations confirm the experimental results of strong magnetoelastic coupling revealing itself in the strong dependence of the magnetic properties on the distortion of the atomic structure.
Topological magnetism in low-dimensional systems is of fundamental and practical importance in condensed-matter physics and material science. Here, using first-principles and Monte-Carlo simulations, we present that multiple topological magnetism (i.e., skyrmion and bimeron) can survive in van der Waals Heterostructure of MnTe2ZrS2. Arising from interlayer coupling, MnTe2ZrS2 can harbor a large Dzyaloshinskii-Moriya interaction. This, combined with ferromagnetic exchange interaction, yields an intriguing skyrmion phase consisting of sub-10 nm magnetic skyrmions under a tiny magnetic field of 7…
Manipulating the interlayer magnetic coupling in van der Waals magnetic materials and heterostructures is the key to tailoring their magnetic and electronic properties for various electronic applications and fundamental studies in condensed matter physics. By utilizing the MnBi2Te4-family compounds and their heterostructures as a model system, we systematically studied the dependence of the sign and strength of interlayer magnetic coupling on constituent elements by using first-principles calculations. It was found that the coupling is a long-range superexchange interaction mediated by the cha…
Recent developments in two-dimensional (2D) magnetism have motivated the search for novel van-der Waals (vdWs) magnetic materials to explore new magnetic phenomenon in the 2D limit. Metal thiophosphates, MPX3, is a class of magnetic vdWs materials with antiferromagnetic (AFM) ordering persisting down to the atomically thin limit. The magnetism in this material family has been found to be highly dependent on the choice of transition metal M. In this work, we have synthesized the intermediate compounds Ni1-xMnxPS3 (0 < x < 1) and investigated their magnetic properties. Our study reveals that the…
The recent emergence of two-dimensional (2D) van der Waals ferromagnets has provided a new platform for exploring magnetism in the flatland and for designing 2D ferromagnet-based spintronics devices. Despite intensive studies, the anomalous Hall effect (AHE) mechanisms in 2D van der Waals ferromagnets have not been investigated yet. In this paper, we report the AHE mechanisms in quasi-2D van der Waals ferromagnet Fe0.29TaS2 via systematically measuring Fe0.29TaS2 devices with thickness from 14 nm to bulk single crystal. The AHE mechanisms are investigated via the scaling relationship between t…
The van der Waals ferromagnet Fe$_3$GeTe$_2$ has recently attracted extensive research attention due to its intertwined magnetic, electronic and topological properties. Here, using high-resolution angle-resolved photoemission spectroscopy, we systematically investigate the temperature evolution of the electronic structure of bulk Fe$_3$GeTe$_2$. We observe largely dispersive energy bands that are narrowed by a factor of 1.6 compared with ab-initio calculation. Upon heating towards the ferromagnetic transition near 225 K, we observe a massive reduction of quasiparticle coherence in a large ener…
We study the enhancement of the magnetic transition temperature $T_c$ due to incident light in ferromagnetic semiconductors such as EuS. The photoexcited carriers mediate an extra ferromagnetic interaction due to the coupling with the localized magnetic moments. The Hamiltonian consists of a Heisenberg model for the localized moments and an interaction term between the photoexcited carriers and the localized moments. The model predicts a small enhancement of the transition temperature in semi-quantitative agreement with the experiments.
We examined what interactions control the sign and strength of the interlayer coupling in van der Waals ferromagnets such as Fe3-xGeTe2, Cr2Ge2Te6, CrI3 and VI3, to find that high-spin orbital interaction across the van der Waals gaps are a key to understanding their ferromagnetism. Interlayer ferromagnetic coupling in Fe3-xGeTe2 Cr2Ge2Te6, and CrI3 is governed by the high-spin two-orbital two-electron destabilization, but that in VI3 by the high-spin four-orbital two-electron stabilization. These interactions explain a number of seemingly puzzling observations in van der Waals fer-romagnets.
