Few layers Fe$_3$GeTe$_2$ is currently the only atomically thin ferromagnetic metal, and thus has drawn huge attention in the field of two-dimensional (2D) magnetism. In this paper, we perform a systematic first principle study on the electronic structure, magnetic anisotropy energy (MAE), and magneto-optical (MO) effects in monolayer (ML), bilayer (BL) and trilayer (TL) as well as bulk Fe$_3$GeTe$_2$. All the considered structures of Fe$_3$GeTe$_2$ are predicted to have large MAE of order $\sim$3.0 meV/f.u., being larger than reported 2D ferromagnetic semiconductors Cr$_2$Ge$_2$Te$_6$ and CrI$_3$ and also being comparable to that of FePt which has the largest MAE among the transition metal alloys. This large MAE thus stabilizes the long range ferromagnetic order down to atomically thin layers and also suggests promising applications of 2D Fe$_3$GeTe$_2$ in high density data storage. Furthermore, the calculated magneto-optical spectra show large magnetic circular dichroism, thus resulting in large MO Kerr rotation and Faraday rotation angles. In visible frequency range, Kerr rotation angles up to $\sim$1.0$^\circ$ for TL Fe$_3$GeTe$_2$ are found. Such values are larger than famous MO transition metal alloy MnBi. Also, large Faraday rotation angles are predicted for all considered Fe$_3$GeTe$_2$ structures. In particular, ML Fe$_3$GeTe$_2$ has a Faraday rotation angle of -156$^\circ$/$μm$, which is three times larger than famous MO oxide Bi$_3$Fe$_5$O$_{12}$. These important findings are analysed in terms of the calculated orbital-decomposed density of states and dipole selection rule derived from the group theory. Our findings thus suggest that few layers and bulk Fe$_3$GeTe$_2$ are promising MO materials and could be widely applied to nano MO devices in the future.
Utilizing the first-principles calculations together with the group theory analysis, we systematically investigate the magnetocrystalline anisotropy energy, magneto-optical effect, and anomalous transport properties (including anomalous Hall, Nernst, and thermal Hall effects) of monolayer and bilayer Fe$_n$GeTe$_2$ ($n$ = 3, 4, 5). The monolayer Fe$_n$GeTe$_2$ ($n$ = 3, 4, 5) exhibits the out-of-plane, in-plane, and in-plane ferromagnetic orders with considerable magnetocrystalline anisotropy energies of -3.17, 4.42, and 0.58 meV/f.u., respectively. Ferromagnetic order is predicted in bilayer …
Material research has been a major driving force in the development of modern nano-electronic devices. In particular, research in magnetic thin films has revolutionized the development of spintronic devices; identifying new magnetic materials is key to better device performance and new device paradigm. The advent of two-dimensional van der Waals crystals creates new possibilities. This family of materials retain their chemical stability and structural integrity down to monolayers and, being atomically thin, are readily tuned by various kinds of gate modulation. Recent experiments have demonstr…
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…
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…
To understand the magnetic properties of Fe$_3$GeTe$_2$, we performed the detailed first-principles study. Contrary to the conventional wisdom, it is unambiguously shown that Fe$_3$GeTe$_2$ is not ferromagnetic but antiferromagnetic carrying zero net moment in its stoichiometric phase. Fe defect and hole doping are the keys to make this material ferromagnetic, which are shown by the magnetic force response as well as the total energy calculation with the explicit Fe defects and the varied system charges. Further, we found that the electron doping also induces the antiferro- to ferromagnetic tr…
