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Using angle-resolved photoelectron spectroscopy (ARPES), we investigate the surface electronic structure of the magnetic van der Waals compounds MnBi_{4}Te_{7} and MnBi_{6}Te_{10}, the n=1 and 2 members of a modular (Bi_{2}Te_{3})_{n}(MnBi_{2}Te_{4}) series, which have attracted recent interest as intrinsic magnetic topological insulators. Combining circular dichroic, spin-resolved and photon-energy-dependent ARPES measurements with calculations based on density functional theory, we unveil complex momentum-dependent orbital and spin textures in the surface electronic structure and disentangle topological from trivial surface bands. We find that the Dirac-cone dispersion of the topologial surface state is strongly perturbed by hybridization with valence-band states for Bi_{2}Te_{3}-terminated surfaces but remains preserved for MnBi_{2}Te_{4}-terminated surfaces. Our results firmly establish the topologically nontrivial nature of these magnetic van der Waals materials and indicate that the possibility of realizing a quantized anomalous Hall conductivity depends on surface termination.

New heterometallic In-Fe alkoxides [InFe(O

The sensitivity of photoemission tomography (PT) to directly probe single molecule on-surface intramolecular reactions will be shown here. PT application in the study of molecules possessing peripheral ligands and structural flexibility is tested on the temperature-induced dehydrogenation intramolecular reaction on Ag(100), leading from CoOEP to the final product CoTBP. Along with the ring-closure reaction, the electronic occupancy and energy level alignment of the frontier orbitals, as well as the oxidation state of the metal ion, are elucidated for both the CoOEP and CoTBP systems.

Quantum well (QW) heterostructures have been extensively used for the realization of a wide range of optical and electronic devices. Exploiting their potential for further improvement and development requires a fundamental understanding of their electronic structure. So far, the most commonly used experimental techniques for this purpose have been all-optical spectroscopy methods that, however, are generally averaging in momentum space. Additional information can be gained by angle-resolved photoelectron spectroscopy (ARPES), which measures the electronic structure with momentum resolution. Here we report on the use of extremely low-energy ARPES (photon energy ~ 7 eV) to increase depth sensitivity and access buried QW states, located at 3 nm and 6 nm below the surface of cubic-GaN/AlN and GaAs/AlGaAs heterostructures, respectively. We find that the QW states in cubic-GaN/AlN can indeed be observed, but not their energy dispersion, because of the high surface roughness. The GaAs/AlGaAs QW states, on the other hand, are buried too deep to be detected by extremely low-energy ARPES. Since the sample surface is much flatter, the ARPES spectra of the GaAs/AlGaAs show distinct features in momentum space, which can be reconducted to the band structure of the topmost surface layer of the QW structure. Our results provide important information about the samples’ properties required to perform extremely low-energy ARPES experiments on electronic states buried in semiconductor heterostructures.

We present the main parameters of the 1.2 TW Ti:Sapphire laser amplifier system operating at 1 kHz. The high quality beam output allows generation of linearly- and circularly polarized photons in the extreme ultraviolet range.

Due to its unique magnetic properties offered by the open‐shell electronic structure of the central metal ion, and for being an effective catalyst in a wide variety of reactions, iron phthalocyanine has drawn significant interest from the scientific community. Nevertheless, upon surface deposition, the magnetic properties of the molecular layer can be significantly affected by the coupling occurring at the interface, and the more reactive the surface, the stronger is the impact on the spin state. Here, we show that on Cu(100), indeed, the strong hybridization between the Fe d‐states of FePc and the sp‐band of the copper substrate modifies the charge distribution in the molecule, significantly influencing the magnetic properties of the iron ion. The FeII ion is stabilized in the low singlet spin state (S=0), leading to the complete quenching of the molecule magnetic moment. By exploiting the FePc/Cu(100) interface, we demonstrate that NO2 dissociation can be used to gradually change the magnetic properties of the iron ion, by trimming the gas dosage. For lower doses, the FePc film is decoupled from the copper substrate, restoring the gas phase triplet spin state (S=1). A higher dose induces the transition from ferrous to ferric phthalocyanine, in its intermediate spin state, with enhanced magnetic moment due to the interaction with the atomic ligands. Remarkably, in this way, three different spin configurations have been observed within the same metalorganic/metal interface by exposing it to different doses of NO2 at room temperature., By exploiting the FePc/Cu(100) interface, the possibility to control, via nitrogen dioxide (NO2) dissociation at the interface, the magnetic properties of the chelated ion the FePc array is demonstrated. Three different spin configurations have been stabilized within the same metalorganic/metal interface.

We explored THz emission from laser illuminated Fe/heavy-metal bilayer spintronic emitters grown epitaxially on top of GaAs(001) substrates. The fabricated emitters show an overall enhancement of the transient THz radiation compared to structures fabricated on insulating substrates. In addition, the THz amplitude shows strongly vertically off-set hysteretic behavior in varying magnetic field. We ascribe these observations to the interplay of several mechanisms including inverse spin Hall effect, optical rectification, Lorentz force and chemical bonding at the epitaxial Fe/GaAs(001) interface.