In order to investigate the electronic properties of the semiconducting van der Waals ferromagnet Cr$_2$Ge$_2$Te$_6$ (CGT), where ferromagnetic layers are bonded through van der Waals forces, we have performed angle-resolved photoemission spectroscopy (ARPES) measurements and density-functional-theory (DFT+U) calculations. The valence-band maximum at the Γ point is located $\sim$ 0.2 eV below the Fermi level, consistent with the semiconducting property of CGT. Comparison of the experimental density of states with the DFT calculation has suggested that Coulomb interaction between the Cr 3d elec…
Among the recently widely studied van der Waals layered magnets CrX3 (X=Cl, Br, I), CrCl3 monolayer (ML) is particularly puzzling as it is solely shown by experiments to have an in-plane magnetic easy axis and, furthermore, all of previous first-principles calculation results contradict this. Through systematical first-principles calculations,we unveil that its in-plane shape anisotropy that dominates over its weak perpendicular magnetocrystalline anisotropy is responsible for the in-plane magnetic easy axis of CrCl3 ML. To tune the in-plane ferromagnetism of CrCl3 ML into the desirable perpen…
Two-dimensional (2D) van der Waals (vdWs) materials have gathered a lot of attention recently. However, the majority of these materials have Curie temperatures that are well below room temperature, making it challenging to incorporate them into device applications. In this work, we synthesized a room-temperature vdW magnetic crystal Fe$_5$GeTe$_2$ with a Curie temperature T$_c = 332$ K, and studied its magnetic properties by vibrating sample magnetometry (VSM) and broadband ferromagnetic resonance (FMR) spectroscopy. The experiments were performed with external magnetic fields applied along th…
We investigated the Dirac-cone state and its modulation when an ultrathin film of topological insulator Bi2Se3 was epitaxially grown on a van-der-Waals ferromagnet Cr2Si2Te6 (CST) by angle-resolved photoemission spectroscopy. We observed a gapless Dirac-cone surface state in 6 quintuple-layer (QL) Bi2Se3 on CST, whereas the Dirac cone exhibits a gap of 0.37 eV in 2QL counterpart. Intriguingly, this gap is much larger than those for Bi2Se3 films on Si(111). We also revealed no discernible change in the gap magnitude across the ferromagnetic transition of CST, suggesting the very small character…
We address the importance of the modern theory of orbital magnetization for spintronics. Based on an all-electron first-principles approach, we demonstrate that the predictive power of the routinely employed "atom-centered" approximation is limited to materials like elemental bulk ferromagnets, while the application of the modern theory of orbital magnetization is crucial in chemically or structurally inhomogeneous systems such as magnetic thin films, and materials exhibiting non-trivial topology in reciprocal and real space,~e.g.,~Chern insulators or non-collinear systems. We find that the mo…
Electronic structure of layered van der Waals ferromagnet CrI$_{3}$ is studied with self consistent diagrammatic approaches beyond GW approximation. Considerable improvement in the calculated band gap as compared to the non-self-consistent G0W0 results has been found. Certain spectral features in the valence bands discovered recently by the angle resolved photoemission spectroscopy, are reproduced better when we use full frequency dependent self energy. Density functional theory and quasiparticle self-consistent GW method which are based on frequency-independent self energy are unable to resol…
Laser-Induced Topological Toggle Switching at Room Temperature in the van der Waals Ferromagnet Fe3GaTe2
University College London（伦敦大学学院）；Korea Advanced Institute of Science and Technology (KAIST)（韩国科学技术院）；University of Exeter（埃克塞特大学）；The University of Edinburgh（爱丁堡大学）
Manipulating quantum state via electrostatic gating has been intriguing for many model systems in nanoelectronics. When it comes to the question of controlling the electron spins, more specifically, the magnetism of a system, tuning with electric field has been proven to be elusive. Recently, magnetic layered semiconductors have attracted much attention due to their emerging new physical phenomena. However, challenges still remain in the demonstration of a gate controllable magnetism based on them. Here, we show that, via ionic gating, strong field effect can be observed in few-layered semicon…
