Over the past two years, we have made dramatic progress in the scientific program of the CAT. At our science review in November 2000 we reported no publications, three papers submitted for publication and six talks involving work done in the MUCAT sector. As of November 2002, we count seventeen articles in print, one more in press, and six student theses based, in part, on work done in the MUCAT Sector. Furthermore, there are currently fourteen papers submitted for publication in various stages of the review process. Finally, we count over 60 invited and contributed talks at institutes, universities and national and international conferences.
The scattering cross-sections for resonant and non-resonant magnetic x-ray scattering provide fertile ground for investigations of magnetism in solids that are just now being sown. The development of x-ray scattering methods for solving magnetic structure provides an important alternative to traditional neutron diffraction measurements. The work in our sector over the past few years, in exploiting resonant and non-resonant magnetic x-ray scattering for magnetic structure determination, has contributed very significantly to this goal. Indeed, we now routinely solve magnetic structures via x-ray measurements in our sector.
Several features of resonant and non-resonant magnetic scattering make the x-ray technique a particularly attractive complement to traditional neutron diffraction methods for magnetic structure determinations. For example, several of the technologically important magnetic rare-earth elements (e.g. Sm, Eu and Gd) present very large neutron absorption cross-sections making neutron diffraction measurements difficult or impossible without expensive isotopic substitutions. Second, the high momentum resolution available at the APS has allowed us to study several interesting magnetic systems more closely and provide more detail concerning the nature of magnetic order in these systems. Finally, the brilliance of the source has enabled routine measurements of sub-millimeter sized single crystal samples, allowing us to perform single crystal measurements on interesting systems that, previously, could only be investigated by neutron powder measurements.
Our group has shown that by exploiting the angular dependence of the cross- sections for resonant and non-resonant scattering, a complete determination of the spatial components of the ordered moment can be accomplished. The importance of measuring the dipole and quadrupole resonances, along with the non-resonant scattering, lies in the fact that for a given incident polarization of the x-ray beam, they are sensitive to different spatial components of the ordered moment. In magnetic neutron diffraction, for example, one measures only the component of the ordered moment that is perpendicular to the scattering vector, Q. In contrast, for dipole allowed transitions, resonant magnetic scattering probes the component of the ordered moment in the scattering plane (determined by k and k'), while for quadrupole transitions, all three components of the magnetic moment contribute to the scattered intensity. The non-resonant magnetic cross-section is primarily sensitive to the component of the ordered moment perpendicular to the scattering plane. The utility of these new methods for magnetic structure determination has been demonstrated by measurements on several interesting antiferromagnetic systems:
In TbNi2B2C, a member of the RNi2B2C family that includes magnetic superconductors, previous neutron measurements did not provide sufficient resolution to detect a tetragonal-to-orthorhombic distortion that occurs at the antiferromagnetic transition. Using resonant magnetic scattering, we were able to unambiguously determine the direction of the modulation in the basal plane of the orthorhombic structure and demonstrate, through an analysis of the order parameters for the structural distortion and sublattice magnetization, that the distortion arose from magnetoelastic coupling. In another experiment using X-ray circular magnetic dichroism, we confirmed that the onset of a ferromagnetic component below 5K in this system arose from polarization of the Tb ions.
Using resonant and non-resonant magnetic scattering, we were able to demonstrate that an anomaly in the magnetic susceptibility of TbCu2Ge2 below the antiferromagnetic transition arose from a reorientation of the magnetic moment within the basal plane of the tetragonal structure. The sublattice magnetization, as a function of temperature, was measured using resonant dipole and non-resonant magnetic scattering. These order parameters were simultaneously fit using a model that accounted for the rotation angle of the moment in the plane.
The magnetic structure of GdAgSb2 was solved using resonant dipole and quadrupole scattering. The series of compounds, RAgSb2, exhibit a wide variety of low-temperature ground states ranging from Kondo-lattice behavior with ground state ferromagnetism in CeAgSb2 and exotic metamagnetic behavior in low applied fields in DyAgSb2, to charge density wave ordering in LaAgSb2. These compounds fall within a general category of systems where CEF effects have significant impact upon the nature of the magnetic structure. While virtually the entire range of rare-earth constituents were studied by neutron scattering methods, GdAgSb2 had not been investigated until our measurement (this is actually a fairly typical situation due to the neutron opacity of naturally occurring Gd). In addition to determining the antiferromagnetic ordering wave vector, quadrupole resonant scattering was used to determine the direction of the magnetic moment.
