Sequential single-shot imaging of nanoscale dynamic interactions with a table-top soft x-ray laser

We demonstrate the first real-space recording of nanoscale dynamic interactions using single-shot soft x-ray (SXR) full-field laser microscopy. A sequence of real-space flash images acquired with a table-top SXR laser was used to capture the motion of a rapidly oscillating magnetic nanoprobe. Changes of 30 nm in the oscillation amplitude were detected when the nanoprobe was made to interact with stray fields from a magnetic sample. The table-top visualization of nanoscale dynamics in real space can significantly contribute to the understanding of nanoscale processes and can accelerate the development of new nanodevices.     

Full-field digital image correlation with Kriging regression

A full-field Digital Image Correlation (DIC) method with integrated Kriging regression is presented in this article. The displacement field is formulated as a best linear unbiased model that includes the correlations between all the locations in the Region of Interest (RoI). A global error factor is employed to extend conventional Kriging interpolation to quantify displacement errors of the control points. An updating strategy for the self-adaptive control grid is developed on the basis of the Mean Squared Error (MSE) determined from the Kriging model. Kriging DIC is shown to outperform several other full-field DIC methods when using open-access experimental data. Numerical examples are used to demonstrate the robustness of Kriging DIC to different choices of initial control points and to speckle pattern variability. Finally Kriging DIC is tested on an experimental example.     

Stochastic nature of magnetic processes studied by full-field soft X-ray microscopy

In nanomagnetism, one of the crucial scientific questions is whether magnetic behaviors are deterministic or stochastic on a nanoscale. Apart from the exciting physical issue, this question is also of paramount highest relevance for using magnetic materials in a wealth of technological applications such as magnetic storage and sensor devices. In the past, the research on the stochasticity of a magnetic process has been mainly done by macroscopic measurements, which only offer ensemble-averaged information. To give more accurate answer for the question and to fully understand related underlying physics, the direct observation of statistical behaviors in magnetic structures and magnetic phenomena utilizing advanced characterization techniques is highly required. One of the ideal tools for such study is a full-field soft X-ray microscope since it enables imaging of magnetic structures on the large field of view within a few seconds. Here we review the stochastic behaviors of various magnetic processes including magnetization reversal process in thin films, magnetic domain wall motions in nanowires, and magnetic vortex formations in nanodisks studied by full-field soft X-ray microscopy. The origin triggering the stochastic nature witnessed in each magnetic process and the way to control the intrinsic nature are also discussed.     

X-ray diffraction microscopy of magnetic structures

We report the first proof-of-principle experiment of iterative phase retrieval from magnetic x-ray diffraction. By using the resonant x-ray excitation process and coherent x-ray scattering, we show that linearly polarized soft x rays can be used to image both the amplitude and the phase of magnetic domain structures. We recovered the magnetic structure of an amorphous terbium-cobalt thin film with a spatial resolution of about 75 nm at the Co L3 edge at 778 eV. In comparison with soft x-ray microscopy images recorded with Fresnel zone plate optics at better than 25 nm spatial resolution, we find qualitative agreement in the observed magnetic structure.     

Study of sawtooth oscillations on the HT-7 tokamak using 2D tomography of soft x-ray signal

It is the first time so far as we know that two arrays of multi-channel soft x-ray detectors are used to generate twodimensional （2D） images of sawtooth oscillation on the HT-7 tokamak using the Fourier-Bessel harmonic reconstruction method, and using the singular value decomposition to analyse the data from soft x-ray cameras. By these two arrays, 2D image reconstruction of soft x-ray emissivity can be obtained without assumption of plasma rigid rotation. Tomographic reconstruction of the m=1 mode structure is obtained during the precursor oscillation of the sawtooth crash. The crescent-shaped mode structure appearing on the contour map of the soft x-ray emissivity is consistent with the quasiinterchange mode. The characteristics of the m=1/n=1 mode structure observed in the soft x-ray tomography are as follows： the magnetic surface is made up of the crescent-shaped “hot core” and the circular “cold bubble”. The structure of the magnetic surface rotates in the direction of the electron diamagnetic drift and the rotation frequency is the oscillation frequency of soft x-ray signals.     

Layer resolved magnetization reversal study in SmCo5/Fe nanocomposite bilayers

A hard/soft SmCo5/Fe nanocomposite magnetic bilayer system is fabricated on x-ray transparent 100-200 nm thin Si3N4 films by magnetron sputtering.The microscopic magnetic domain pattern and its behaviours during magnetization reversal in the hard and the soft magnetic phases are studied separately by element specific magnetic soft x-ray microscopy at a spatial resolution of better than 25 nm.We observe that the domain patterns for the soft and hard phases show coherent behaviours in varying magnetic fields.We derive local M(H) curves from the images of Fe and SmCo5 separately and find the switches for hard and soft phases to be the same.     

