Imaging of magnetic domains with the X-ray microscope at BESSY using X-ray magnetic circular dichroism

X-ray Magnetic Circular Dichroism (X-MCD), i.e. the change of the absorption of circular polarized X-rays for reversed sample magnetization amounts at the L_2,3_{2,3}-edges of 3d transition metals up to 50% percent. This can be used to obtain in energy-dispersive X-ray imaging techniques a considerable, element-specific magnetic contrast. On the other hand, with the transmission X-ray microscope (TXM) based on the zone-plate technique spatial resolutions of 30 nm30 nm can be achieved. In this communication it is shown for the first time that the combination of the TXM with X-MCD provides a huge contrast and is therefore powerful new method to visualize in a quantitative and element-specific manner magnetic domains. Using soft X-rays with a wavelength of 1.7 nm1.7 nm corresponding to the energy of the Fe L₃_{3}-edge the variation of the shape and magnetization of domains in a magneto-optical GdFe layer system was studied with a lateral resolution of 60 nm60 nm.     

Exploring nanomagnetism with soft X-ray microscopy

Magnetic soft X-ray microscopy images magnetism in nanoscale systems with a spatial resolution down to 15 nm provided by state-of-the-art Fresnel zone plate optics. X-ray magnetic circular dichroism (X-MCD) is used as the element-specific magnetic contrast mechanism similar to photoemission electron microscopy (PEEM), however, with volume sensitivity and the ability to record the images in varying applied magnetic fields which allows study of magnetization reversal processes at fundamental length scales. Utilizing a stroboscopic pump-probe scheme one can investigate fast spin dynamics with a time resolution down to 70 ps which gives access to precessional and relaxation phenomena as well as spin torque driven domain wall dynamics in nanoscale systems. Current developments in zone plate optics aim for a spatial resolution towards 10 nm and at next generation X-ray sources a time resolution in the fs regime can be envisioned.     

Imaging of magnetic domains with the X-ray microscope at BESSY using X-ray magnetic circular dichroism

X-ray Magnetic Circular Dichroism (X-MCD), i.e. the change of the absorption of circular polarized Xrays for reversed sample magnetization amounts at the L_{2, 3}-edges of 3d transition metals up to 50% percent. This can be used to obtain in energy-dispersive X-ray imaging techniques a considerable, element-specific magnetic contrast. On the other hand, with the transmission X-ray microscope (TXM) based on the zone-plate technique spatial resolutions of 30nm can be achieved. In this communication it is shown for the first time that the combination of the TXM with XMCD provides a huge contrast and is therefore a powerful new method to visualize in a quantitative and elementspecific manner magnetic domains. Using soft X-rays with a wavelength of 1:7nm corresponding to the energy of the Fe L₃-edge the variation of the shape and magnetization of domains in a magneto-optical GdFe layer system was studied with a lateral resolution of 60nm.     

Characterization of fluorinated multiwalled carbon nanotubes with X-ray absorption,photoelectron and emission spectroscopies

The combined investigation of the chemical bond formation in fluorinated multiwalled carbon nanotubes with 15 wt.% fluorine concentrations (MWCNTs + F 15 wt.%) using X-ray absorption, emission and photoelectron spectroscopy at C 1s and F 1s thresholds is presented. All measurements were performed at BESSY II. The analysis of the soft X-ray and photoelectron spectra point to the formation of covalent chemical bonding between fluorine and carbon atoms in the fluorinated nanotubes. Based on results of this combined study the depth dependent effects are discussed.     

Electric current control of creation and annihilation of sub-100 nm magnetic bubbles examined by full-field transmission soft X-ray microscopy

The effect of electric current pulses on a sub-100 nm magnetic bubble state in a symmetric Pt/Co multilayer was directly observed using a full-field transmission soft X-ray microscope (MTXM). Field-induced evolution of the magnetic stripe domains into isolated bubbles with their sizes down to 100 nm was imaged under varying external magnetic fields. Electric current pulses were then applied to the created magnetic bubbles, and it was observed that the bubbles could be either created or annihilated by the current pulse depending on the strength of applied magnetic field. The results suggest that the Joule heating plays a critical role in the formation and/or elimination of the bubbles and skyrmions. Finally, the schematic phase diagram for the creation and annihilation of bubbles is presented, suggesting an optimized scheme with the combination of magnetic field and electric current necessary to utilize skyrmions in the practical devices.     

