Mapping the phase-separated state in a 2D magnet.

Intrinsic 2D magnets have recently been established as a playground for studies on fundamentals of magnetism, quantum phases, and spintronic applications. The inherent instability at low dimensionality often results in coexistence and/or competition of different magnetic orders. Such instability of magnetic ordering may manifest itself as phase-separated states. In 4f 2D materials, magnetic phase separation is expressed in various experiments; however, the experimental evidence is circumstantial. Here, we employ a high-sensitivity MFM technique to probe the spatial distribution of magnetic states in the paradigmatic 4f 2D ferromagnet EuGe2. Below the ferromagnetic transition temperature, we discover the phase-separated state and follow its evolution with temperature and magnetic field. The characteristic length-scale of magnetic domains amounts to hundreds of nanometers. These observations strongly shape our understanding of the magnetic states in 2D materials at the monolayer limit and contribute to engineering of ultra-compact spintronics.

Thickness-Dependent Superconductivity in a Layered Electride on Silicon.

Layered materials exhibit a plethora of fascinating properties. The challenge is to make the materials into epitaxial films, preferably integrated with mature technological platforms to facilitate their potential applications. Progress in this direction can establish the film thickness as a valuable parameter to control various phenomena, superconductivity in particular. Here, a synthetic route to epitaxial films of SrAlSi, a layered superconducting electride, on silicon is designed. A set of films ranging in thickness is synthesized employing a silicene-based template. Their structure and superconductivity are explored by a combination of techniques. Two regimes of TC dependence on the film thickness are identified, the coherence length being the crossover parameter. The results can be extended to syntheses of other honeycomb-lattice ternary compounds on Si or Ge exhibiting superconducting, magnetic, and other properties.

Proximity Coupling of Graphene to a Submonolayer 2D Magnet.

Imprinting magnetism into graphene may lead to unconventional electron states and enable the design of spin logic devices with low power consumption. The ongoing active development of 2D magnets suggests their coupling with graphene to induce spin-dependent properties via proximity effects. In particular, the recent discovery of submonolayer 2D magnets on surfaces of industrial semiconductors provides an opportunity to magnetize graphene coupled with silicon. Here, synthesis and characterization of large-area graphene/Eu/Si(001) heterostructures combining graphene with a submonolayer magnetic superstructure of Eu on silicon are reported. Eu intercalation at the interface of the graphene/Si(001) system results in a Eu superstructure different from those formed on pristine Si in terms of symmetry. The resulting system graphene/Eu/Si(001) exhibits 2D magnetism with the transition temperature controlled by low magnetic fields. Negative magnetoresistance and the anomalous Hall effect in the graphene layer provide evidence for spin polarization of the carriers. Most importantly, the graphene/Eu/Si system seeds a class of graphene heterostructures based on submonolayer magnets aiming at applications in graphene spintronics.

Intrinsic exchange bias state in silicene and germanene materials EuX2.

2D magnets have recently emerged as a host for unconventional phases and related phenomena. The prominence of 2D magnetism stems from its high amenability to external stimuli and structural variations. The low dimensionality facilitates competition between magnetic orders which may give rise to exchange bias, in particular in magnetic heterostructures. Here, we propose a strategy for the search of exchange bias state in 2D individual compounds. We track the evolution of magnetic orders driven by the number of monolayers in a system exhibiting antiferromagnetism in the multilayer and ferromagnetism in the monolayer limit. The material, EuSi2, has the structure of multilayer silicene intercalated by Eu. A strong intrinsic exchange bias effect accompanies the dimensional crossover. Comparison with silicene-based GdSi2 and germanene-based EuGe2 suggests the competition between magnetic orders to be a common property of this class of materials that may be useful in spintronic applications.

Femtosecond optical orientation triggering magnetization precession in epitaxial EuO films.