Identifying intrinsic low-dimensional ferromagnets with high transition temperature and electrically tunable magnetism is crucial for the development of miniaturized spintronics and magnetoelectrics. Recently long-range 2D ferromagnetism was observed in van der Waals crystals CrI$_3$ and Cr$_2$Ge$_2$Te$_6$, however their Curie temperature is significantly lowered when reducing down to monolayer/few layers. Herein, using renormalized spin-wave theory and first-principles electronic structure theory, we present a theoretical study of electrically tunable 2D ferromagnetism in van der Waals layere…
The discoveries of intrinsic ferromagnetism in atomically-thin van der Waals crystals have opened up a new research field enabling fundamental studies on magnetism at two-dimensional (2D) limit as well as development of magnetic van der Waals heterostructures. To date, a variety of 2D ferromagnetism has been explored mainly by mechanically exfoliating 'originally ferromagnetic (FM)' van der Waals crystals, while bottom-up approach by thin film growth technique has demonstrated emergent 2D ferromagnetism in a variety of 'originally non-FM' van der Waals materials. Here we demonstrate that V5Se8…
We use the synergy of infrared, terahertz, and Raman spectroscopies with DFT calculations to shed light on the magnetic and lattice properties of VI3. The structural transition at TS1 = 79 K is accompanied by a large splitting of polar phonon modes. Below TS1, strong ferromagnetic fluctuations are observed. The variations of phonon frequencies at 55 K induced by magnetoelastic coupling enhanced by spin-orbit interaction indicate the proximity of long-range ferromagnetic order. Below TC = 50 K, two Raman modes simultaneously appear and show dramatic softening in the narrow interval around the t…
A recent experimental study (Phys. Rev. Appl. 9, 024023, 2018) on paramagnetic CeCo$_3$ finds that Magnesium alloying induces a ferromagnetic transition with intrinsic properties large enough for permanent magnet applications. Here we explain these surprising results \textit{via} a first principles study of the electronic structure and magnetism of Magnesium-alloyed CeCo$_3$. We find the origin of this Magnesium-induced ferromagnetic transition to be Stoner physics - the substantial increase in the Fermi-level density-of-states $N(E_F)$ with Mg alloying. Our calculations suggest that both Ce a…
The discovery of an ever increasing family of atomic layered magnetic materials, together with the already established vast catalogue of strong spin-orbit coupling (SOC) and topological systems, calls for some guiding principles to tailor and optimize novel spin transport and optical properties at their interfaces. Here we focus on the latest developments in both fields that have brought them closer together and make them ripe for future fruitful synergy. After outlining fundamentals on van der Waals (vdW) magnetism and SOC effects, we discuss how their coexistence, manipulation and competitio…
Manipulation of long-range order in two-dimensional (2D) van der Waals (vdW) magnetic materials (e.g., CrI$_3$, CrSiTe$_3$ etc.), exfoliated in few-atomic layer, can be achieved via application of electric field, mechanical-constraint, interface engineering, or even by chemical substitution/doping. Usually, active surface oxidation due to the exposure in the ambient condition and hydrolysis in the presence of water/moisture causes degradation in magnetic nanosheets which, in turn, affects the nanoelectronic/spintronic device performance. Counterintuitively, our current study reveals that expos…
Low temperature magnetization of CrI3, CrSiTe3 and CrGeTe3 single crystals were systematically studied. Based on the temperature dependence of extrapolated spontaneous magnetization from magnetic isotherms measured at different temperatures, the spin stiffness constant (D) and spin excitation gap ($Δ$) were extracted according to Bloch's law. For spin stiffness, D is estimated to be 27${\pm}$6 meV $Å^2$, 20${\pm}$3 meV $Å^2$ and 38${\pm}$7 meV $Å^2$ for CrI3, CrSiTe3 and CrGeTe3 respectively. Spin excitation gaps determined via Bloch's formulation have larger error bars yielding 0.59${\pm}$0.3…