By performing density functional theory (DFT) and Green's functions calculations, complemented by X-ray Photoemission Spectroscopy, we investigate the electronic structure of Fe/GeTe(111), a prototypical ferromagnetic/Rashba-ferroelectric interface. We reveal that such system exhibits several intriguing properties resulting from the complex interplay of exchange interaction, electric polarization and spin-orbit coupling. Despite a rather strong interfacial hybridization between Fe and GeTe bands, resulting in a complete suppression of the surface states of the latter, the bulk Rashba bands are…
Long lifetimes of nanoscale skyrmions in lithium-decorated van der Waals ferromagnet Fe$_3$GeTe$_2$
Institute of Theoretical Physics and Astrophysics, University of Kiel（基尔大学理论物理与天体物理研究所）；CEMES, Université de Toulouse, CNRS（图卢兹大学CNRS CEMES）；Kiel Nano, Surface, and Interface Science (KiNSIS), University of Kiel（基尔大学基尔纳米、表面与界面科学(KiNSIS)）
Scalable fabrication of magnetic 2D materials and heterostructures constitutes a crucial step for scaling down current spintronic devices and the development of novel spintronic applications. Here, we report on van der Waals (vdW) epitaxy of the layered magnetic metal Fe$_3$GeTe$_2$ - a 2D crystal with highly tunable properties and a high prospect for room temperature ferromagnetism - directly on graphene by employing molecular beam epitaxy. Morphological and structural characterization confirmed the realization of large-area, continuous Fe$_3$GeTe$_2$/graphene heterostructure films with stabl…
Magnetic anisotropy in the near-stoichiometric van der Waals ferromagnet Fe$_3$GeTe$_2$
Leibniz Institute for Solid State and Materials Research（莱布尼茨固体与材料研究所）；Institute for Solid State and Materials Physics, TU Dresden（德累斯顿工业大学固体与材料物理研究所）；Department of Condensed Matter and Materials Physics, S. N. Bose National Centre for Basic Sciences（S.N. 玻色基础科学国家中心凝聚态与材料物理系）；School of Physical Sciences, Indian Association for the Cultivation of Science（印度科学促进协会物理科学学院）
Magnetic van der Waals (vdW) materials, including ferromagnets (FM) and antiferromagnets (AFM), have given access to the investigation of magnetism in two-dimensional (2D) limit and attracted broad interests recently. However, most of them are semiconducting or insulating and the vdW itinerant magnets, especially vdW itinerant AFM, are very rare. Here, we studied the anomalous Hall effect of a vdW itinerant magnet Fe$_5$GeTe$_2$ (F5GT) with various thicknesses down to 6.8 nm (two unit cells). Despite the robust ferromagnetic ground state in thin-layer F5GT, however, we show that the electron d…
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…
Topologically protected magnetic structures provide a robust platform for low power consumption devices for computation and data storage. Examples of these structures are skyrmions, chiral domain walls, and spin spirals. Here we use scanning electron microscopy with polarization analysis to unveil the presence of chiral counterclockwise Néel spin spirals at the surface of a bulk van der Waals ferromagnet Fe$_3$GeTe$_2$ (FGT), at zero magnetic field. These Néel spin spirals survive up to FGT's Curie temperature $T_\mathrm{C}= 220 \mathrm{~K}$, with little change in the periodicity $p=300 \mathr…
We demonstrate that kinetics play an important role in the epitaxial growth of Fe$_3$GeTe$_2$ (FGT) van der Waals (vdW) ferromagnetic films by molecular beam epitaxy. By varying the deposition rate, we control the formation or suppression of an initial tellurium-deficient non-van der Waals phase (Fe$_3$Ge$_2$) prior to realizing epitaxial growth of the vdW FGT phase. Using cross-sectional scanning transmission electron microscopy and scanning tunneling microscopy, we optimize the FGT films to have atomically smooth surfaces and abrupt interfaces with the Ge(111) substrate. The magnetic propert…