Quantum well states formed by d electrons in metallic thin films are responsible for many fundamental phenomena that oscillate with layer thickness, such as magnetic anisotropy or magnetoresistance. Using momentum microscopy and angle-resolved photoemission, we mapped in unprecedented detail the quantized electronic states of Fe(001) in a broad photon energy range starting from soft x-ray (160 eV) down to vacuum ultraviolet (8.4 eV). We show that it is possible to simulate the experimentally observed photoemission spectra with high accuracy by using the ab initio electronic bulk band structure as the initial state, taking into account that free electron final electronic states are intrinsically broadened along the wave vector direction perpendicular to the sample surface. To simulate the thin-film case, we take into account a subset of the initial electronic states, which results in the reproduction of the quantized electronic structure observed in the experiment. In addition, we present results of the spin-sensitive measurements, which are confronted with the photoemission simulation that takes into account the spin degree of freedom. We demonstrate electronic states that can be responsible for the oscillations of the magnetic anisotropy in Fe(001) thin films with periods of about 5 and 9 monolayers. We show that these quantum well states change position in reciprocal space depending on the magnetization direction. Our photoemission simulation reproduces this effect, which highlights its origin in the relativistic bulk electronic band structure of bcc Fe. We also observed magnetization-dependent spin-orbit gaps with the symmetry lower than the bulk symmetry. We believe that the same method of simulating photoemission spectra might facilitate interpretation of the photoemission intensities measured for other three-dimensional materials, especially when the spin-polarized quantized electronic states are considered.

Near total reflection regime has been widely used in X-ray science, specifically in grazing incidence small angle X-ray scattering and in hard X-ray photoelectron spectroscopy. In this work, we introduce some practical aspects of using near total reflection in ambient pressure X-ray photoelectron spectroscopy and apply this technique to study chemical concentration gradients in a substrate/photoresist system. Experimental data are accompanied by X-ray optical and photoemission simulations to quantitatively probe the photoresist and the interface with the depth accuracy of ~1 nm. Together, our calculations and experiments confirm that near total reflection X-ray photoelectron spectroscopy is a suitable method to extract information from buried interfaces with highest depth-resolution, which can help address open research questions regarding our understanding of concentration profiles, electrical gradients, and charge transfer phenomena at such interfaces. The presented methodology is especially attractive for solid/liquid interface studies, since it provides all the strengths of a Bragg-reflection standing-wave spectroscopy without the need of an artificial multilayer mirror serving as a standing wave generator, thus dramatically simplifying the sample synthesis., Comment: 13 pages, 4 figures Supplemental Information

We generated pulses of electromagnetic radiation with a frequency content up to three terahertz (THz) by optical excitation of Co2Fe0.4Mn0.6Si Heusler alloy/heavy metal bilayers (CFMS/HM) using fs-laser pulses. We attribute the generation process to the conversion of a spin current, generated by the illumination with a fs-laser pulse, to a charge current via the inverse spin Hall effect. We compared the THz emission efficiency in CFMS/Pt and CFMS/Ta bilayers due to their high spin-orbit coupling of Pt and Ta. Surprisingly, our data reveal that CFMS/Pt shows substantially larger THz amplitudes compared to CFMS/Ta. Both bilayers exhibit a tunability of the THz amplitude by external magnetic field both at 300 K and 100 K. Ferromagnetic resonance measurements demonstrate that CFMS/Ta has a high effective spin mixing conductance, describing the efficiency of interfacial spin transport. We observe that the efficiency of the THz radiation cannot be solely described by the spin-orbit coupling strength and the spin diffusion length in the HM material plays an important role.

We report on the generation of electromagnetic transients of picosecond duration from La-Sr-Mn-O/Au and La-Sr-Mn-O/lr nanobilayers, illuminated by femtosecond laser pulses in the presence of an external magnetic field. Fourier analysis revealed that the frequency content of these transients extended up 4 THz. The amplitude of THz transients followed the functional dependence of La-Sr-Mn-O magnetization on the magnetic field and was also tunable by varying the intensity of the optical pulses. We ascribe the observed emission of THz transients to the inverse spin Hall effect, earlier demonstrated in metallic ferromagnet/metal spintronic nanobilayers.

We have generated intense electromagnetic transients by femtosecond laser pulse illumination of ferromagnet/metal (F/M) nanobilayers, in the presence of an external magnetic field. Fourier analysis revealed that the frequency content of these transients extended up to ~5 THz. We have also observed that upon the increase of the magnetic field, the entire THz transient shifts towards earlier times by up to 110 fs. We ascribe this magnetically tunable onset-time shift to extra acceleration of photoelectrons induced due to the Lorenz force.

Irradiation of semiconductor surfaces with femtosecond optical laser pulses is one of the common techniques for broadband, free-space THz transient generation. We demonstrate that the amplitude of surface-emitted THz pulses scales linearly with an applied, external, in-plane magnetic field. We studied the effect in several highly resistive GaAs samples and ascribe it to the Lorentz force that additionally accelerates optically excited carriers.