The existence of long range magnetic order in low dimensional magnetic systems, such as the quasi-two-dimensional (2D) van der Waals (vdW) magnets, has attracted intensive studies of new physical phenomena. The vdW Fe$_N$GeTe$_2$ ($N$ = 3, 4, 5; FGT) family is exceptional owing to its vast tunability of magnetic properties. Particularly, a ferromagnetic ordering temperature ($T_{\text{C}}$) above room temperature at $N$ = 5 (F5GT) is observed. Here, our study shows that, by nickel (Ni) substitution of iron (Fe) in F5GT, a record high $T_{\text{C}}$ = 478(6) K is achieved. Importantly, pervasiv…
Recently intrinsic ferromagnetism in two-dimensional(2D) van der Waals materials was discovered [1, 2, 3]. A monolayer of Chromiun triiodide(CrI3) is ferromagnetic while a bilayer structure was reported to be anti-ferro magnetic, moreover an external electric field changes its magnetic phase [4]. We have studied the two found in nature stackings of CrI3 bilayers and found that indeed the magnetic phase of one of them can be tuned by an external electric field while the other remains ferromagnetic. We simulate those results with ab initio calculations and explain them with a simple model based …
In recent years, two-dimensional van der Waals materials have emerged as an important platform for the observation of long-range ferromagnetic order in atomically thin layers. Although heterostructures of such materials can be conceived to harness and couple a wide range of magneto-optical and magneto-electrical properties, technologically relevant applications require Curie temperatures at or above room-temperature and the ability to grow films over large areas. Here we demonstrate the large-area growth of single-crystal ultrathin films of stoichiometric Fe5GeTe2 on an insulating substrate us…
Coexistence of ferromagnetism and ferroelectricity in the van der Waals multiferroic CuIn0.2V0.8P2S6
D crystal consortium, Materials Research Institute, Pennsylvania State University（宾夕法尼亚州立大学材料研究所）；Department of Physics, Pennsylvania State University（宾夕法尼亚州立大学物理系）；Department of Electrical and Computer Engineering and Quantum Technology Center, University of Maryland（马里兰大学电气与计算机工程系及量子技术中心）；Department of Chemistry, Michigan State University（密歇根州立大学化学系）；Department of Materials Science and Engineering, Pennsylvania State University（宾夕法尼亚州立大学材料科学与工程系）
The recent emergence of magnetic van der Waals materials allows for the investigation of current induced magnetization manipulation in two dimensional materials. Uniquely, Fe3GeTe2 has a crystalline structure that allows for the presence of bulk spin-orbit torques (SOTs), that we quantify in a Fe3GeTe2 flake. From the symmetry of the measured torques, we identify the current induced effective fields using harmonic analysis and find dominant bulk SOTs, which arise from the symmetry in the crystal structure. Our results show that Fe3GeTe2 uniquely can exhibit bulk SOTs in addition to the convent…
Recently, van der Waals (vdW) magnetic heterostructures have received increasing research attention in spintronics. However, the lack of room-temperature magnetic order of vdW material has largely impedes its development in practical spintronics devices. Inspired by the recently discovered vdW ferromagnet Fe3GaTe2, which has been shown to have magnetic order above room temperature and sizable perpendicular magnetic anisotropy, we investigate the basic electronic structure and magnetic properties of Fe3GaTe2 as well as tunneling magnetoresistance effect in magnetic tunnel junctions (MTJs) with …
Charge transfer plays a key role at the interfaces of heterostructures, which can affect electronic structures and ultimately the physical properties of the materials. However, charge transfer is difficult to manipulate externally once the interface formed. Here, we report electrically tunable charge transfer in Fe3GeTe2/Cr2Ge2Te6/Fe3GeTe2 all-magnetic van der Waals heterostructures, which can be exploited to selectively modify the magnetic properties of the top or bottom Fe3GeTe2 electrodes. The directional charge transfer from metallic Fe3GeTe2 to semiconducting Cr2Ge2Te6 remarkably modifies…