Anomalous Nernst effect, a result of charge current driven by temperature gradient, provides a probe of the topological nature of materials due to its sensitivity to the Berry curvature near the Fermi level. Fe3GeTe2, one important member of the recently discovered two-dimensional van der Waals magnetic materials, offers a unique platform for anomalous Nernst effect because of its metallic and topological nature. Here, we report the observation of large anomalous Nernst effect in Fe3GeTe2. The anomalous Hall angle and anomalous Nernst angle are about 0.07 and 0.09 respectively, far larger than…
Anisotropic electron-phonon coupling and chiral phonons in van der Waals room temperature ferromagnet Fe$_{5}$GeTe$_{2}$
Indian Institute of Technology Kharagpur（印度理工学院卡拉格普尔分校）；Institute of Nuclear Physics, Polish Academy of Sciences（波兰科学院核物理研究所）；Jawaharlal Nehru University（贾瓦哈拉尔·尼赫鲁大学）；Sungkyunkwan University（成均馆大学）
The weak interlayer coupling in van der Waals (vdW) magnets has confined their application to two dimensional (2D) spintronic devices. Here, we demonstrate that the interlayer coupling in a vdW magnet Fe$_3$GeTe$_2$ (FGT) can be largely modulated by a protonic gate.With the increase of the protons intercalated among vdW layers,interlayer magnetic coupling increases.Because of the existence of antiferromagnetic layers in FGT nanoflakes, the increasing interlayer magnetic coupling induces exchange bias in protonated FGT nanoflakes. Most strikingly, a rarely seen zero-field cooled (ZFC) exchange …
We investigate the impact of mechanical strain, stacking order, and external electric fields on the magnetic interactions of a two-dimensional (2D) van der Waals (vdW) heterostructure in which a 2D ferromagnetic metallic Fe$_3$GeTe$_2$ monolayer is deposited on germanene. Three distinct computational approaches based on \textit{ab initio} methods are used, and a careful comparison is given: (i) The Green's function method, (ii) the generalized Bloch theorem, and (iii) the supercell approach. First, the shell-resolved exchange constants are calculated for the three Fe atoms within the unit cell…
In the local or itinerant extreme, magnetic excitations can be described by the Heisenberg model which treats electron spins as localized moments, or by the itinerant-electron model where the exchange interaction between electrons leads to unequal numbers of electrons with up and down spins. However, it has been elusive when both local moments and itinerant electrons are present in the intermediate range. Using inelastic neutron scattering, we provide direct spectroscopic evidence on the coexistence of and interplay between local moments and itinerant electrons in a van der Waals metallic ferr…
Intrinsic magnetic van der Waals (vdW) materials have attracted much attention, especially Fe$_{3}$GeTe$_{2}$ (FGT), which exhibits highly tunable properties. However, despite vast efforts, there are still several challenging issues to be resolved. Here using a first-principles linear-response approach, we carry out comprehensive investigation of both bulk and monolayer FGT. We find that although the magnetic exchange interactions in FGT are frustrated, our Monte Carlo simulations agree with the total energy calculations, confirming that the ground state of bulk FGT is indeed ferromagnetic (FM…
Fe$_{3-x}$GeTe$_2$ is a layered van der Waals magnetic material with a relatively high ordering temperature and large anisotropy. While most studies have concluded the interlayer ordering to be ferromagnetic, there have also been reports of interlayer antiferromagnetism in Fe$_{3-x}$GeTe$_2$. Here, we investigate the interlayer magnetic ordering by neutron diffraction experiments, scanning tunneling microscopy (STM) and spin-polarized STM measurements, density functional theory plus U calculations and STM simulations. We conclude that the layers of Fe$_{3-x}$GeTe$_2$ are coupled ferromagnetica…