Light-induced magnetization response unfolding on a temporal scale down to femtoseconds presents a way to convey information via spin manipulation. The advancement of the field requires exploration of new materials implementing various mechanisms for ultrafast magnetization dynamics. Here, pump-probe measurements of EuO-based ferromagnets by a time-resolved two-colour stroboscopic technique are reported. Epitaxial films of the pristine semiconductor and metallic Gd-doped EuO demonstrate photo-induced magnetization precession. Comparative experimental studies of both systems are carried out varying temperature, magnetic field, and polarization light helicity of the pump beam, followed by numerical estimates. The study establishes optical spin orientation by the electronic transition 4f75d0 → 4f65d1 as a mechanism triggering collective magnetization precession in these materials. The results suggest applications of EuO-based systems in optoelectronics and spintronics.

Exchange Bias State at the Crossover to 2D Ferromagnetism.

The inherent malleability of 2D magnetism provides access to unconventional quantum phases, in particular those with coexisting magnetic orders. Incidentally, in a number of materials, the magnetic state in the bulk undergoes a fundamental change when the system is pushed to the monolayer limit. Therefore, a competition of magnetic states can be expected in the crossover region. Here, an exchange bias state is observed at the crossover from 3D antiferromagnetism to 2D ferromagnetism driven by the number of monolayers in the metalloxene GdSi2. The material constitutes a stack of alternating monolayers of Gd and silicene, the Si analogue of graphene. The exchange bias manifests itself as a shift of the hysteresis loop signifying coupling of magnetic systems, as evidenced by magnetization studies. Two features distinguish the phenomenon: (i) it is intrinsic, i.e. it is detected in an individual compound; (ii) the exchange bias field, 1.5 kOe, is unusually high, which is conducive to applications. The results suggest magnetic derivatives of 2D-Xenes to be prospective materials for ultracompact spintronics.

Layer-controlled evolution of electron state in the silicene intercalation compound SrSi2.

Silicene, a Si-based analogue of graphene, holds a high promise for electronics because of its exceptional properties but a high chemical reactivity makes it a very challenging material to work with. The silicene lattice can be stabilized by active metals to form stoichiometric compounds MSi2. Being candidate topological semimetals, these materials provide an opportunity to probe layer dependence of unconventional electronic structures. It is demonstrated here that in the silicene compound SrSi2, the number of monolayers controls the electronic state. A series of films ranging from bulk-like multilayers down to a single monolayer have been synthesized on silicon and characterized with a combination of techniques - from electron and X-ray diffraction to high-resolution electron microscopy. Transport measurements reveal evolution of the chiral anomaly in bulk SrSi2 to weak localization in ultrathin films down to 3 monolayers followed by 3D and 2D strong localization in 2 and 1 monolayers, respectively. The results outline the range of stability of the chiral state, important for practical applications, and shed light on the localization phenomena in the limit of a few monolayers.

2D magnetic phases of Eu on Ge(110).

2D magnetic materials are at the forefront of research on fundamentals of magnetism; they exhibit unconventional phases and properties controlled by external stimuli. 2D magnets offer a solution to the problem of miniaturization of spintronic devices. A technological target of materials science is to find suitable magnetic materials and scale their thickness down as much as possible, a single monolayer being a natural limit. However, magnetism does not halt at one monolayer - it may persist beyond this boundary, to sparse but regular lattices of magnetic atoms. Here, we report 2D magnetic phases of Eu on the Ge(110) surface. We synthesized two submonolayer structures Eu/Ge(110) employing molecular beam epitaxy. The phases, identified by electron diffraction, differ in the surface density of Eu atoms. At low temperature, they exhibit magnetic ordering with magnetic moments lying in-plane. Strong dependence of the effective magnetic transition temperature on weak magnetic fields points at the 2D nature of the observed magnetism. The results are set against those on the Eu/Si system. The study of Eu/Ge(110) magnets demonstrates that a variety of substrates of different structure and symmetry can host submonolayer 2D magnetic phases, suggesting the phenomenon to be rather general.