Two-dimensional van der Waals MnBi$_{2n}$Te$_{3n+1}$ (n = 1, 2, 3, 4) compounds have been recently found to be intrinsic magnetic topological insulators rendering quantum anomalous Hall effect and diverse topological states. Here, we summarize and compare the crystal and magnetic structures of this family, and discuss the effects of chemical composition on their magnetism. We found that a considerable fraction of Bi occupies at the Mn sites in MnBi$_{2n}$Te$_{3n+1}$ (n = 1, 2, 3, 4) while Mn is no detectable at the non-magnetic atomic sites within the resolution of neutron diffraction experime…
The recent experimental discovery of intrinsic ferromagnetism in single-layer CrI3 opens a new avenue to low-dimensional spintronics. However, the low Curie temperature Tc=45 K is still a large obstacle to its realistic device application. In this work, we demonstrate that the Tc and magnetic moment of CrX3(X=Br, I) can be enhanced simultaneously by coupling them to buckled two-dimensional Mene (M=Si, Ge) to form magnetic van der Waals (vdW) heterostructures. Our first-principles calculations reveal that n-doping of CrX3, induced by a significant spin-dependent interlayer charge transfer from …
Among several well-known transition metal-based compounds, the van der Waals (vdW) Fe3-xGeTe2 (FGT) magnet is a strong candidate for use in two-dimensional (2D) magnetic devices due to its strong perpendicular magnetic anisotropy, sizeable Curie temperature (TC ~ 154 K), and versatile magnetic character that is retained in the low-dimensional limit. While the TC remains far too low for practical applications, there has been a successful push toward improving it via external driving forces such as pressure, irradiation, and doping. Here we present experimental evidence of a novel room-temperatu…
We study the microscopical origin of anisotropic ferromagnetism in the van der Waals magnet CrI3. We conclude that the nearest neighbors exchange is well described by the Heisenberg-Kitaev-Gamma (HKGamma) model, and we also found a nonzero Dzyaloshinskii-Moriya interaction (DMI) on next nearest neighbors. Both Kitaev and DMI are known to generate a non-trivial topology of the magnons in the honeycomb lattice and have been used separately to describe the low energy regime of this material. We discuss that including one or the other leads to different signs of the Chern number. Furthermore, the …
The recent discovery of magnetic van der Waals materials has triggered a wealth of investigations in materials science, and now offers genuinely new prospects for both fundamental and applied research. Although the catalogue of van der Waals ferromagnets is rapidly expanding, most of them have a Curie temperature below 300 K, a notable disadvantage for potential applications. Combining element-selective x-ray magnetic imaging and magnetic force microscopy, we resolve at room temperature the magnetic domains and domains walls in micron-sized flakes of the CrTe$_2$ van der Waals ferromagnet. Flu…
Magnetic insulators are a key resource for next-generation spintronic and topological devices. The family of layered metal halides promises ultrathin insulating multiferroics, spin liquids, and ferromagnets, but new characterization methods are required to unlock their potential. Here, we report tunneling through the layered magnetic insulator CrI3 as a function of temperature and applied magnetic field. We electrically detect the magnetic ground state and inter-layer coupling and observe a field-induced metamagnetic transition. The metamagnetic transition results in magnetoresistances of 95%,…
Ferromagnetic semiconductors discovered in two-dimensional (2D) materials open an avenue for highly integrated and multifunctional spintronics. The Curie temperature (TC) of existed 2D ferromagnetic semiconductors is extremely low and the modulation effect of their magnetism is limited compared with their 2D metallic counterparts. The interfacial effect was found to effectively manipulate the three-dimensional magnetism, providing a unique opportunity for tailoring the 2D magnetism. Here we demonstrate that the TC of 2D ferromagnetic semiconductor Cr2Ge2Te6 can be enhanced by 130% (from ~65 K …