A novel furnace for high-energy/high-temperature powder diffraction has been developed by members of MUCAT for the investigation of materials and materials processing at high temperature. The furnace design was developed to closely emulate tube furnaces found in materials laboratories used to synthesize materials and measure material properties. This design minimizes any thermal gradients across the sample volume. The use of high energy x-rays allows the sample bulk to be probed and allows high Q regions of the diffraction pattern to be measured. Data can be taken in several modes, using several detectors including point detectors and an analyzer for high-resolution diffraction measurements and area detectors for time resolved experiments on processing kinetics and the phase evolution. We are currently able to take full data sets in a time-resolved mode of between 1 to 5s. One of the key features of the furnace is the provision for sample rotation while maintaining a controlled atmosphere through a magnetic coupling with an external drive motor. Sample spinning is essential for texture-free diffraction peaks that can then be used in Rietveld refinements of the structure. The furnace itself has been highlighted in several publications including the March 12, 2001 edition of DOE's The Pulse. Some of the most notable experiments using this furnace over the past two years include:
Time-resolved HTXRD has provided a unique insight into the phase selection process in nominally amorphous melt-spun Nd-Fe-B alloys. The role of alloy additions of Ti/C to control formation of properitectic alpha-Fe was demonstrated. In addition, work on extracting precise values for the Fe-Fe distances during the magnetostriction is nearly complete. While there is a wide range in 2-14-1:alpha-Fe in the as quenched alloys, the HTXRD data provides direct evidence that the Ti/C alloying additions to Nd-Fe-B alters the rapid solidification pathway and suppress the formation of properitectic alpha-Fe in favor of 2-14-1. This conclusion is supported by a comparison between the small proportion of alpha-Fe that forms upon quenching and the relatively large increase in the alpha-Fe proportion that forms above the crystallization temperature. This increase in alpha-Fe is transient since after 30 min. at 1123 K the phase fraction of alpha-Fe is reduced to 2.5 wt. %. These data indicate that nucleation and growth of the small proportion of quenched-in clusters of Fe compete with the 2-14-1 phase in both the quenched glass and during solid state devitrification.
We have demonstrated that we can observe the structural relaxation in glass alloys. Coupling this information to conventional thermal analysis, we have show that the devitrification pathways in a Zr-Pd alloy can be radically altered by changing the short-range order (SRO). Since novel nanostructures may be formed by devitrification of glassy alloys, understanding these transformation pathways is vital to understanding and controlling these processes. The reason for the obvious differences in the devitrification pathways is due to changes in the short-range order (SRO) observed in the atomic pair distribution function. The melt-spun alloy retains the 'icosahedral' like clusters typically observed in liquids. The mechanically alloyed (or milled) samples appear amorphous by every measure but have a SRO which is closer in its pair-correlations to the stable high temperature phase (Zr2Ni). The implications are that processing induced changes in the SRO can radically alter the nanostructure phases formed by devitrification of glasses.
Key tools in materials science for the investigation of high temperature processes include Differential Scanning Calorimetry (DSC) and Differential Thermal Analysis (DTA), where the sample is heated at a uniform rate (typically 10 K/minute) and the heat flow is monitored for endothermic or exothermic signatures. Thermal analysis only provides reaction temperatures but no insight into the actual phase selection process. Using the HTXRD furnace in time resolved mode, the structure of these products can be directly measured and compared to DTA and DSC scans. For example, time-resolved devitrification studies of a Ti-Cu-Zr-Ni-Si metallic glass were performed during heating at a rate of 40 K/min between 623 and 1073 K. The crystallization behavior could be directly compared with DSC measurements and the structural components of the four-phase microstructure at 1073 K were all identified. These metastable high temperature phases differed from those observed using more conventional heat and quench methods.
The X-ray liquid-surface diffractometer on the 6-ID beam line enables in situ structural investigations of free liquid-gas interfaces. The diffractometer offers unique research opportunities for the investigation of monomolecular layers and the ability to monitor and characterize interfacial phenomena at biomimetic membranes. The structural determination of interfaces between two media, membrane-solution or imiscible liquid-liquid, for instance, is gaining considerable attention in a range of areas such as friction and lubrication, phase transfer catalysis, solvent extraction of metals and as model systems for biological membranes.
The apparatus is currently used to conduct reflectivity and grazing incidence X-ray diffraction (GIXD) experiments to determine the structure of monomolecular films at gas-liquid interfaces. From reflectivity data, the electron density across the interface can be determined and related to the molecular arrangements at the interface. One can also use the same technique to monitor the adsorption of molecules to functionalized interfaces. Although reflectivity measurements can be performed using laboratory-based x-ray generators, the quality of the data (error-bars) and the extended Qz range accessible at the APS results in data sets that are far superior. Furthermore, the high rate of data collection possible at the APS enables studies of kinetics and metastable systems. The determination of in-plane ordering of monolayers using the GIXD technique can be realistically achieved only at a synchrotron source. Typical slit sizes before the sample are 100 microns (and approx. 1mm wide) passing about 10¹¹ photon/sec to illuminate the surface. Some highlights of our work over the past few years include:
Investigations of Dendrimers at Air/Water Interfaces. Dendrimers have stimulated great interest in recent years because of their ability to organize into supramolecular structures of various shapes and sizes. The structure of monodendrons with appropriate amphiphilic balance between hydrophilic cores and hydrophobic shells allows for the organization of the molecules at interfaces and surfaces in the form of self-assembled or Langmuir monolayers. By varying the chemical architecture of the branches and peripheral tails the interfacial behavior and microstructure can be dramatically altered. The stability of the dendrimer monolayers and their internal ordering are directly related to the molecular dimensions of the hydrophobic and hydrophilic parts of the molecules. GIXD and reflectivity have been used on Langmuir monolayers of low generation monodendrons containing a crown-ether polar group, azobenzene spacer, and varying number of peripheral alkyl chains of 1, 2, 4 and 8. We observe that the cross-sectional mismatch between the bulky polar head and the alkyl tails has profound effect on the local ordering of the alkyl tails. It is found that the alkyl chains in a single-tail molecule are significantly tilted away from the surface normal. The tilt is eliminated in molecules with two or more alkyl chains where the cross-sectional mismatch is in favor of the peripheral tails. The molecule with one tail possesses a supercell orthorhombic packing caused by structural non-equivalency on the neighboring tails.