The first layer of atoms on an oxide catalyst provides the first sites for adsorption of reactants and the last sites before products or oxygen are desorbed. We employ a unique combination of morphological, structural, and chemical analyses of a model ceria catalyst with different surface terminations under an H2 environment to unequivocally establish the effect of the last layer of atoms on surface reduction. (111) and (100) terminated epitaxial islands of ceria are simultaneously studied in situ allowing for a direct investigation of the structure–reducibility relationship under identical conditions. Kinetic rate constants of Ce4+ to Ce3+ transformation and equilibrium concentrations are extracted for both surface terminations. Unlike the kinetic rate constants, which are practically the same for both types of islands, more pronounced oxygen release, and overall higher reducibility were observed for (100) islands compared to (111) ones. The findings are in agreement with coordination-limited oxygen vacancy formation energies calculated by density functional theory. The results point out the important aspect of surface terminations in redox processes, with particular impact on the catalytic reactions of a variety of catalysts.

Cavity optomechanics, where photons are coupled to mechanical motion, provides the tools to control mechanical motion near the fundamental quantum limits. Reaching single-photon strong coupling would allow to prepare the mechanical resonator in non-Gaussian quantum states. Preparing massive mechanical resonators in such states is of particular interest for testing the boundaries of quantum mechanics. This goal remains however challenging due to the small optomechanical couplings usually achieved with massive devices. Here we demonstrate a novel approach where a mechanical resonator is magnetically coupled to a microwave cavity. We measure a single-photon coupling of $g_0/2 \pi \sim 3$ kHz, an improvement of one order of magnitude over current microwave optomechanical systems. At this coupling we measure a large single-photon cooperativity with $C_0 \gtrsim 10$, an important step to reach single-photon strong coupling. Such a strong interaction allows us to cool the massive mechanical resonator to a third of its steady state phonon population with less than two photons in the microwave cavity. Beyond tests for quantum foundations, our approach is also well suited as a quantum sensor or a microwave to optical transducer., Comment: 5 pages, 4 figures

The discovery of a two-dimensional electron system (2DES) at the interfaces of perovskite oxides such as LaAlO3 and SrTiO3 has motivated enormous efforts in engineering interfacial functionalities with this type of oxide heterostructures. However, its fundamental origins are still not understood, e.g. the microscopic mechanisms of coexisting interface conductivity and magnetism. Here we report a comprehensive spectroscopic investigation of the depth profile of 2DES-relevant Ti 3d interface carriers using depth- and element-specific techniques, standing-wave excited photoemission and resonant inelastic scattering. We found that one type of Ti 3d interface carriers, which give rise to the 2DES are located within 3 unit cells from the n-type interface in the SrTiO3 layer. Unexpectedly, another type of interface carriers, which are polarity-induced Ti-on-Al antisite defects, reside in the first 3 unit cells of the opposing LaAlO3 layer (~10 {\AA}). Our findings provide a microscopic picture of how the localized and mobile Ti 3d interface carriers distribute across the interface and suggest that the 2DES and 2D magnetism at the LaAlO3/SrTiO3 interface have disparate explanations as originating from different types of interface carriers., Comment: 28 pages, 4 figures; Supplemental Materials 19 pages

JuSPARC, the Jülich Short-Pulsed Particle and Radiation Center, is a laser-driven facility to enable research with short-pulsed photon and particle beams to be performed at the Forschungszentrum Jülich. The conceptual design of JuSPARC is determined by a set of state-of-the-art time-resolved instruments, which are designed to address the electronic, spin, and structural states of matter and their dynamic behaviour. From these instruments and experiments JuSPARC derives the need of operating several dedicated high pulse-power laser systems at highest possible repetition rates. They serve as core units for optimized photon up-conversion techniques generating the light pulses for the respective experiments. The applications also include experiments with spin polarized particle beams, which require the use of laser-based polarized gas targets. Thus, in its rst stage JuSPARC comprises four driving laser systems, called JuSPARC_VEGA, JuSPARC_DENEB, JuSPARC_SIRIUS and JuSPARC_MIRA, which are outlined in this article.

The 2D self-assembly of Ni-containing tetrapyrroles on Cu(100) allows control of the Ni atom oxidation state, yielding inactive Ni(II) or active Ni(I) upon modification of the molecule-substrate interaction, resembling the behavior of the biochemical counterpart. Ni(I) is indeed the active site of methanogenic bacteria in the tetrahydrocorphin of the F-430 coenzyme of methyl-coenzyme reductase. Tuning of the electronic configuration of the Ni atom in the 2D system is accomplished by exploiting the surface trans effect, by analogy to the biologic enzymatic pocket, which is activated by a molecular trans effect. In this report, we identify the vibrational fingerprint of the molecular macrocycle that reflects the actual Ni oxidation state in the 2D system showing that, despite the apparent differences of the two cases, the fact that the Ni-porphin in the F-430 pocket is accessible to the reactants but not to the solvent makes the two situations more similar than expected.