Chemisorption on ferromagnetic and non-magnetic surfaces is discussed within the Newns-Anderson-Grimley model along with the Stoner model of ferromagnetism. In the case of ferromagnetic surfaces, the adsorption energy is formulated in terms of the change in surface magnetic moments. Using such a formulation, we address the issue of how an adsorbate's binding strength depends on the magnetic moments of the surface and how the adsorption process reduces/enhances the magnetic moments of the surface. Our results indicates a possible adsorption energy scaling relationship in terms surface magnetic …
Defect-induced ferromagnetism has triggered a lot of investigations and controversies. The major issue is that the induced ferromagnetic signal is so weak that it can sufficiently be accounted for by trace contamination. To resolve this issue, we studied the variation of the magnetic properties of SiC after neutron irradiation with fluence covering four orders of magnitude. A large paramagnetic component has been induced and scales up with defect concentration, which can be well accounted for by uncoupled divacancies. However, the ferromagnetic contribution is still weak and only appears in th…
Flexible and efficient manipulation of magnetic configurations can be challenging. In the design of practical devices, achieving a high effective magnetic field with a low working current is under tight demand. Here, we report a unique method for efficient magnetism modulation by direct current injection in magnetic Weyl semimetal Co3Sn2S2. We demonstrate that the modulation process stems from current-assisted domain wall motion. Through two independent methods, we reveal that the spin-transfer torque efficiency of Co3Sn2S2 reaches as high as 2.4-5.6 kOe MA^(-1) cm^2, and the threshold current…
Incorporating magnetism into two dimensional (2D) van der Waals (VdW) heterostrutures is crucial for the development of functional electronic and magnetic devices. Here we show that Nb3X8 (X = Cl, Br) is a family of 2D layered trimerized kagomé magnets that are paramagnetic at high temperatures and undergo a first order phase transition on cooling to a singlet magnetic state. X-ray diffraction shows that a rearrangement of the VdW stacking accompanies the magnetic transition, with high and low temperature phases consistent with STEM images of the end members α-Nb3Cl8 and \b{eta}-Nb3Br8. The te…
The recent isolation of two-dimensional (2D) magnets offers tantalizing opportunities for spintronics and magnonics at the limit of miniaturization. One of the key advantages of atomically-thin materials is their outstanding deformation capacity, which provides an exciting avenue to control their properties by strain engineering. Herein, we investigate the magnetic properties, magnon dispersion and spin dynamics of the air-stable 2D magnetic semiconductor CrSBr ($T_C$ = 146 K) under mechanical strain using first-principles calculations. Our results provide a deep microscopic analysis of the co…
We report a density functional study of the electronic structure and magnetism of Y2Ni7. The results show itinerant magnetism very similar to that in the weak itinerant ferromagnet Ni3Al. The electropositive Y atoms in Y2Ni7 serves to donate charge to the Ni host mostly in the form of $s$ electrons. The non-spin-polarized state shows a high density of states at the Fermi level, N(EF) due to flat bands. This leads to the ferromagnetic instability. However, there are also several much more dispersive bands crossing E(F), which should promote the conductivity. Spin fluctuation effects appear to b…
We review recent progress on spins and magnetism in 2D materials including graphene, transition metal dichalcogenides, and 2D magnets. We also discuss challenges and prospects for the future of spintronics with 2D van der Waals heterostructures.
We have synthesized unique colloidal nanoplatelets of the ferromagnetic two-dimensional (2D) van der Waals material CrI3 and have characterized these nanoplatelets structurally, magnetically, and by magnetic circular dichroism spectroscopy. The isolated CrI3 nanoplatelets have lateral dimensions of ~25 nm and ensemble thicknesses of only ~4 nm, corresponding to just a few CrI3 monolayers. Magnetic and magneto-optical measurements demonstrate robust 2D ferromagnetic ordering in these nanoplatelets with Curie temperatures similar to those observed in bulk CrI3, despite the strong spatial confine…