Accurate kinematic measurement at interfaces between dissimilar materials using conforming finite-element-based digital image correlation

Digital image correlation (DIC) is now an extensively applied full-field measurement technique with subpixel accuracy. A systematic drawback of this technique, however, is the smoothening of the kinematic field (e.g., displacement and strains) across interfaces between dissimilar materials, where the deformation gradient is known to be large. This can become an issue when a high level of accuracy is needed, for example, in the interfacial region of composites or joints. In this work, we described the application of global conforming finite-element-based DIC technique to obtain precise kinematic fields at interfaces between dissimilar materials. Speckle images from both numerical and actual experiments processed by the described global DIC technique better captured sharp strain gradient at the interface than local subset-based DIC.     

Path-independent digital image correlation with high accuracy,speed and robustness

The initial guess transferring mechanism is widely used in iterative DIC algorithms and leads to path-dependence. Using the known deformation at a processed point to estimate the initial guess at its neighboring points could save considerable computation time, and a cogitatively-selected processing path contributes to the improved robustness. In this work, our experimental study demonstrates that a path-independent DIC method is capable to achieve high accuracy, efficiency and robustness in full-field measurement of deformation, by combining an inverse compositional Gauss–Newton (IC-GN) algorithm for sub-pixel registration with a fast Fourier transform-based cross correlation (FFT-CC) algorithm to estimate the initial guess. In the proposed DIC method, the determination of initial guess accelerated by well developed software library can be a negligible burden of computation. The path-independence also endows the DIC method with the ability to handle the images containing large discontinuity of deformation without manual intervention. Furthermore, the possible performance of the proposed path-independent DIC method on parallel computing device is estimated, which shows the feasibility of the development of real-time DIC with high-accuracy.     

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.     

Manipulating the magnetic structure with electric fields in multiferroic ErMn2O5

Based on measurements of soft x-ray magnetic diffraction under in situ applied electric field, we report on significant manipulation and exciting of commensurate magnetic order in multiferroic ErMn2O5. The induced magnetic scattering intensity arises at the commensurate magnetic Bragg position whereas the initial magnetic signal almost persists. We demonstrate the possibility to imprint a magnetic response function in ErMn2O5 by applying an electric field.     

Magnetic properties of soft layer/FePt--MgO exchange coupled composite perpendicular recording media

The magnetic properties of exchange coupled composite （ECC） media that are composed of perpendicular magnetic recording media FePt MgO and two kinds of soft layers have been studied by using an x-ray diffractometer, a polar Kerr magneto-optical system （PMOKE） and a vibrating sample magnetometer （VSM）. The results show that ECC media can reduce the coercivities of perpendicular magnetic recording media FePt-MgO. The ECC media with granular-type soft layers have weaker exchange couplings between magnetic grains and the magnetization process, for ECC media of this kind mainly follow the Stoner Wohlfarth model.     

Origin of the large polarization in multiferroic YMnO3 thin films revealed by soft- and hard-X-ray diffraction

We investigated the magnetic structure of an orthorhombic YMnO(3) thin film by resonant soft x-ray and hard x-ray diffraction. We observed a temperature-dependent incommensurate magnetic reflection below 45 K and a commensurate lattice-distortion reflection below 35 K. These results demonstrate that the ground state is composed of coexisting E-type and cycloidal states. Their different ordering temperatures clarify the origin of the large polarization to be caused by the E-type antiferromagnetic states in the orthorhombic YMnO(3) thin film.     

Condenser for Koehler-like illumination in transmission x-ray microscopes at undulator sources

We report on a novel condenser for full-field transmission x-ray microscopes that use synchrotron radiation from an undulator source. The condenser produces a Koehler-like homogeneous intensity distribution in the sample plane and eliminates object illumination problems connected with the high degree of spatial coherence in an undulator beam. The optic consists of a large number of small linear diffraction gratings and is therefore relatively easy to manufacture. First imaging experiments with a prototype condenser were successfully performed with the Twinmic x-ray microscope at the Elettra synchrotron facility in Italy.     

Soft X-ray resonant magnetic diffraction

We have conducted the first soft x-ray diffraction experiments from a bulk single crystal, studying the bilayer manganite La2-2xSr1+2xMn2O7 with x=0.475 in which we were able to access the (002) Bragg reflection using soft x rays. The Bragg reflection displays a strong resonant enhancement at the L(III) and L(II) manganese absorption edges. We demonstrate that the resonant enhancement of the magnetic diffraction of the (001) is extremely large, indeed so large that it exceeds that of the nonresonant Bragg diffraction. Resonant soft x-ray scattering of 3d transition metal oxides is the only technique for the atomic selective measurement of spin, charge, and orbital correlations in materials, such as high temperature superconductors, colossal magnetoresistance manganites, and charge stripe nickelates.     