Low-temperature X-ray photoemission electron microscopy (X-PEEM) is used to measure the electric potential at domain walls in improper ferroelectric Er0.99Ca0.01MnO3. By combining X-PEEM with scanning probe microscopy and theory, we develop a model that relates the detected X-PEEM contrast to the emergence of uncompensated bound charges, explaining the image formation based on intrinsic electronic domain-wall properties. In contrast to previously applied low-temperature electrostatic force microscopy (EFM), X-PEEM readily distinguishes between positive and negative bound charges at domain walls. Our study introduces an X-PEEM based approach for low-temperature electrostatic potential mapping, facilitating nanoscale spatial resolution and data acquisition times in the order of 0.1-1 sec., Comment: 15 pages, 6 figures

Photoelectron spectroscopy is our main tool to explore the electronic structure of novel material systems, the properties of which are often determined by an intricate interplay of competing interactions. Elucidating the role of this interactions requires studies over an extensive range of energy, momentum, length, and time scales. We show that immersion lens-based momentum microscopy with spin-resolution is able to combine these seemingly divergent requirements in a unifying experimental approach. We will discuss applications to different areas in information research, for example, resistive switching and spintronics. The analysis of resistive switching phenomena in oxides requires high lateral resolution and chemical selectivity, as the processes involve local redox processes and oxygen vacancy migration. In spintronics topological phenomena are currently a hot topic, which lead to complex band structures and spin textures in reciprocal space. Spin-resolved momentum microscopy is uniquely suited to address these aspects.

It is well known that intercalated species can strongly affect the graphene-substrate interaction. As repeatedly shown by experiment and theory, the intercalation of atomic species may establish a free-standing character in chemisorbed graphene systems. Here, we focus on graphene grown on a strongly interacting support, cobalt, and demonstrate that the film electronic structure and doping can be tuned via the intercalation/removal of interfacial oxygen. Importantly, cathode lens microscopy reveals the main mechanism of oxygen intercalation, and in particular how microscopic openings in the mesh enable oxygen accumulation at the graphene-cobalt interface. Our experiments show that this process can be carefully controlled through temperature, without affecting the film morphology and crystalline quality. The presence of oxygen at the interface induces an upward shift of the graphene π band, moving its crossing above the Fermi level, accompanied by an increased Fermi velocity and reduced momentum width. Control on the graphene coupling to cobalt may enable one to alter the induced spin polarization in graphene’s electronic states.

Many applications of molecular layers deposited on metal surfaces, ranging from single-atom catalysis to on-surface magnetochemistry and biosensing, rely on the use of thermal cycles to regenerate the pristine properties of the system. Thus, understanding the microscopic origin behind the thermal stability of organic/metal interfaces is fundamental for engineering reliable organic-based devices. Here, we study nickel porphyrin molecules on a copper surface as an archetypal system containing a metal center whose oxidation state can be controlled through the interaction with the metal substrate. We demonstrate that the strong molecule-surface interaction, followed by charge transfer at the interface, plays a fundamental role in the thermal stability of the layer by rigidly anchoring the porphyrin to the substrate. Upon thermal treatment, the molecules undergo an irreversible transition at 420 K, which is associated with an increase of the charge transfer from the substrate, mostly localized on the phenyl substituents, and a downward tilting of the latters without any chemical modification. This journal is

Recent developments in environmental and liquid cells equipped with electron transparent graphene windows have enabled traditional surface science spectromicroscopy tools, such as scanning X-ray photoelectron microscopy, X-ray photoemission electron microscopy (XPEEM), and scanning electron microscopy to be applied for studying solid–liquid and liquid–gas interfaces. Here, we focus on the experimental implementation of XPEEM to probe electrified graphene–liquid interfaces using electrolyte-filled microchannel arrays as a new sample platform. We demonstrate the important methodological advantage of these multi-sample arrays: they combine the wide field of view hyperspectral imaging capabilities from XPEEM with the use of powerful data mining algorithms to reveal spectroscopic and temporal behaviors at the level of the individual microsample or the entire array ensemble.

We present a study of temperature dependent growth of nano-sized ceria islands on a Cu (100) substrate. Low-energy electron microscopy, micro-electron diffraction, X-ray absorption spectroscopy, and photoemission electron microscopy are used to determine the morphology, shape, chemical state, and crystal structure of the grown islands. Utilizing real-time observation capabilities, we reveal a three-way interaction between the ceria, substrate, and local oxygen chemical potential. The interaction manifests in the reorientation of terrace boundaries on the Cu (100) substrate, characteristic of the transition between oxidized and metallic surface. The reorientation is initiated at nucleation sites of ceria islands, whose growth direction is influenced by the proximity of the terrace boundaries. The grown ceria islands were identified as fully stoichiometric CeO2 (100) surfaces with a (2 × 2) reconstruction.

