Session H6: Coherent Scattering and Imaging

Using Bragg Coherent Diffraction Imaging to See Strain in a Tensile Loaded Copper Film

Coherent Diffraction Imaging (CDI) is a novel imaging technique using coherent light sources and iterative phase retrieval (IPR) algorithms instead of lenses to form high resolution images. Bragg coherent diffraction imaging (BCDI) is a variation of CDI that measures coherent diffraction near a Bragg peak of a crystalline sample. Since the Bragg peak contains information about lattice strain, the IPR retrieves nanometer scale images of crystalline strain. We present three dimensional BCDI reconstructions of the strain in a single grain in polycrystalline copper thin films under tensile loading measured at sector 34 of the Advanced Photon Source.   

Coherent Diffractive Imaging in the Near Field at Large Angles

—Coherent diffraction imaging (CDI) is a rapidly developing form of lensless imaging where the intensity of the diffraction pattern is directly imaged on a CCD and iterative phase retrieval (IPR) algorithms are used to reconstruct a high resolution image of the sample. This is especially useful at x-ray wavelengths, where lenses are inefficient and difficult to manufacture. However, one challenge with CDI is that the exact relationship between light emerging from the sample and arriving at the detector is nearly impossible to determine, even numerically, but becomes tractable with various assumptions. The standard far-field assumptions require the detector to be placed hundreds or thousands of meters from the sample at hard x-ray energies, which is not practical. A different set of assumptions called the “distorted object” approach allows imaging at any distance, but has the strict requirement that a value called the small angle number, An, needs to be much smaller than one. Here we examine where the distorted object approach fails, specifically in regards to An. We have found that we can obtain good quality images with very large An values at visible wavelengths.   

X-Ray Diffraction and Partial Distribution Function Analyses on Barium Hexaferrite

There is a growing interest in investigating the structural properties of Barium Hexaferrite (BaFe12O19) because it exhibits both ferroelectric and ferromagnetic phenomena. These properties are expected to aid the performance and efficiency of electronic data storage devices. The mechanism of both ferroelectric and ferromagnetic phenomena would be better understood, and perhaps enhanced, if the structure could be deciphered. The X-ray diffraction pattern was used to investigate the average structure of the material. In addition, pair distribution analysis using neutron scattering as the probe was conducted to investigate the short, medium, and long range order of Barium Hexaferrite’s atomic structure. The neutron scattering data was analyzed by fitting the experimental data to idealized models. The X-ray diffraction pattern indicated the presence of crystalline phase separation. By combining XRD and PDF analyses we were able to provide an insight into the evolution of BaFe12O19 at different ambient temperatures (300K, 315K). Future studies include The use of coherent X-ray diffraction imaging to understand the functionality of individual nanoparticles and how it correlates to the global structure.   

Analysis of Topological Defects in Multiferroic.

Topological defects (TDs), which include domain walls, vortices and skyrmions, are actively being studied. These TDs are of fundamental importance in condensed matter and particle physics with technological implications in the design of future devices for data storage and information. Many theoretical models based on phase-field computation have been used to predict the presence of such TDs in multiferroics (MF). However, the experimental observation of TDs in nanoparticles requires probes capable of probing the volume as a whole of such nanostructures. Here, we use Coherent X-ray diffraction Imaging (CXDI) to map the 3D distribution of strain, TD density and their interaction in a highly inhomogeneous MF nanoparticle exhibiting room temperature magneto-electric coupling; Our choice of CXDI over other scanning optical and electron microscopic approach will be addressed   

Iterative Phase Retrieval Coherent Diffraction Imaging Algorithm

The rapid growth of nanoscience and nanoscale materials requires non-destructive probes capable of mapping the local structure (shape, chemical homogeneity and density modulations) in three dimensions of materials functional properties such as strain and electron density distribution. The Bragg Coherent Diffraction Imaging (BCDI) method has been developed for nondestructive imaging of 3D strain evolution within small crystals. This has widespread applications for medicine, material science and condensed matter systems. The inversion of diffraction data is a critical step that uses a computer algorithm that takes advantage of internal redundancies when the measurement points are spaced close enough together to meet the ``oversampling'' requirement. The first step is to postulate a 3D ``support'' volume in which all the sample density will be constrained to exist. Arguably, the bestmethod so far for phase. Here we demonstrate the development of a BCDI inversion algorithm that combines HIO, ER and phase constraint HIO to retrieve the phases and amplitudes from measured diffraction patterns. We demonstrate the applicability of the technique to imaging polar distortions in complex BaFe12O19 nanoparticles showing room temperature magneto-electric coupling.   

Imaging Magnetic Domain Morphologies in Co/Pt Multilayer with 4Å of Cobalt

We have studied the magnetic domains that form perpendicular to the film of our multilayer Co/Pt thin film with 4Å of Cobalt. When an external magnetic field is applied, the domains create morphologies that vary based on the strength of the magnetic field. These morphologies vary from maze-like to bubble patterns. We used a Vibrating Sample Magnetometer (VSM) to apply the fields to our 4Å Co/Pt thin film. A Magnetic Force Microscope (MFM) was also used to obtain images of the magnetic domains of the sample in the absence of the field. We were able to find the field at which greatest number of domains was achieved by analyzing these MFM images. This research can be applied to data storage.   

Topological defects in the solid state

Yttrium Manganite (YMnO3) exhibits fascinating one-dimensional features in which ferroelectric and antiferromagnetic domains come together to form a topologically stable defect structure. We're exploring the underlying conditions that make features like this possible in solid-state materials.   

Holographic imaging of nano-scale objects using soft x-ray laser light at 13.9nm and 18.9nm.

The short wavelength of soft x-ray light enables extending holographic techniques to the nano-scale. ~Holographic imaging requires coherent beams. We describe the generation of highly coherent x-ray beam using an x-ray laser with a separate seed and amplifier stage. ~The 13.9nm or 18.9nm coherent x-ray beams produced by this laser were used to image nanowires using Fourier holography. Part of the soft x-ray laser beam was focused with a Fresnel zone plate into a 1-2 um hole to generated spherical reference wave. The open middle~zone of the zone plate lets light pass through to illuminate the object. The interferogram resulting from the interference of the two beams is then inverse Fourier transformed to produce an image of the objective. Holograms of 50-70 nm diameter nanowires were obtained.