Magnetic interface polarization of the La-5d states in Fe/LaHx multilayers

For rare-earth (RE)-hydrides (REHx), a metal-to-insulator transition is observed if the hydrogen concentration is increased from the dihydride (x=2) towards the trihydride (x=3). This transition provides an object for studies of the magnetic interface polarization in layered ferromagnet/insulator structures. For different samples with a fixed Fe and varied LaH_x sublayer thickness t_LaHx ([15 ? Fe/t? LaH_x]xn), the H concentration x in the LaH_x sublayer is gradually increased in the experiment. Starting from the as prepared dihydride REH_2-δ, the modification of the magnetic polarization of the La-5d states at the Fe/La_x interface is studied across the transition by magnetic circular dichroism measurements at the La-L₂ and L₃ edges. The experiments reveal an induced magnetic polarization of the La-5d hole states on a length scale of 9 ? that is independent of the altered electronic structure of the La-5d states. Received: 23 May 2001 / Accepted: 4 July 2001 / Published online: 5 October 2001     

Element-specific hard X-ray micro-magnetometry of magnetic modifications in Co–Pt dots fabricated by ion etching

We developed a micro-magnetometry with a 2.5 μm spatial resolution based on micro X-ray magnetic circular dichroism (XMCD) technique in order to study magnetic properties of dot arrays for bit-patterned media. This micro-magnetometer was applied to the magnetic characterization of Co–Pt dot arrays fabricated by ion beam etching. As the dot size became small, the intensity of XMCD drastically decreased for dots fabricated by Ga-focused ion beam. This suggested that the dot edges were damaged magnetically by implantation of Ga ions. The damaged width of the dot edge was estimated to be about 13 nm from the decrease in XMCD intensities. This damaged edge width agreed with the ion-implanted area estimated by Monte-Carlo simulation. The less-damaged effect of Ar ion etching was verified by the XMCD measurement of Co–Pt dots with diameter of 20 and 70 nm. It was concluded that ions with inertness, lower energy and smaller atomic number should be used to fabricate dot arrays with an areal density of 1 Tbit/in².     

Nanocylinder-array structure greatly increases the soft X-ray intensity generated from femtosecond-laser-produced plasma

In order to increase the soft X-ray conversion efficiency for the femtosecond-laser-produced plasma, we adopted a nanocylinder-array structure target. Gold nanocylinder-array targets with 70–90 nm cylinder diameter and 100 nm cylinder pitch were made. A continuous smooth soft X-ray spectrum adequate for X-ray absorption spectroscopy was obtained. An around 20-fold soft X-ray (7–20 nm) fluence enhancement compared with a flat-surface gold foil target was obtained when the cylinder height was 18 μm. X-ray (>0.06 keV) pulse duration was 17 ps, which is much shorter than that obtained by using the pre-pulse technique. The X-ray pulse peak intensity was 7-fold higher than that of a gold foil target. Received: 2 May 2001 / Revised version: 1 June 2001 / Published online: 18 July 2001     

High-resolution X-ray imaging—a powerful nondestructive technique for applications in semiconductor industry

The availability of high-brilliance X-ray sources, high-precision X-ray focusing optics and very efficient CCD area detectors has contributed essentially to the development of transmission X-ray microscopy (TXM) and X-ray computed tomography (XCT) with sub-50 nm resolution. Particularly, the fabrication of high aspect ratio Fresnel zone plates with zone widths approaching 15 nm has contributed to the enormous improvement in spatial resolution during the previous years. Currently, Fresnel zone plates give the ability to reach spatial resolutions of 15 to 20 nm in the soft and of about 30 to 50 nm in the hard X-ray energy range. X-ray microscopes with rotating anode X-ray sources that can be installed in an analytical lab next to a semiconductor fab have been developed recently. These unique TXM/XCT systems provide an important new capability of nondestructive 3D imaging of internal circuit structures without destructive sample preparation such as cross sectioning. These lab systems can be used for failure localization in micro- and nanoelectronic structures and devices, e.g., to visualize voids and residuals in on-chip metal interconnects without physical modification of the chip. Synchrotron radiation experiments have been used to study new processes and materials that have to be introduced into the semiconductor industry. The potential of TXM using synchrotron radiation in the soft X-ray energy range is shown for the nondestructive in situ imaging of void evolution in embedded on-chip copper interconnect structures during electromigration and for the imaging of different types of insulating thin films between the on-chip interconnects (spectromicroscopy).     