Tuning Structure and Magnetism in Large-Scale 2D Ferromagnet Fe$_3$GeTe$_2$ through Ni Doping
Department of Physics and Astronomy, Uppsala University（乌普萨拉大学物理与天文系）；Zernike Institute for Advanced Materials, University of Groningen（格罗宁根大学泽尔尼克先进材料研究所）；ALBA Synchrotron Light Facility, CELLS（阿尔巴同步辐射光源设施，CELLS）；Helmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research（德累斯顿-罗斯多夫亥姆霍兹中心，离子束物理与材料研究所）；Departament de Física de la Matéria Condensada, Universitat de Barcelona（巴塞罗那大学凝聚态物理系）；Institut de Nanociéncia i Nanotecnologia (IN2UB), Universitat de Barcelona（巴塞罗那大学纳米科学与纳米技术研究所（IN2UB））
High-Temperature ferromagnetism from site-selective filling in (Fe,Ni)$_{6-δ}$GeTe$_2$
Yale University（耶鲁大学）；University of California, Berkeley（加州大学伯克利分校）；Lawrence Berkeley National Lab（劳伦斯伯克利国家实验室）；Cornell University（康奈尔大学）；University of Southern California（南加州大学）；Boston College（波士顿学院）
The magnetic structure and phase diagram of the layered ferromagnetic compound Fe$_3$GeTe$_2$ has been investigated by a combination of synthesis, x-ray and neutron diffraction, high resolution microscopy, and magnetization measurements. Single crystals were synthesized by self-flux reactions, and single crystal neutron diffraction finds ferromagnetic order with moments of 1.11(5)$μ_B$/Fe aligned along the $c$-axis at 4K. These flux-grown crystals have a lower Curie temperature $T_{\textrm{c}}\approx$150K compared to crystals previously grown by vapor transport ($T_{\textrm{c}}$=220K). The dif…
In order to tune the magnetic properties of the cleavable high-Curie temperature ferromagnet Fe$_{5-x}$GeTe$_2$, the effect of increasing the electron count through arsenic substitution has been investigated. Small additions of arsenic (2.5 and 5%) seemingly enhance ferromagnetic order in polycrystalline samples by quenching fluctuations on one of the three magnetic sublattices, whereas larger As concentrations decrease the ferromagnetic Curie temperature ($T_{\rm C}$) and saturation magnetization. This work also describes the growth and characterization of Fe$_{4.8}$AsTe$_2$ single crystals t…
Thin van der Waals (vdW) layered magnetic materials disclose the possibility to realize vdW heterostructures with new functionalities. Here we report on the realization and investigation of tunneling spin valves based on van der Waals heterostructures consisting of an atomically thin hBN layer acting as tunnel barrier and two exfoliated Fe3GeTe2 crystals acting as ferromagnetic electrodes. Low-temperature anomalous Hall effect measurements show that thin Fe3GeTe2 crystals are metallic ferromagnets with an easy axis perpendicular to the layers, and a very sharp magnetization switching at magnet…
We systematically investigate the influence of high pressure on the electronic transport properties of layered ferromagnetic materials, in particular, those of Fe$_3$GeTe$_2$. Its crystal sustains a hexagonal phase under high pressures up to 25.9 GPa, while the Curie temperature decreases monotonously with the increasing pressure. By applying appropriate pressures, the experimentally measured anomalous Hall conductivity, $σ_{xy}^A$, can be efficiently controlled. Our theoretical study reveals that this finding can be attributed to the shift of the spin--orbit-coupling-induced splitting bands o…
Among two-dimensional materials, Fe$_3$GeTe$_2$ has come to occupy a very important place owing to its ferromagnetic nature with one of the highest Curie temperatures among known van der Waals materials and the potential for hosting skyrmions. In this combined experimental and theoretical work, we investigate the magnetic bubble domains as well as the microscopic domain wall profile using spin-polarized scanning tunneling microscopy in combination with atomistic spin-dynamics simulations performed with parameters from density functional theory calculations. We find a weak magneto-electric effe…