The complementary capabilities of the Scanning PhotoElectron Microscopes (SPEM) and X-ray PhotoEmission Electron Microscopes (XPEEM), operated at Elettra, in terms of imaging and micro-spectroscopy have opened unique opportunities to explore properties of functional materials as a function of their morphology and dimensions and to follow modifications in their properties during their operation. This paper describes the present performance of SPEMs and XPEEMs at Elettra, illustrated by selected recent studies relevant to graphene science. Ongoing efforts for implementing SPEM set-ups allowing for in-situ investigations under realistic operating conditions and PEEM set-up for spin-filtered momentum microscopy are outlined and discussed as well.

A major obstacle for the implementation of redox-based memristive memory or logic technology is the large cycle-to-cycle and device-to-device variability. Here, we use spectromicroscopic photoemission threshold analysis and operando XAS analysis to experimentally investigate the microscopic origin of the variability. We find that some devices exhibit variations in the shape of the conductive filament or in the oxygen vacancy distribution at and around the filament. In other cases, even the location of the active filament changes from one cycle to the next. We propose that both effects originate from the coexistence of multiple (sub)filaments and that the active, current-carrying filament may change from cycle to cycle. These findings account for the observed variability in device performance and represent the scientific basis, rather than prior purely empirical engineering approaches, for developing stable memristive devices.

Plasma physics and controlled fusion 61(11), 115012 (2019). doi:10.1088/1361-6587/ab4613, Published by IOP Publ., Bristol

Magnetic field-assisted CVD offers a direct pathway to manipulate the evolution of microstructure, phase composition, and magnetic properties of the as-prepared film. We report on the role of applied magnetic fields (0.5 T) during a cold-wall CVD deposition of iron oxide from [FeIII(OtBu)3]2 leading to higher crystallinity, larger particulates, and better out-of-plane magnetic anisotropy, if compared with zero-field depositions. Whereas selective formation of homogeneous magnetite films was observed for the field-assisted process, coexistence of hematite and amorphous iron(III) oxide was confirmed under zero-field conditions. Comparison of the coercive field (11 vs 60 mT) indicated lower defect concentration for the field-assisted process with nearly superparamagnetic behavior. X-ray photoemission electron microscopy (X-PEEM) in absorption mode at the O-K and Fe-L3,2 edges confirmed the selective formation of magnetite (field-assisted) and hematite (zero-field) with coexisting amorphous phases, respectively, emphasizing the importance of field-matter interactions in the phase-selective synthesis of magnetic thin films.

Laterally structured materials can exhibit properties uniquely suited for applications in electronics, magnetoelectric memory, photonics, and nanoionics. Here, a patterning approach is presented that combines the precise geometric control enabled by lithography with topochemical anionic manipulation of complex oxide films. Utilizing oxidation and fluorination reactions, striped patterns of SrFeO2.5/SrFeO3,SrFeO2.5/SrFeO2F, and SrFeO3/SrFeO2F have been prepared with lateral periodicities of 200, 20, and 4 μm. Coexistence of the distinct chemical phases is confirmed through x-ray diffraction, optical and photoemission microscopies, and optical spectroscopy. The lateral heterostructures exhibit highly anisotropic electronic transport and also enable transience and regeneration of patterns through reversible redox reactions. This approach can be broadly applied to a variety of metal-oxide systems, enabling chemically reconfigurable lateral heterostructures tailored for specific electronic, optical, ionic, thermal, or magnetic functionalities.

Motivated by recent studies on current-driven domain-wall motion we have explored the dispersion of spin waves (magnons) in ultrathin nickel/cobalt multilayers. The layers are grown epitaxially on Cu(100) surfaces and consist of $n$ nickel single-atom layers, each topped by a cobalt bilayer with $n=2$, 3, and $4\phantom{\rule{0.16em}{0ex}}(n\ifmmode\times\else\texttimes\fi{}\mathrm{N}{\mathrm{i}}_{1}\mathrm{C}{\mathrm{o}}_{2})$. Layers of the type $2\ifmmode\times\else\texttimes\fi{}{\mathrm{Ni}}_{2}{\mathrm{Co}}_{1}$ are also studied. While parallel to the film plane the magnon dispersion is nearly equal to that of pure cobalt films, magnons with wave vectors perpendicular to the film plane (standing waves) are considerably softened. The softening is attributed to a reduction of the effective interlayer exchange coupling between the two layers next to the interface with the Cu(100) substrate.

Donor-doped transition metal oxides such as donor-doped strontium titanate ($n\ensuremath{-}{\mathrm{SrTiO}}_{3}$) are of fundamental importance for oxide electronic devices as well as for electronic surface and interface engineering. Here we quantitatively analyze the variable band alignment and the resulting space charge layer at the surface of $n\ensuremath{-}{\mathrm{SrTiO}}_{3}$, determined by its surface redox chemistry. Synchrotron-based ambient-pressure x-ray photoelectron spectroscopy conducted under applied thermodynamic bias is used to access electronic structure and chemistry of the surface. We find an electron depletion layer driven by cationic surface point defects that are controlled by adjusting the ambient atmosphere ($p{\mathrm{O}}_{2}$). We correlate the $p{\mathrm{O}}_{2}$ dependence to a response of the strontium sublattice, namely the precipitation of strontium oxide and the formation of charged strontium vacancies at the surface. We suggest the reversible conversion of surface-terminating strontium oxide into extended strontium oxide clusters as the responsible process by resolving chemical dynamics in situ. As we show, atomic control of these subtle changes in the surface redox chemistry allows us to tailor electrical transport properties along the $n\ensuremath{-}{\mathrm{SrTiO}}_{3}$ surface. Our study thereby gives access to engineering electronic band bending in transition metal oxides by the control of the surface chemistry.