Deposition and characterization of ITO films produced by laser ablation at 355 nm

Indium tin oxide (ITO) films have been deposited by pulsed laser deposition (PLD) at 355 nm. Even though the absorption of laser light at the wavelength 355 nm is much smaller than that of the standard excimer lasers for PLD at 248 nm and 193 nm, high-quality films can be produced. At high fluence and at high substrate temperatures, the specific resistivity of the films, 2–3×10^-4 Ω cm, is comparable to values obtained with excimer lasers, whereas the resistivities obtained at room temperature are somewhat higher than those of films produced by excimer lasers. The transmission coefficient of visible light, about 0.9, is also comparable to values for films deposited by excimer lasers. The crystalline structure of films produced at 355 nm is similar to that of samples produced by these lasers. Received: 16 January 2001 / Accepted: 24 July 2001 / Published online: 17 October 2001     

Microstructural properties of bulk nanocrystalline Ag–Ni alloy prepared by hot pressing of mechanically pre-alloyed powders

A bulk nanocrystallined Ag50Ni alloy has been prepared by hot-pressing the mechanically pre-alloyed powders at 620 °C under a normal pressure of 58 MPa in vacuum. The microstructural characteristics of the alloy were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The results of the precise determination of the lattice parameters of the phases in the powders and in the alloy by XRD show that, after mechanical alloying for 200 h, the solid solubility of Ag in Ni reaches 4.85±0.21 at %, while that of Ni in Ag reaches 0.84±0.30 at %. After hot pressing, the Ag- and Ni-rich phases in the alloy still show a certain degree of supersaturation, with a solid solubility of 0.45±0.11 at % of Ag in Ni. After further annealing of the alloy at 700 °C for 24 h, the solubility decreases to a value of 0.21±0.11 at % for Ag in Ni and to less than 0.1 at % for Ni in Ag. The grain size of the mechanically alloyed powders was of ca 6 nm. After hot pressing, the grain size of the alloy increased to 40–60 nm and then grew further to 100–110 nm after annealing. The influence of the variation of the grain size and the internal stress on the line breadth of the X-ray diffraction peaks has been evaluated in detail. Finally, the role of the nanocrystalline structure in the fast densification process of the powders is also discussed. Received: 12 September 2001 / Accepted: 18 Febraury 2002 / Published online: 10 September 2002 RID="*" ID="*"Corresponding author. Fax: +86-24/2389-3624, E-mail: wwt@icpm.syb.ac.cn     

Enhanced imaging of magnetic structures in micropatterned arrays of Co dots and antidots

A specific technique of numerical treatment of atomic force microscopy (AFM) and magnetic force microscopy (MFM) signal has been developed to enhance the quality of raw images, in order both to improve their contrast and to gain better insight on the sample topography and on the local arrangement of the magnetisation vector. Basically, the technique consists in computing the optimum conformal transformation that allows one to superimpose two AFM images of the same area, acquired performing subsequent scans whose fast scan axis were mutually perpendicular, and applying the inverse transform to the second image. After MFM image superposition, the two datasets were either summed or subtracted, in order to improve the magnetic contrast. Computations have been done in a Matlab^® workspace with the help of Image Processing Toolbox 4.2. Improved MFM images obtained on both dots and antidots thin evaporated Co arrays in the demagnetised state (after performing alternate field demagnetisation parallel and perpendicular to the array plane) have been interpreted. Samples consisting of large-size patterns (1×1 mm) of circular dots/antidots with square/hexagonal lattices and minimum diameters of 1 μm were prepared by optical lithography. The magnetic film thickness was chosen depending on resist thickness, and varied between 25 and 150 nm, with a fixed ratio 1:4 between metal/resist film thickness. MFM was exploited to obtain images of either intra-dot or inter-antidot magnetic structures.     