Fe$_{3}$GeTe$_{2}$ is a two-dimensional van der Waals material with a ferromagnetic ground state and a maximum transition temperature $T_{\mathrm{c}}\sim225$ K. However, when Fe$_{3}$GeTe$_{2}$ is synthesized lower values of $T_{\mathrm{c}}$ are often reported. This is attributed to a deficiency in the Fe at the 2c site in the crystal structure. Here we investigate the effect of Fe deficiency and the substitution of Co for Fe on the magnetic properties of this system. We have synthesized both polycrystalline material and single crystals by chemical vapor transport and the flux method, with the…
We report the doping effects of cobalt on van der Waals (vdW) magnet Fe$_5$GeTe$_2$. A series of (Fe$_{1-x}$Co$_x$)$_5$GeTe$_2$ (0$\leq$x$\leq$0.44) single crystals have been successfully grown, their structural, magnetic and transport properties are investigated. For x=0.20, The Curie temperature $T_C$ increases from 276~K to 337~K. Moreover, the magnetic easy-axis is reoriented to the $ab$-plane from the $c$-axis in undoped Fe$_5$GeTe$_2$ with largely enhanced magnetic anisotropy. These magnetic properties would make (Fe$_{0.8}$Co$_{0.2}$)$_5$GeTe$_2$ more effective in stabilizing magnetic o…
The quasi-two-dimensional van der Waals magnet Fe$_{5-δ}$GeTe$_2$ has emerged as a promising platform for electronic and spintronic functionalities at room temperature, owing to its large ferromagnetic ordering temperature $T_{\text{C}}$ $\sim$ 315 K. Interestingly, by cobalt (Co) substitution of iron in F5GT, $i.e.$ $({\text{Fe}}_{1-x}{\text{Co}}_x)_{5-δ}{\text{GeTe}}_2$ (Co-F5GT), not only can its magnetic transition temperature be further enhanced, but the magnetic and structural ground states can also be tuned. Specifically, an antiferromagnetic (AFM) order is induced beyond the Co doping …
Fe$_{5-x}$GeTe$_2$ is a centrosymmetric, layered van der Waals (vdW) ferromagnet that displays Curie temperatures $T_c$ (270-330 K) that are within the useful range for spintronic applications. However, little is known about the interplay between its topological spin textures (e.g., merons, skyrmions) with technologically relevant transport properties such as the topological Hall effect (THE), or topological thermal transport. Here, we show via high-resolution Lorentz transmission electron microscopy that merons and anti-meron pairs coexist with Néel skyrmions in Fe$_{5-x}$GeTe$_2$ over a wide…
Suppression of local magnetic moment formation and paramagnetic exchange interactions in monolayer Fe$_3$GeTe$_2$
Moscow Institute of Physics and Technology（莫斯科物理技术学院）；M. N. Mikheev Institute of Metal Physics of Ural Branch of Russian Academy of Sciences（俄罗斯科学院乌拉尔分校 M.N. 米凯耶夫金属物理研究所）；Radboud University, Institute for Molecules and Materials（拉德堡德大学分子与材料研究所）
Magnetic skyrmions are chiral nanoscale spin textures which are usually induced by Dzyaloshinskii-Moriya interaction (DMI). Recently, magnetic skyrmions have been observed in two-dimensional (2D) van der Waals (vdW) ferromagnetic materials, such as Fe$_{3}$GeTe$_{2}$. The electric control of skyrmions is important for their potential application in low-power memory technologies. Here, we predict that DMI and magnetic skyrmions in a Fe$_{3}$GeTe$_{2}$ monolayer can be controlled by ferroelectric polarization of an adjacent 2D vdW ferroelectric In$_{2}$Se$_{3}$. Based on density functional theor…
It is shown theoretically that a giant magnetoelectric susceptibility exceeding 10^-6 s/m may be achieved in the ferromagnetic/ferroelectric epitaxial systems via the magnetization rotation induced by an electric field applied to the substrate. The predicted magnetoelectric anomaly results from the strain-driven spin-reorientation transitions in ferromagnetic films, which take place at experimentally accessible misfit strains in CoFe2O4 and Ni films.