The specific determination of adsorbed ions by resonance reflectivity. Ions at cell membrane surfaces impact the function and conformation of nearby molecules and play an important role in inter- and intra-cellular transport and recognition processes. We have use resonant x-ray reflectivity measurements to provide detailed information about the position of ions that adsorb at bio-membranes. As a first model system, Ba2+ ions in solution interacting with DMPA (1,2-dimystroyl-sn-glycero-3-phosphate) monolayers at the air-water interface were studied. From three sets of reflectivities at, and removed from, the Ba LIII absorption edge (E = 5247 eV), the effective electron density profile of the surface film at each energy was determined. From their differences, the contribution of the adsorbed ions could be extracted.
Surface properties of Sphingolipids in relation to their function. Sphingolipids are long chain amphiphilic molecules that are found in animal cell membranes. In addition to constituting cellular membranes of eukaryotic cells, sphingolipids play an important role in the life cycle of cells, however very little is know about their interfacial properties. Synchrotron x-ray studies, taken together with surface pressure versus molecular area isotherms of C18- and C20-sphingosines spread at air/water interfaces, reveal unique surface interfacial properties with considerable differences between the two single-hydrocarbon chain amino-alcohols. The C20-sphingosine forms a crystalline monolayer with structural characteristics that are dominated by hydrogen bonding in the head group (common to its sphingolipid derivatives). In contrast, its natural counterpart, the C18-sphingosine, forms a disordered liquid-like metastable monolayer and has to be spread in excess with a floating reservoir on the water surface to compensate for the high dissolution rate of molecules into the water sub-phase. The marginal affinity of the C18-sphingosine for residing at the interface, the micro-crystallization at very low densities, the corrugated monolayers it forms and the strong interaction with the water surface help to explain some of the functions of sphingosine and its derivatives in the life cycle of eukaryotic cells.
Investigations of self assembled Magnetic-molecules at water/gas interfaces. Controlling the arrangements of functional molecules on solid substrates is a crucial step in turning the system (substrate-molecule) into a device, such as sensor, memory, detector, etc. There are several routes to achieving such control depending on the molecules, their sensitivities to the conditions under which the controlling technique is applied, on aggregation properties, etc. One route to bringing organic molecules one layer at a time to a solid substrate, (with some control of the in-plane arrangement) is the Langmuir-Blodgett technique, but a prerequisite is that the molecule under consideration must form a homogeneous monolayer at the gas-water interface, a Langmuir monolayer. We have investigated the feasibility and the quality of a Langmuir monolayer formed by a typical magnetic molecule (Cr8 see below), and subsequently transferred the monolayer to a solid surface. Magnetic molecules are organometallic based, consisting of 3d and/or 4f magnetic ions coupled by electronic superexchange interactions, exhibiting interesting magnetic properties (magnetic quantum tunneling for instance), that might be exploited in future devices. X-ray Reflectivity (XR) and grazing incidence X-ray diffraction (GIXD) measurements were conducted to determine the structure of Cr8O4(O2CPh)16 Langmuir films at the air/water interface at various surface pressures. The molecular area versus pressure isotherm, reflectivity, and GIXD variation (width and peak position) with compression, all imply the formation of a homogeneous monolayer at very low surface pressures. The film is disordered with a liquid-like or glassy structure factor at all pressures, with correlation lengths that extend over a few molecular distances. The liquid-like state of the monolayer and an incomplete formation of a second layer suggest difficulty in forming high quality Langmuir-Blodgett films of this system.
Citations to publications resulting from work in the MUCAT sector may be found by directing your browser to: The APS Publication Database. Select "Search Database", then select "MU" from the "CAT where work was done" pop-up menu. Press the "Search The System" button to retrieve citations for MUCAT publications.
Use of the Advanced Photon Source is supported by the U.S. Department of Energy, Basic Energy Sciences, Office of Science, under Contract No. W-31-109-Eng-38.
The MUCAT sector at the APS is supported by the Department of Energy, Office of Science through the Ames Laboratory Contract No. W-7405-Eng-82.