High Carrier Mobility in a Layered Antiferromagnet Integrated with Silicon.

Coupling various functional properties in one material is always a challenge, more so if the material should be nanostructured for practical applications. Magnetism and high carrier mobility are key components for spintronic applications but rather difficult to bundle together. Here, we establish EuAl2Si2 as a layered antiferromagnet supporting high carrier mobility. Its topotactic synthesis via a sacrificial two-dimensional template results in epitaxial nanoscale films on silicon. Their outstanding structural quality and atomically sharp interfaces are demonstrated by diffraction and microscopy techniques. EuAl2Si2 films exhibit extreme magnetoresistance and a carrier mobility of above 10,000 cm2 V-1 s-1. The marriage of these properties and magnetism makes EuAl2Si2 a promising spintronic material. Importantly, the seamless integration of EuAl2Si2 with silicon technology is particularly appealing for applications.

Two-Dimensional Magnets beyond the Monolayer Limit.

Intrinsic two-dimensional (2D) magnetism has been demonstrated in various materials scaled down to a single monolayer. However, the question is whether 2D magnetism extends beyond the monolayer limit, to chemical species formed by sparse but regular 2D arrays of magnetic atoms. Here we show that sub-monolayer superstructures of Eu atoms self-assembled on the silicon surface exhibit strong magnetic signals. Robust easy-plane magnetism is discovered in both one- and two-dimensionally ordered structures with Eu coverage of half monolayer and above. The emergence of 2D magnetism manifests itself by a strong dependence of the effective transition temperature on weak magnetic fields. The results constitute a versatile platform for miniaturization of 2D magnetic systems and seed an expandable class of atomically thin magnets for applications in information technologies.

Interface-Induced Anomalous Hall Conductivity in a Confined Metal.

The mature silicon technological platform is actively explored for spintronic applications. Metal silicides are an integral part of the Si technology used as interconnects, gate electrodes, and diffusion barriers; their epitaxial integration with Si results in premier contacts. Recent studies highlight the exceptional role of electronic discontinuities at interfaces in the spin-dependent transport properties. Here, we report a new type of Hall conductivity driven by sharp interfaces of Eu silicide, an antiferromagnetic metal, sandwiched between two insulators - Si and SiO x. Quasi-ballistic transport probes spin-orbit coupling at the interfaces, in particular, charge-spin interconversion. Transverse magnetic field results in anomalous Hall effect signals of an unusual line shape. The interplay between opposite-sign signals from the two interfaces allows efficient control over the magnitude and sign of the overall effect. Selective engineering of interfaces singles out a particular spin signal. The two-channel nature of the effect and its high tunability offer new functional possibilities for future spintronic devices.

High-Temperature Magnetism in Graphene Induced by Proximity to EuO.

Addition of magnetism to spectacular properties of graphene may lead to novel topological states and design of spin logic devices enjoying low power consumption. A significant progress is made in defect-induced magnetism in graphene-selective elimination of p z orbitals (by vacancies or adatoms) at triangular sublattices tailors graphene magnetism. Proximity to a magnetic insulator is a less invasive way, which is being actively explored now. Integration of graphene with the ferromagnetic semiconductor EuO has much to offer, especially in terms of proximity-induced spin-orbit interactions. Here, we synthesize films of EuO on graphene using reactive molecular beam epitaxy. Their quality is attested by electron and X-ray diffraction, cross-sectional electron microscopy, and Raman and magnetization measurements. Studies of electron transport reveal a magnetic transition at TC* ≈ 220 K, well above the Curie temperature 69 K of EuO. Up to TC*, the dependence R xy( B) is strongly nonlinear, suggesting the presence of the anomalous Hall effect. The role of synthesis conditions is highlighted by studies of an overdoped structure. The results justify the use of the EuO/graphene system in spintronics.