We report the anomalous magnetocaloric behavior in the polycrystalline compound Tb2Ni2Sn. The magnetization measurements show that this compound shows multiple magnetic transitions, which are attributed to the coexistence of ferromagnetic and antiferromagnetic phases at low temperatures. With increase in field and temperature, the compound undergoes a metamagnetic transition to a ferromagnetic state. In the temperature range where the antiferromagnetic phase is dominant, it exhibits inverse (negative) magnetocaloric effect. At temperatures close to the Neel temperature, the compound shows posi…
We report on the effects of spin fluctuations, magnetic ordering, and external magnetic field on the dielectric constant of the ferromagnet SeCuO3, and the antiferromagnet TeCuO3. A model based on the coupling between uniform polarization and the q-dependent spin-spin correlation function is presented to explain the different behaviors for these isostructural compounds. The large magnetocapacitance near the transition temperature in the ferromagnet SeCuO3 suggests routes to enhancing the magnetodielectric response for practical applications.
A magneto electric (ME) effect was reported [1] for the insulator material of helimagnetic hexaferrite of Sr1.5Ba0.5Zn2Fe12O22. In this study we are interested in investigating this materials ferromagnetic resonance to define its internal anisotropy fields. By using a vibrating sample magnetometer (VSM) and ferromagnetic resonance (FMR) analysis we realized that this helical hexaferrite has about -0.5 kOe exchange anisotropy field of HE. Also, by using an alternative free energy model we derived this materials Polder tensor and its ferromagnetic resonance condition. Meanwhile, we were able to …
Controlling the magnetic anisotropy of ferromagnetic materials plays a key role in magnetic switching devices and spintronic applications. Examples of spin-orbit torque devices with different magnetic anisotropy geometries (in-plane or out-of-plane directions) have been demonstrated with novel magnetization switching mechanisms for extended device functionalities. Normally, the intrinsic magnetic anisotropy in ferromagnetic materials is unchanged within a fixed direction, and thus, it is difficult to realize multifunctionality devices. Therefore, continuous modulation of magnetic anisotropy in…
The authors have investigated magnetic domain wall induced capacitance variation as a tool for the detection of magnetic reversal in magnetic nanowires for in-plane (NiFe) and out-of-plane (Co/Pd) magnetization configurations. The switching fields in the capacitance measurements match with that of the magnetoresistance measurements in the opposite sense. The origin of the magnetocapacitance has been attributed to magnetoresistance. This magnetocapacitance detection technique can be useful for magnetic domain wall studies.
We discuss magnetotransport in a low density electron gas coupled to spin fluctuations near and above a ferromagnetic transition. Provided the density is low enough ($n \lesssim 1/ξ^{3}(T)$, with $ξ(T)$ the ferromagnetic correlation length), spin polarons form in an intermediate temperature regime above $T_c$. Both in the spin polaron regime, and in the itinerant regime nearer $T_c$, the magnetoresistance is large. We propose that this provides a good model for ``colossal'' magnetoresistance in the pyrochlore Tl$_{2-x}$Sc$_x$Mn$_2$O$_7$, fundamentally different from the mechanism in the perovs…
Giant magnetoresistance (GMR) of sequentially evaporated Fe-Ag structures have been investigated. Direct experimental evidence is given that inserting ferromagnetic layers into a granular structure significantly enhances the magnetoresistance. The increase of the GMR effect is attributed to spin polarization effects. The large enhancement (up to more than a fourfold value) and the linear variation of the GMR in low magnetic fields are explained by scattering of the spin polarized conduction electrons on paramagnetic grains.
In the present work a detailed thermodynamic consideration for the magnetic free energy of the composite material consisting of the ferromagnetic powder particles embedded into a polymer matrix is given. We estimate their magnetostatic interaction energy and its dependence on the microscopic distribution of the magnetization and the magnetic field in the composite material. We also define the hydrostatic component of the mechanical force developed and the volume change effect caused by the magnetostatic interactions in such composites.