The mechanical setup of a novel scanning reflection X-ray microscope is presented. It is based on zone plate optics optimized for reflection mode in the EUV spectral range. The microscope can operate at synchrotron radiation beamlines as well as at laboratory-based plasma light sources. In contrast to established X-ray transmission microscopes that use thin foil samples, the new microscope design presented here allows the investigation of any type of bulk materials. Importantly, this permits the investigation of magnetic materials by employing experimental techniques based on X-ray magnetic circular dichroism, X-ray linear magnetic dichroism or the transversal magneto-optical Kerr effect (T-MOKE). The reliable functionality of the new microscope design has been demonstrated by T-MOKE microscopy spectra of Fe/Cr-wedge/Fe trilayer samples. The spectra were recorded at various photon energies across the Fe 3p edge revealing the orientation of magnetic domains in the sample.

We demonstrate the emergence and control of magnetic phases between magnetite (Fe

Measuring magnetic moments in ferromagnetic materials with atomic column resolution is theoretically possible using the electron magnetic circular dichroism (EMCD) technique in a (scanning) transmission electron microscope ((S)TEM). However, experimental and data processing hurdles currently hamper the realization of this goal. Experimentally, the sample must be tilted to a zone-axis orientation, yielding a complex distribution of magnetic scattering intensity, and the same sample region must be scanned multiple times with sub-atomic spatial registration necessary at each pass. Furthermore, the weak nature of the EMCD signal requires advanced data processing techniques to reliably detect and quantify the result. In this manuscript, we detail our experimental and data processing progress towards achieving single-pass zone-axis EMCD using a patterned aperture. First, we provide a comprehensive data acquisition and analysis strategy for this and other EMCD experiments that should scale down to atomic resolution experiments. Second, we demonstrate that, at low spatial resolution, promising EMCD candidate signals can be extracted, and that these are sensitive to both crystallographic orientation and momentum transfer., Comment: 17 pages, 9 figures. Data available at DOI: 10.5281/zenodo.3361582 Code available at DOI: 10.5281/zenodo.3374648

We demonstrate the emergence and control of magnetic phases between magnetite (Fe3O4), a ferrimagnetic halfmetal, and SrTiO3, a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. The Verwey transition (TV) was detected to persist from bulk-like down to ultrathin Fe3O4 films, decreasing from 117 ± 4 K (38 nm) to 25 ± 4 K (2 nm), respectively. Element-selective electronic and magnetic properties of the ultrathin films and buried interfaces are studied by angle-dependent hard X-ray photoelectron spectroscopy and X-ray magnetic circular dichroism techniques. We observe a reduction of Fe2+ ions with decreasing film thickness, accompanied by an increase of Fe3+ ions in both tetrahedral and octahedral sites and conclude on the formation of a magnetically active ferrimagnetic 2 u.c. γ-Fe2O3 intralayer. To manipulate the interfacial magnetic phase, a postannealing process causes the controlled reduction of the γ-Fe2O3 that finally leads to stoichiometric and ferrimagnetic Fe3O4/Sr...

International audience; Functional oxides displaying phenomena such as 2D electron gas (2DEG) at oxide interfaces represent potential technological breakthroughs for post‐CMOS (Complementary Metal Oxide Semiconductor) electronics. Noninvasive techniques are required to study the surface chemistry and electronic structure underlying their often unique electrical properties. The sensitivity of photoemission electron microscopy (PEEM) to local potential, chemistry, and electronic structure makes it an invaluable tool for probing the near surface region of microscopic regions and domains of functional materials. In particular, PEEM allows single shot acquisition of the 2D Fermi surface and full angular probing of the symmetry‐induced intensity modulations. We present results demonstrating a 2DEG at the surface of SrTiO3(001) at 140 K. The 2DEG is created by soft X‐ray irradiation and can be reversibly controlled by a combination of soft X‐rays and oxygen partial pressure.

