Michael S. Pierce

Disorder-induced microscopic magnetic memory

Using coherent x-ray speckle metrology, we have measured the influence of disorder on major loop return point memory (RPM) and complementary point memory (CPM) for a series of perpendicular anisotropy Co/Pt multilayer films. In the low disorder limit, the domain structures show no memory with field cycling--no RPM and no CPM. With increasing disorder, we observe the onset and the saturation of both the RPM and the CPM. These results provide the first direct ensemble-sensitive experimental study of the effects of varying disorder on microscopic magnetic memory and are compared against the predictions of existing theories.

Quasistatic X-Ray Speckle Metrology of Microscopic Magnetic Return-Point Memory

We have used coherent, resonant, x-ray magnetic speckle patterns to measure the statistical evolution of the microscopic magnetic domains in perpendicular magnetic films as a function of the applied magnetic field. Our work constitutes the first direct, ensemble-averaged study of microscopic magnetic return-point memory, and demonstrates the profound impact of interfacial roughness on this phenomenon. At low fields, the microscopic magnetic domains forget their past history with an exponential field dependence.

Disorder-induced magnetic memory: Experiments and theories

Beautiful theories of magnetic hysteresis based on random microscopic disorder have been developed over the past ten years. Our goal was to directly compare these theories with precise experiments. To do so, we first developed and then applied coherent x-ray speckle metrology to a series of thin multilayer perpendicular magnetic materials. To directly observe the effects of disorder, we deliberately introduced increasing degrees of disorder into our films. We used coherent x rays, produced at the Advanced Light Source at Lawrence Berkeley National Laboratory, to generate highly speckled magnetic scattering patterns. The apparently “random” arrangement of the speckles is due to the exact configuration of the magnetic domains in the sample. In effect, each speckle pattern acts as a unique fingerprint for the magnetic domain configuration. Small changes in the domain structure change the speckles, and comparison of the different speckle patterns provides a quantitative determination of how much the domain structure has changed. Our experiments quickly answered one longstanding question: How is the magnetic domain configuration at one point on the major hysteresis loop related to the configurations at the same point on the loop during subsequent cycles? This is called microscopic return-point memory RPM. We found that the RPM is partial and imperfect in the disordered samples, and completely absent when the disorder is below a threshold level. We also introduced and answered a second important question: How are the magnetic domains at one point on the major loop related to the domains at the complementary point, the inversion symmetric point on the loop, during the same and during subsequent cycles? This is called microscopic complementary-point memory CPM. We found that the CPM is also partial and imperfect in the disordered samples and completely absent when the disorder is not present. In addition, we found that the RPM is always a little larger than the CPM. We also studied the correlations between the domains within a single ascending or descending loop. This is called microscopic half-loop memory and enabled us to measure the degree of change in the domain structure due to changes in the applied field. No existing theory was capable of reproducing our experimental results. So we developed theoretical models that do fit our experiments. Our experimental and theoretical results set benchmarks for future work.

Surface X-Ray Speckles: Coherent Surface Diffraction from Au(001)

We present coherent speckled x-ray diffraction patterns obtained from a monolayer of surface atoms. We measured both the specular anti-Bragg reflection and the off-specular hexagonal reconstruction peak for the Au(001) surface reconstruction. We observed fluctuations of the speckle patterns even when the integrated intensity appears static. By autocorrelating the speckle patterns, we were able to identify two qualitatively different surface dynamic behaviors of the hex reconstruction depending on the sample temperature.

Persistent oscillations of x-ray speckles: Pt (001) step flow

We observed well-defined oscillations of speckle intensities from Pt (001) surfaces at high temperatures, persisting for tens of minutes. We used a model of hex-reconstructed terraces to show that the coherent x-rays reflected from the terraces retain their phases relative to the illumination boundary and the observed oscillations come from surface dynamics due to “step-flow” motion. Our results demonstrate a possibility that x-ray speckles can be applied to monitor the real-time evolution of surfaces.

Ptychographic x-ray imaging of surfaces on crystal truncation rod

Ptychography is a high-resolution imaging technique, which does not require lenses for image magnification and which provides phase contrast with high sensitivity. Here, we propose to use x-ray ptychography for the imaging of surface structure in crystalline samples. We show that ptychography can be used to image atomic step structures using coherent diffraction patterns recorded along the crystal truncation rod of a crystal surface. In a proof-of-concept experiment on a Pt (111) sample, we present ptychographic reconstructions showing features consistent with surface steps. Due to the penetration power of x-rays, this method could find interesting applications for the study of surface structures under buried interfaces or in harsh environments.

Layering and Ordering in Electrochemical Double Layers

Electrochemical double layers (EDL) form at electrified interfaces. Whereas the Gouy-Chapman model describes moderately charged EDL, the formation of Stern layers was predicted for highly charged EDL. Our results provide structural evidence for a Stern layer of cations at potentials close to hydrogen evolution in alkali fluoride and chloride electrolytes. Layering was observed by X-ray crystal truncation rods and atomic-scale recoil responses of Pt(111) surface layers. Ordering in the layer was confirmed by glancing-incidence in-plane diffraction measurements.

Epitaxial oxide bilayer on Pt (001) nanofacets

We observed an epitaxial, air-stable, partially registered (2 × 1) oxide bilayer on Pt (001) nanofacets [V. Komanicky, A. Menzel, K.-C. Chang, and H. You, J. Phys. Chem. 109, 23543 (2005)]. The bilayer is made of two half Pt layers; the top layer has four oxygen bonds and the second layer two. The positions and oxidation states of the Pt atoms are determined by analyzing crystal truncation rods and resonance scattering data. The positions of oxygen atoms are determined by density functional theory (DFT) calculations. Partial registry on the nanofacets and the absence of such registry on the extended Pt (001) surface prepared similarly are explained in DFT calculations by strain relief that can be accommodated only by nanoscale facets.

Direct determination of one-dimensional interphase structures using normalized crystal truncation rod analysis

A one-dimensional non-iterative direct method was employed for normalized crystal truncation rod. The non-iterative approach, utilizing Kramers-Kronig relation, avoids the ambiguities due to the improper initial model or the incomplete convergence in the conventional iterative methods. The validity and limitation of the present method are demonstrated through both numerical simulations and experiments with Pt (111) in 0.1 M CsF aqueous solution. The present method is compared to conventional iterative phase-retrieval methods.