Magnetic microstructures and their dynamics studied by X-ray microscopy

Full-field soft X-ray microscopy in combination with X-ray magnetic circular dichroism as contrast mechanism is a powerful technique to image with elemental specificity magnetic nanostructures and multilayered thin films at high lateral resolution down to 15nm by using Fresnel zone plates as X-ray optical elements. Magnetization reversal phenomena on a microscopic level are studied by recording the images in varying external magnetic fields. Local spin dynamics at a time resolution below 100ps can be addressed by engaging a stroboscopic pump-and-probe scheme taking into account the time pattern of synchrotron storage rings. Characteristic features of magnetic soft X-ray microscopy are reviewed and an outlook into future perspectives with regard to increased lateral and temporal resolution is given.     

Temperature dependent magnetic spin and orbital moments of mass-filtered cobalt clusters on Au(111)

Mass-filtered cobalt clusters with a size of 8 nm have been deposited in-situ under soft-landing conditions onto Au(111). The spin and orbital moments of the Co nanoparticles on a Au (111) single crystal have been investigated as a function of the temperature using the element-specific method of X-ray magnetic circular dichroism in photoabsorption. The results hint at an temperature-dependent spin-reorientation transition which is discussed with respect to different contribution to the magnetic anisotropy. Furthermore, by means of an in-situ oxidation experiment, the influence of an exposure to oxygen on the properties of the cobalt clusters has been investigated.     

Low-energy ion emission from a xenon gas-puff laser-plasma X-ray source

We have measured low-energy ion emission from a gas-puff laser-plasma X-ray source. The ions may cause the degradation of the condenser mirror of the extreme ultra-violet projection lithography system. A 0.7 J in 8 ns Nd:YAG laser at 1.06 μm was focused onto the xenon gas-puff target with an intensity of ∼10¹² W/cm². The silicon (111) plates, placed at a distance of 32 mm from the laser-interaction region, were exposed with the xenon ions. The average ion energy was measured to be less than 50 eV with a Faraday-cup detector placed close to the silicon plates. The xenon deposition occurred in the silicon plates with a depth of less than 40 nm. The deposition density was measured with a quadrupole secondary ion mass spectrometer to be 10²¹ /cm³ after 1500 laser shots. The energy-conversion efficiency from the laser energy into the ions is ∼0.1%/4 π sr/shot. For the lithography system, if we can remove such ion bombardment completely using novel techniques such as electro-magnetic devices or gas flow curtain techniques, the lifetime of the condenser mirror will be extended significantly. Received: 20 November 2000 / Published online: 9 February 2001     

Photoluminescence characteristics of ZnS nanocrystallites doped with Ti3+and Ti4+

Direct synthesis of ZnS nanocrystallites doped with Ti³⁺ or Ti⁴⁺ by precipitation has led to novel photoluminescence properties. Detailed X-ray diffraction (XRD), fluorescence spectrophotometry, UV–vis spectrophotometry and X-ray photoelectron spectroscopy (XPS) analysis reveal the crystal lattice structure, average size, emission spectra, absorption spectra and composition. The average crystallite size doped with different mole ratios, estimated from the Debye–Scherrer formula, is about 2.6±0.2 nm. The nanoparticles can be doped with Ti³⁺ and Ti⁴⁺ during the synthesis without the X-ray diffraction pattern being altered. The strong and stable visible-light emission has been observed from ZnS nanocrystallites doped with Ti³⁺ (its maximum fluorescence intensity is about twice that of undoped ZnS nanoparticles). However, the fluorescence intensity of the ZnS nanocrystallites doped with Ti⁴⁺ is almost the same as that of the undoped ZnS nanoparticles. The emission peak of the undoped sample is at 440–450 nm. The emission spectrum of the doped sample consists of two emission peaks, one at 420–430 nm and the other at 510 nm. Received: 27 April 2001 / Accepted: 16 August 2001 / Published online: 17 October 2001     