Experimental determination of Representative Volume Element (RVE) size in woven composites

A systematic approach is proposed to estimate the length scales of the representative volume element (RVE) in orthogonal plain woven composites. The approach is based on experimental full-field deformation measurements at mesoscopic scales. Stereovision digital image correlation (DIC) is conducted to determine the full-field strain distribution in on- and off-axis specimens loaded axially in tension. A sensitivity analysis is carried out to optimize the image correlation parameters. Using the optimized set of image correlation parameters, full-field strains are measured and used in conjunction with a simple strain averaging algorithm to identify the length scales at which globally applied and spatially-averaged local strains converge in values. The size of a virtual window containing local strain data, the average of which has the same value as the global strain, is identified as the RVE dimensions for the examined material. The smallest RVE sizes found in this work are shown to be both strain and angle dependent. The largest RVE dimension obtained is reported as a unique, strain and orientation insensitive RVE size for the woven composite examined.     

On the origin of soft x-ray and microwave sources of emission in solar magnetic loops

One possible mechanism is proposed for the appearance of high-temperature sources of emission in solar magnetic loops visible sometimes in the microwave and soft x-ray bands in active regions, even out of flares. The main concept of this mechanism is as follows. Due to the existence of current in the magnetic loop, there is a component of the electric field E which is longitudinal to the magnetic field. In this case, at some heights the parameters of solar atmospheric plasma (density and temperature) appear to cause a certain number of electrons to be in a runaway regime. These electrons can be accelerated to energies of about 1–2 keV. This corresponds to the temperature of the observed microwave and x-ray source. Accelerated electrons satisfying the trapping condition will gradually fill the magnetic tube, and after a while a hot x-ray loop can occur. Similar mechanisms of the appearance of hightemperature sources of emissions in active regions probably take place in the simple loop flare for the class of thermal flares visible in the microwave and soft x-ray bands.Department of Astrophysics and Cosmic Plasma Physics, Applied Physics Institute, Russian Academy of Sciences, Nizhnii Norgorod. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Radiofizika, Vol. 37, No. 7, pp. 836–850, July, 1994.     

Isolating the interface magnetocrystalline anisotropy contributions in magnetic multilayers

The interface magnetocrystalline anisotropy energy (MAE) in Fe/CeH(2) multilayers has been site and element-specifically isolated by combining soft x-ray resonant magnetic scattering (SXRMS) with soft x-ray standing waves. Using the different temperature evolutions of the Fe and Ce SXRMS contributions, following an in-plane to out-of-plane spin reorientation, the interface Fe 3d MAE and Ce 4f single-ion anisotropy have been separated. The results demonstrate that the transition metal interface MAE dominates the spin reorientation while the rare-earth contribution becomes significant only at much lower temperatures.     

Comparison of subset-based local and FE-based global digital image correlation: Theoretical error analysis and validation

Subset-based local and finite-element-based (FE-based) global digital image correlation (DIC) approaches are the two primary image matching algorithms widely used for full-field displacement mapping. Very recently, the performances of these different DIC approaches have been experimentally investigated using numerical and real-world experimental tests. The results have shown that in typical cases, where the subset (element) size is no less than a few pixels and the local deformation within a subset (element) can be well approximated by the adopted shape functions, the subset-based local DIC outperforms FE-based global DIC approaches because the former provides slightly smaller root-mean-square errors and offers much higher computation efficiency. Here we investigate the theoretical origin and lay a solid theoretical basis for the previous comparison. We assume that systematic errors due to imperfect intensity interpolation and undermatched shape functions are negligibly small, and perform a theoretical analysis of the random errors or standard deviation (SD) errors in the displacements measured by two local DIC approaches (i.e., a subset-based local DIC and an element-based local DIC) and two FE-based global DIC approaches (i.e., Q4-DIC and Q8-DIC). The equations that govern the random errors in the displacements measured by these local and global DIC approaches are theoretically derived. The correctness of the theoretically predicted SD errors is validated through numerical translation tests under various noise levels. We demonstrate that the SD errors induced by the Q4-element-based local DIC, the global Q4-DIC and the global Q8-DIC are 4, 1.8–2.2 and 1.2–1.6 times greater, respectively, than that associated with the subset-based local DIC, which is consistent with our conclusions from previous work.     

A novel speckle pattern—Adaptive digital image correlation approach with robust strain calculation

Digital image correlation (DIC) has seen widespread acceptance and usage as a non-contact method for the determination of full-field displacements and strains in experimental mechanics. The advances of imaging hardware in the last decades led to high resolution and speed cameras being more affordable than in the past making large amounts of data image available for typical DIC experimental scenarios. The work presented in this paper is aimed at maximizing both the accuracy and speed of DIC methods when employed with such images. A low-level framework for speckle image partitioning which replaces regularly shaped blocks with image-adaptive cells in the displacement calculation is introduced. The Newton-Raphson DIC method is modified to use the image pixels of the cells and to perform adaptive regularization to increase the spatial consistency of the displacements. Furthermore, a novel robust framework for strain calculation based also on the Newton-Raphson algorithm is introduced. The proposed methods are evaluated in five experimental scenarios, out of which four use numerically deformed images and one uses real experimental data. Results indicate that, as the desired strain density increases, significant computational gains can be obtained while maintaining or improving accuracy and rigid-body rotation sensitivity.