Magnetostatic coupling of 90° domain walls in Fe19Ni81/Cu/Co trilayers

The magnetic interlayer coupling of Fe19Ni81/Cu/Co trilayered microstructures has been studied by means of x-ray magnetic circular dichroism in combination with photoelectron emission microscopy (XMCD-PEEM). We find that a parallel coupling between magnetic domains coexists with a non-parallel coupling between magnetic domain walls (DWs) of each ferromagnetic layer. We attribute the non-parallel coupling of the two magnetic layers to local magnetic stray fields arising at DWs in the magnetically harder Co layer. In the magnetically softer FeNi layer, non-ordinary DWs, such as 270° and 90° DWs with overshoot of the magnetization either inwards or outwards relative to the turning direction of the Co magnetization, are identified. Micromagnetic simulations reveal that in the absence of magnetic anisotropy, both types of overshooting DWs are energetically equivalent. However, if a uniaxial in-plane anisotropy is present, the relative orientation of the DWs with respect to the anisotropy axis determines which of these DWs is energetically favorable

Sample cartridge with built in miniature molecule evaporator for in situ measurement with a photoemission electron microscope

We present a new sample holder that is compatible with Photoemission Electron Microscopes PEEMs and contains a molecule evaporator. With the integrated low cost evaporator, a local and controlled material deposition in clean ultra high vacuum conditions can be achieved minimizing the contamination of the analysis chamber. Different molecule systems can easily be studied by exchanging the sample holder. This opens up new possibilities for in situ investigation of thin film growth by means of spectromicroscopy and element selective imaging at the nanometer scale. As an example of the performances of the setup, we present a study of the hybrid inorganic organic system HIOS consisting of the organic acceptor molecule 2,2 amp; 8242; perfluoronaphthalene 2,6 diylidene dimalononitrile F6TCNNQ and ZnO, which is of great interest for novel HIOS based optoelectronic devices. Here, the ZnO surface work function modification by F6TCNNQ adsorption is investigated in situ in a spatially resolved manner. In addition, we employ PEEM to selectively probe the chemical state of F6TCNNQ molecules in contact with ZnO in the first monolayer and those molecules located in multilayers in 3D island

Direct observation of the band gap transition in atomically thin ReS2_2

ReS$_2$ is considered as a promising candidate for novel electronic and sensor applications. The low crystal symmetry of the van der Waals compound ReS$_2$ leads to a highly anisotropic optical, vibrational, and transport behavior. However, the details of the electronic band structure of this fascinating material are still largely unexplored. We present a momentum-resolved study of the electronic structure of monolayer, bilayer, and bulk ReS$_2$ using k-space photoemission microscopy in combination with first-principles calculations. We demonstrate that the valence electrons in bulk ReS$_2$ are - contrary to assumptions in recent literature - significantly delocalized across the van der Waals gap. Furthermore, we directly observe the evolution of the valence band dispersion as a function of the number of layers, revealing a significantly increased effective electron mass in single-layer crystals. We also find that only bilayer ReS$_2$ has a direct band gap. Our results establish bilayer ReS$_2$ as a advantageous building block for two-dimensional devices and van der Waals heterostructures

Strong interlayer coupling in van der Waals heterostructures built from single-layer chalcogenides

Semiconductor heterostructures are the fundamental platform for many important device applications such as lasers, light-emitting diodes, solar cells and high-electron-mobility transistors. Analogous to traditional heterostructures, layered transition metal dichalcogenide (TMDC) heterostructures can be designed and built by assembling individual single-layers into functional multilayer structures, but in principle with atomically sharp interfaces, no interdiffusion of atoms, digitally controlled layered components and no lattice parameter constraints. Nonetheless, the optoelectronic behavior of this new type of van der Waals (vdW) semiconductor heterostructure is unknown at the single-layer limit. Specifically, it is experimentally unknown whether the optical transitions will be spatially direct or indirect in such hetero-bilayers. Here, we investigate artificial semiconductor heterostructures built from single layer WSe2 and MoS2 building blocks. We observe a large Stokes-like shift of ~100 meV between the photoluminescence peak and the lowest absorption peak that is consistent with a type II band alignment with spatially direct absorption but spatially indirect emission. Notably, the photoluminescence intensity of this spatially indirect transition is strong, suggesting strong interlayer coupling of charge carriers. The coupling at the hetero-interface can be readily tuned by inserting hexagonal BN (h-BN) dielectric layers into the vdW gap. The generic nature of this interlayer coupling consequently provides a new degree of freedom in band engineering and is expected to yield a new family of semiconductor heterostructures having tunable optoelectronic properties with customized composite layers.Comment: http://www.pnas.org/content/early/2014/04/10/1405435111.abstrac