Phase transition in YbAl<Subscript>3</Subscript>C<Subscript>3</Subscript>

¹⁷⁰Yb M?ssbauer spectroscopy, temperature dependent X-ray, magnetisation and specific heat data are presented in the hexagonal intermetallic YbAl₃C₃, in order to shed light on the isostructural transition occurring near 80 K and to investigate the electronic state of the Yb ion above and below the transition. In the low temperature phase, we find that there occurs an atomic rearrangement in the hexagonal unit cell, leading to a strong symmetry lowering at the Yb site. We show that no magnetic ordering of the Yb³⁺ moments occurs down to 0.04 K, and we discuss this finding in terms of 4f-conduction electron hybridisation and geometric frustration.     

Laser induced threshold photoemission magnetic circular dichroism and its application to photoelectron microscope

This work enlightens the threshold photoemission magnetic circular dichroism (MCD) and its adaption on photoemission electron microscopy (PEEM) using lasers. MCD is a simple and efficient way to investigate magnetic properties since it does not need any spin analyzers with low efficiency, and thus the MCD related techniques have developed to observe magnetic domains. Usually, MCD in a total yield measurement in the valence band with weak spin–orbit coupling (SOC) excited by low photon energy (hν≤ 6 eV) does not compete with the X-ray magnetic circular dichroism (XMCD) with strong SOC. XMCD PEEM observation of magnetic domains has been successfully established while MCD PEEM derived from valence bands has not been. However, using angle and energy resolved photoelectron, valence band MCD provides large asymmetry similar to that by XMCD. Threshold measurement of photoelectron in a total electron yield procedure can take advantage of the measurement of photoelectrons with a limited angle and energy mode. This restriction of the photoelectron makes the threshold MCD technique an efficient way to get magnetic information and gives more than 10% asymmetry for Ni/Cu(0 0 1), which is comparable to that obtained by angle resolved photoemission. Thus the threshold MCD technique is a suitable method to observe magnetic domains by PEEM. For threshold MCD, incident angle dependence and high sensitivity to out-of-plane magnetized films compared with in-plane ones are discussed. Ultrashort pulse lasers make it feasible to measure two photon photoemission MCD combined with PEEM, where resonant excitation has a possibility to enhance dichroic asymmetry. Recent results for valence band magnetic dichroism PEEM are presented.     

Small angle X-ray scattering studies on structures of alkylthiol stabilized-silver nanoparticles in solution

Silver nanoparticles (SNPs), which were stabilized with various alkylthiols, were studied in detail with synchrotron small angle X-ray scattering, UV-Vis spectroscopy and transmission electron microscopy (TEM). UV spectra of SNPs capped with various alkylthiols showed the characteristic surface plasmon bands at ∼ 450 nm. However, the spectral shapes of SNPs with relatively short butanethiols became weaker and asymmetrically broader in the longer wavelength region. Their nanostructures were deliberately characterized by small angle X-ray scattering analysis based on a hard sphere model and TEM results. PACS 61.05.cf; 61.46.Df; 81.07.Pr     

Fluorescent Carbon Dots Capped with PEG<Subscript>200</Subscript> and Mercaptosuccinic Acid

The synthesis and functionalization of carbon nanoparticles with PEG₂₀₀ and mercaptosuccinic acid, rendering fluorescent carbon dots, is described. Fluorescent carbon dots (maximum excitation and emission at 320 and 430 nm, respectively) with average dimension 267 nm were obtained. The lifetime decay of the functionalized carbon dots is complex and a three component decay time model originated a good fit with the following lifetimes: τ ₁ = 2.71 ns; τ ₂ = 7.36 ns; τ ₃ = 0.38 ns. The fluorescence intensity of the carbon dots is affected by the solvent, pH (apparent pK _a of 7.4 ± 0.2) and iodide (Stern-Volmer constant of 78 ± 2 M⁻¹).