Emerging two-dimensional ferromagnetism in silicene materials.

The appeal of ultra-compact spintronics drives intense research on magnetism in low-dimensional materials. Recent years have witnessed remarkable progress in engineering two-dimensional (2D) magnetism via defects, edges, adatoms, and magnetic proximity. However, intrinsic 2D ferromagnetism remained elusive until recent discovery of out-of-plane magneto-optical response in Cr-based layers, stimulating the search for 2D magnets with tunable and diverse properties. Here we employ a bottom-up approach to produce layered structures of silicene (a Si counterpart of graphene) functionalized by rare-earth atoms, ranging from the bulk down to one monolayer. We track the evolution from the antiferromagnetism of the bulk to intrinsic 2D in-plane ferromagnetism of ultrathin layers, with its characteristic dependence of the transition temperature on low magnetic fields. The emerging ferromagnetism manifests itself in the electron transport. The discovery of a class of robust 2D magnets, compatible with the mature Si technology, is instrumental for engineering new devices and understanding spin phenomena.

Fine structure of metal-insulator transition in EuO resolved by doping engineering.

Metal-insulator transitions (MITs) offer new functionalities for nanoelectronics. However, ongoing attempts to control the resistivity by external stimuli are hindered by strong coupling of spin, charge, orbital and lattice degrees of freedom. This difficulty presents a quest for materials which exhibit MIT caused by a single degree of freedom. In the archetypal ferromagnetic semiconductor EuO, magnetic orders dominate the MIT. Here we report a new approach to take doping under control in this material on the nanoscale: formation of oxygen vacancies is strongly suppressed to exhibit the highest MIT resistivity jump and magnetoresistance among thin films. The nature of the MIT is revealed in Gd doped films. The critical doping is determined to be more than an order of magnitude lower than in all previous studies. In lightly doped films, a remarkable thermal hysteresis in resistivity is discovered. It extends over 100 K in the paramagnetic phase reaching 3 orders of magnitude. In the warming mode, the MIT is shown to be a two-step process. The resistivity patterns are consistent with an active role of magnetic polarons-formation of a narrow band and its thermal destruction. High-temperature magnetic polaron effects include large negative magnetoresistance and ferromagnetic droplets revealed by x-ray magnetic circular dichroism. Our findings have wide-range implications for the understanding of strongly correlated oxides and establish fundamental benchmarks to guide theoretical models of the MIT.

Anomalous Hall effect in the prospective spintronic material Eu1-x Gd x O integrated with Si.

Remarkable properties of EuO make it a versatile spintronic material. Despite numerous experimental and theoretical studies of EuO, little is known about the anomalous Hall effect in this ferromagnet. So far, the effect has not been observed in bulk EuO, though has been detected in EuO films with uncontrolled distribution of defects. In the present work doping is taken under control: epitaxial films of Gd-doped EuO are synthesized integrated with Si using molecular beam epitaxy and characterized with x-ray diffraction and magnetization measurements. Nanoscale transport studies reveal the anomalous Hall effect in the ferromagnetic region for samples with different Gd concentration. The saturated anomalous Hall effect conductivity value of 5.0 S·cm(-1) in Gd-doped EuO is more than an order of magnitude larger than those reported so far for Eu chalcogenides doped with anion vacancies.

Europium Silicide - a Prospective Material for Contacts with Silicon.

Metal-silicon junctions are crucial to the operation of semiconductor devices: aggressive scaling demands low-resistive metallic terminals to replace high-doped silicon in transistors. It suggests an efficient charge injection through a low Schottky barrier between a metal and Si. Tremendous efforts invested into engineering metal-silicon junctions reveal the major role of chemical bonding at the interface: premier contacts entail epitaxial integration of metal silicides with Si. Here we present epitaxially grown EuSi2/Si junction characterized by RHEED, XRD, transmission electron microscopy, magnetization and transport measurements. Structural perfection leads to superb conductivity and a record-low Schottky barrier with n-Si while an antiferromagnetic phase invites spin-related applications. This development opens brand-new opportunities in electronics.