Electronic structure and spectroscopy of the quaternary Heusler alloy Co2_2Cr1−x_{1-x}Fex_{x}Al

Quaternary Heusler alloys Co$_2$Cr$_{1-x}$Fe$_{x}$Al with varying Cr to Fe ratio $x$ were investigated experimentally and theoretically. The electronic structure and spectroscopic properties were calculated using the full relativistic Korringa-Kohn-Rostocker method with coherent potential approximation to account for the random distribution of Cr and Fe atoms as well as random disorder. Magnetic effects are included by the use of spin dependent potentials in the local spin density approximation. Magnetic circular dichroism in X-ray absorption was measured at the $L_{2,3}$ edges of Co, Fe, and Cr of the pure compounds and the $x=0.4$ alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. Resonant (560eV - 800eV) soft X-ray as well as high resolution - high energy ($\geq 3.5$keV) hard X-ray photo emission was used to probe the density of the occupied states in Co$_2$Cr$_{0.6}$Fe$_{0.4}$Al.Comment: J.Phys.D_Appl.Phys. accepte

2D magnetic domain wall ratchet: The limit of submicrometric holes

The study of ratchet and crossed-ratchet effects in magnetic domain wall motion through 2D arrays of asymmetric holes is extended in this article to the submicrometric limit in hole size (small size regime). Therefore, the gap has been closed between the 2D ratchets in the range of tens-of-micrometers (large size regime) and the small size regime 1D ratchets based on nanowires. The combination of Kerr microscopy, X-ray PhotoEmission Electron Microscopy and micromagnetic simulations has allowed a full magnetic characterisation of both the domain wall (DW) propagation process over the whole array and the local DW morphology and pinning at the holes. It is found that the 2D small size limit is driven by the interplay between DW elasticity and half vortex propagation along hole edges: as hole size becomes comparable to DW width, flat DW propagation modes are favoured over kinked DW propagation due to an enhancement of DW stiffness, and pinned DW segments adopt asymmetric configurations related with Néel DW chirality. Nevertheless, both ratchet and crossed-ratchet effects have been experimentally found, and we propose a new ratchet/inverted-ratchet effect in the submicrometric range driven by magnetic fields and electrical currents respectively

Spatially resolved X ray absorption spectroscopy investigation of individual cation intercalated multi layered Ti3C2Tx MXene particles

Ti3C2Tx MXene is a two dimensional 2D material possessing highly active hydrophilic surfaces coupled with high metallic conductivity. Cations intercalation between the Ti3C2Tx nanosheets has a significant role in many applications such as water purification, desalination, and electrochemical energy storage. The pseudocapacitive charging mechanism involving surface redox reactions at the Ti3C2Tx surface enables higher energy densities than electrical double layer capacitors, and higher power densities than batteries. In this context, the oxidation state of surface Ti atoms involved in redox reactions has a high impact on the capacitance of Ti3C2Tx MXene and this can be impacted by cation intercalation. Thus, the electronic structure of multi layered Ti3C2Tx particles is investigated by X ray absorption XA spectroscopy, while also benefitting from a high spatial resolution of 30 nm from X ray photoemission electron microscopy. In this work, the XA spectra from multi layered intercalated Ti3C2Tx particles of different thicknesses were recorded at the Ti L and O K edges. The Ti oxidation state in pristine, Li , and Mg intercalated Ti3C2Tx was found to be thickness dependent, while Na and K intercalated Ti3C2Tx particles did not reveal differences upon changing thickness. This work demonstrates thickness dependent modification of the MXene surface chemistry upon cation intercalation in different individual Ti3C2Tx particle