The direct and parameter-free measurement of anisotropic electrical resistivity of a magnetic M(n+1)AX(n) (MAX) phase film is presented. A multitip scanning tunneling microscope is used to carry out 4-probe transport measurements with variable probe spacing s. The observation of the crossover from the 3D regime for small s to the 2D regime for large s enables the determination of both in-plane and perpendicular-to-plane resistivities rho(ab) and rho(c). A (Cr0.5Mn0.5)(2)GaC MAX phase film shows a large anisotropy ratio rho(c)/rho(ab) = 525 +/- 49. This is a consequence of the complex bonding scheme of MAX phases with covalent M-X and metallic M-M bonds in the MX planes and predominately covalent, but weaker bonds between the MX and A planes. Published under license by AIP Publishing. Funding Agencies|Knut and Alice Wallenberg (KAW) Foundation; Swedish Research Council [642-2013-8020]

Hemispherical deflection analyzers are the most widely used energy filters for state-of-the-art electron spectroscopy. Due to the high spherical symmetry, they are also well suited as imaging energy filters for electron microscopy. Here, we review the imaging properties of hemispherical deflection analyzers with emphasis on the application for cathode lens microscopy. In particular, it turns out that aberrations, in general limiting the image resolution, cancel out at the entrance and exit of the analyzer. This finding allows more compact imaging energy filters for momentum microscopy or photoelectron emission microscopy. For instance, high resolution imaging is possible, using only a single hemisphere. Conversely, a double pass hemispherical analyzer can double the energy dispersion, which means it can double the energy resolution at certain transmission, or can multiply the transmission at certain energy resolution.

Advanced materials 31(40), 1903391 - (2019). doi:10.1002/adma.201903391, Published by Wiley-VCH, Weinheim

We generate terahertz (THz) transients by illuminating a few-nanometer-thick Ta/NiFe/Pt nanolayers with a train of linearly polarized 100-fs-wide laser pulses. The transients are ∼1-ps-wide free-space propagating bursts of electromagnetic radiations with amplitudes that are magnetically and optically tunable. Their spectral frequency content extends up to 5 THz, and the 3-dB cutoff is at 0.85 THz. The observed transient electromagnetic signals originate from the NiFe/Pt bilayer, and their amplitude dependence on the external magnetic field, applied in the sample plane, very closely follows the static magnetization versus magnetic field dependence of the NiFe film. For the same laser power, excitation with highly energetic, blue light generates THz transients with amplitudes approximately three times larger than the ones resulting from excitation by infrared light. In both cases, the transients exhibit the same spectral characteristics and are linearly polarized in the perpendicular direction to the sample magnetization. The polarization direction can be tuned by rotation of the magnetic field around the laser light propagation axis. The characteristics of our THz spintronic emitter signals confirm that THz transient generation is due to the inverse spin Hall effect in the Pt layer and demonstrate that ferromagnet/metal nanolayers excited by femtosecond laser pulses can serve as efficient sources of magnetically and optically tunable, polarized transient THz radiation.

Electron energy loss spectroscopy has successfully established itself as the experimental method to study exchange-dominated, high-momentum spin waves in ultra-thin films. Because of insufficient energy resolution, previous studies were limited to spin waves in the high energy range and to wave vectors larger than about q|| = 0.4 A−1. In this regime, spin waves are strongly damped by decay into Stoner excitations. Furthermore, the spin wave signal of a multilayer film consists of several overlapping modes. After implementation of several technical modifications, our electron spectrometer now enables the study of spin waves with an energy resolution down to 2 meV, and thereby the discrimination of several spin wave modes in ultra-thin films as well as the study of the intrinsic width of modes down to 1 meV. Examples are presented for fcc and hcp cobalt films and discussed in terms of current theoretical models. Copyright © 2016 John Wiley & Sons, Ltd.

Rapid development of magnetic nanotechnologies calls for experimental techniques capable of providing magnetic information with subnanometre spatial resolution. Available probes of magnetism either detect only surface properties, such as spin-polarized scanning tunnelling microscopy, magnetic force microscopy or spin-polarized low-energy electron microscopy, or they are bulk probes with limited spatial resolution or quantitativeness, such as X-ray magnetic circular dichroism or classical electron magnetic circular dichroism (EMCD). Atomic resolution EMCD methods have been proposed, although not yet experimentally realized. Here, we demonstrate an EMCD technique with an atomic size electron probe utilizing a probe-corrected scanning transmission electron microscope in its standard operation mode. The crucial element of the method is a ramp in the phase of the electron beam wavefunction, introduced by a controlled beam displacement. We detect EMCD signals with atomic-plane resolution, thereby bringing near-atomic resolution magnetic circular dichroism spectroscopy to hundreds of laboratories worldwide.

LaNiO$_3$ (LNO) is an intriguing member of the rare-earth nickelates in exhibiting a metal-insulator transition for a critical film thickness of about 4 unit cells [Son et al., Appl. Phys. Lett. 96, 062114 (2010)]; however, such thin films also show a transition to a metallic state in superlattices with SrTiO$_3$ (STO) [Son et al., Appl. Phys. Lett. 97, 202109 (2010)]. In order to better understand this transition, we have studied a strained LNO/STO superlattice with 10 repeats of [4 unit-cell LNO/3 unit-cell STO] grown on an (LaAlO$_3$)$_{0.3}$(Sr$_2$AlTaO$_6$)$_{0.7}$ substrate using soft x-ray standing-wave-excited angle-resolved photoemission (SWARPES), together with soft- and hard- x-ray photoemission measurements of core levels and densities-of-states valence spectra. The experimental results are compared with state-of-the-art density functional theory (DFT) calculations of band structures and densities of states. Using core-level rocking curves and x-ray optical modeling to assess the position of the standing wave, SWARPES measurements are carried out for various incidence angles and used to determine interface-specific changes in momentum-resolved electronic structure. We further show that the momentum-resolved behavior of the Ni 3d eg and t2g states near the Fermi level, as well as those at the bottom of the valence bands, is very similar to recently published SWARPES results for a related La$_{0.7}$Sr$_{0.3}$MnO$_3$/SrTiO$_3$ superlattice that was studied using the same technique (Gray et al., Europhysics Letters 104, 17004 (2013)), which further validates this experimental approach and our conclusions. Our conclusions are also supported in several ways by comparison to DFT calculations for the parent materials and the superlattice, including layer-resolved density-of-states results.