Atomic-Scale Engineering of Abrupt Interface for Direct Spin Contact of Ferromagnetic Semiconductor with Silicon.

Control and manipulation of the spin of conduction electrons in industrial semiconductors such as silicon are suggested as an operating principle for a new generation of spintronic devices. Coherent injection of spin-polarized carriers into Si is a key to this novel technology. It is contingent on our ability to engineer flawless interfaces of Si with a spin injector to prevent spin-flip scattering. The unique properties of the ferromagnetic semiconductor EuO make it a prospective spin injector into silicon. Recent advances in the epitaxial integration of EuO with Si bring the manufacturing of a direct spin contact within reach. Here we employ transmission electron microscopy to study the interface EuO/Si with atomic-scale resolution. We report techniques for interface control on a submonolayer scale through surface reconstruction. Thus we prevent formation of alien phases and imperfections detrimental to spin injection. This development opens a new avenue for semiconductor spintronics.

Structural coupling across the direct EuO/Si interface.

The ferromagnetic semiconductor EuO is believed to be an effective spin injector when directly integrated with silicon (Si). Injection through spin-selective ohmic contact requires superb structural quality of the interface EuO/Si. A recent breakthrough in manufacturing free-of-buffer-layer EuO/Si junctions calls for structural studies of the interface between the semiconductors. The synthesis of EuO employs an advanced protection of the Si substrate surface and a two-step growth protocol. It prevents unwanted chemical reactions at the interface. Ex situ high-resolution x-ray diffraction (XRD) and reflectivity (XRR) accompanied by in situ reflection high-energy electron diffraction reveal direct coupling at the interface. A combined analysis of XRD and XRR data provides a common structural model. The structural quality of the EuO/Si spin contact far exceeds that of previous reports and thus makes a step forward to the ultimate goals of spintronics.

Direct epitaxial integration of the ferromagnetic semiconductor EuO with silicon for spintronic applications.

Following a remarkable success of metallic spintronics, tremendous efforts have been invested into the less developed semiconductor spintronics, in particular, with the aim to produce three-terminal spintronic devices, e.g., spin transistors. One of the most important prerequisites for such a technology is an effective injection of spin-polarized carriers into a nonmagnetic semiconductor, preferably one of those currently used for industrial applications such as Si-a workhorse of modern electronics. Ferromagnetic semiconductor EuO is long believed to be the best candidate for integration with Si. Although EuO proved to offer optimal conditions for effective spin injection into silicon and in spite of considerable efforts, the direct epitaxial stabilization of stoichiometric EuO thin films on Si without any buffer layer has not been demonstrated to date. Here we report a new technique for control of EuO/Si interface on submonolayer level. Using this technique we solve a long-standing problem of direct epitaxial growth on silicon of thin EuO films which exhibit structural and magnetic properties of EuO bulk material. This result opens up new possibilities in developing all-semiconductor spintronic devices.

Spin polarons in EuO layers: a quantum computational study.

Magnetic semiconductors are highly promising materials for spintronics. Among them europium(ii) oxide makes a unique working host material due to a remarkable combination of magnetic and transport properties. Its electronic structure and various properties are often described in terms of spin polarons - local ferromagnetic clusters in the paramagnetic sea of Eu ions. This concept finds support in numerous experiments and theoretical studies but so far it has not emerged in quantum-chemical calculations of this material. Here we try to find evidence for the existence of these quasiparticles from local cluster calculations for different values of the total spin. Our calculations of EuO layers show that there are low-energy states consistent with quasi-two-dimensional spin polarons as part of their electronic structure. The size of these quasiparticles (radius R ≈ 0.5 nm and spin S ≈ 38) doesn't change with the size of the cluster and is in good accord with experimental results.