We report on quantum transport measurements of a carbon nanotube (CNT) quantum dot that is functionalized with magnetic {Mn4} complexes. The coupling between the spin-5/2 MnII ions within each {Mn4} complex is predominantly antiferromagnetic. Coulomb diamond measurements at T = 4 K reveal that the covalent attachment of the complexes to the CNT framework has only little influence on the carbon nanotube quantum dot. Surprisingly, a strong increase of noise is observed upon cooling the sample to T = 30 mK. Time traces of the current taken at a diamond edge reveal a random telegraph signal. We attribute this to fluctuations of molecular spin states of the attached {Mn4} complexes.

Rational design of low-dimensional electronic phenomena at oxide interfaces is currently considered to be one of the most promising schemes for realizing new energy-efficient logic and memory devices. An atomically abrupt interface between paramagnetic $\mathrm{LaNi}{\mathrm{O}}_{3}$ and antiferromagnetic $\mathrm{CaMn}{\mathrm{O}}_{3}$ exhibits interfacial ferromagnetism, which can be tuned via a thickness-dependent metal-insulator transition in $\mathrm{LaNi}{\mathrm{O}}_{3}$. Once fully understood, such emergent functionality could turn this archetypal Mott-interface system into a key building block for the above-mentioned future devices. Here, we use depth-resolved standing-wave photoemission spectroscopy in conjunction with scanning transmission electron microscopy and x-ray absorption spectroscopy, to demonstrate a depth-dependent charge reconstruction at the $\mathrm{LaNi}{\mathrm{O}}_{3}/\mathrm{CaMn}{\mathrm{O}}_{3}$ interface. Our measurements reveal an increased concentration of $\mathrm{M}{\mathrm{n}}^{3+}$ and $\mathrm{N}{\mathrm{i}}^{2+}$ cations at the interface, which create an electronic environment favorable for the emergence of interfacial ferromagnetism mediated via the $\mathrm{M}{\mathrm{n}}^{4+}\ensuremath{-}\mathrm{M}{\mathrm{n}}^{3+}$ ferromagnetic double exchange and $\mathrm{N}{\mathrm{i}}^{2+}\ensuremath{-}\mathrm{O}\ensuremath{-}\mathrm{M}{\mathrm{n}}^{4+}$ superexchange mechanisms. Our findings suggest a strategy for designing functional Mott oxide heterostructures by tuning the interfacial cation characteristics via controlled manipulation of thickness, strain, and ionic defect states.

Point defects such as oxygen vacancies cause emergent phenomena such as resistive switching in transition-metal oxides, but their influence on the electron-transport properties is far from being understood. Here, we employ direct mapping of the electronic structure of a memristive device by spectromicroscopy. We find that oxygen vacancies result in in-gap states that we use as input for single-band transport simulations. Because the in-gap states are situated below the Fermi level, they do not contribute to the current directly but impact the shape of the conduction band. Accordingly, we can describe our devices with band-like transport and tunneling across the Schottky barrier at the interface.

We have investigated the electronic structure of the dilute magnetic semiconductor (DMS) Ga0.98Mn0.02P and compared it to that of an undoped GaP reference sample, using hard x-ray photoelectron spectroscopy (HXPS) and hard x-ray angle-resolved photoemission spectroscopy (HARPES) at energies of about 3 keV. We present experimentaldata,aswellastheoreticalcalculations,tounderstandtheroleoftheMndopantintheemergenceof ferromagnetism in this material. Both core-level spectra and angle-resolved or angle-integrated valence spectra are discussed. In particular, the HARPES experimental data are compared to free-electron final-state model calculations and to more accurate one-step photoemission theory. The experimental results show differences between Ga0.98Mn0.02P and GaP in both angle-resolved and angle-integrated valence spectra. The Ga0.98Mn0.02P bands are broadened due to the presence of Mn impurities that disturb the long-range translational order of the host GaP crystal. Mn-induced changes of the electronic structure are observed over the entire valence band range, including the presence of a distinct impurity band close to the valence-band maximum of the DMS. These experimental results are in good agreement with the one-step photoemission calculations and a prior HARPES study of Ga0.97Mn0.03As and GaAs [Gray et al., Nat. Mater. 11, 957 (2012)], demonstrating the strong similarity between these two materials. The Mn 2p and 3s core-level spectra also reveal an essentially identical state in doping both GaAs and GaP.