Computer hard drives and other advanced electronic devices depend on layered stacks of magnetic and non-magnetic materials, but researchers don't fully understand why such layered materials exhibit new properties that cannot be predicted from the properties of the individual layers. In a recent publication a team working at SSRL and the ALS describes new methods, based on x-ray spectroscopy and x-ray microscopy, that reveal the magnetic structures at the boundaries between these layers. Their data show that the boundaries are not as clean as previously assumed but a new ultrathin interface layer may be formed by a chemical reaction. The thickness of the interfacial layer is found to change with temperature and this change can be directly correlated with the magnetic properties of the multilayer stack. The work provides the first magnetic images of a buried interface and gives direct experimental evidence for the existence and long-assumed importance of interfacial magnetic spins.
The strong electron correlations in transition metal oxides give rise to such phenomena as high-temperature superconductivity in layered cuprates and to stripe-like order in layered cuprates and nickelates. In the case of the manganites, an additional strong electron-lattice interaction leads to a very rich phase diagram in which structural, magnetic, and transport properties are intimately related. Colossal magnetoresistance (CMR) has been observed in the perovskite and double-layer manganites, but not in the single-layer system La1-xSr1+xMnO4 (Mn214).
Extensive research efforts to study the novel electronic properties of high-Tc superconductors and their related materials by angle-resolved photoemission spectroscopy at a recently commissioned Beam Line 5-4 (led by Z.-X. Shen) continue to be successful, producing many important results. These results, which are highlighted by five articles recently published in Physical Review Letters and one in Science, brought our understanding steps closer to solving the mystery of the high-Tc superconductivity.
Today's laptop computers utilize flat panel displays where the light transmission from the back to the front of the display is modulated by orientation changes in liquid crystal (LC) molecules. Details are discussed in Ref. 2 below. One of the key steps in the manufacture of the displays is the alignment of the LC molecules in the display. Today this is done by mechanical rubbing of two polymer surfaces and then sandwiching the LC between two such surfaces with orthogonal rubbing directions. Over the past years a great challenge of this $20 billion/year industry has been to devise an alternative method of liquid crystal alignment. The rubbing process is plagued with contamination issues and the polymer film is deposited by a wet process that is incompatible with high-tech manufacturing techniques.
Beam line 14-3 is under development and is expected to begin commissioning in the spring of 2010. The microprobe beam line is a bending magnet side station and will use K-B optics with a virtual source to focus the x-ray beam to an estimated size of 2 x 2 microns. BL 14-3 will specialize in the energy range around the sulfur K-edge and have a He purged sample environment. In addition to fluorescence mapping of lower-Z elements, spectroscopy and spectroscopic imaging will also be able to be performed over an ideal energy range of 2 to 6 keV.
The soft x-ray (150 - 1200 eV) coherent scattering beamline is capable of X-ray Fourier Transform Holography (FTH) and resonant Coherent Diffraction Imaging (CDI) with a flux of 2 x 107 photons s-1um-2 at the shortest transverse coherence of 3 um in a spot size of 25 x 250 um on the sample. The distance between the sample and the in-vacuum backside-illuminated CCD detector can be adjusted from 50 to 400 mm. The detector consists of 1340 x 1300 pixels of 20 um. Nanoscale imaging at cryogenic temperatures (15 - 300 K) and the application of magnetic fields up to 0.1 T are also available.
BL13-2 has stations designed for surface and solid state experiments. The SSE station has an electron spectrometer (SES-R3000, VG-Scienta) for photoemission spectroscopy and Auger electron yield X-ray absorption spectroscopy, and a Ni coated elliptical grating spectrometer for C 1s, N 1s and O 1s x-ray emission spectroscopy. A horizontally mounted manipulator is provided for experiments with a minimum sample temperature of about 40 K. The manipulator transfers samples between the preparation chamber and the main chamber.
In a scanning transmission x-ray microscope (STXM), spatial resolution is obtained by using zone plate lenses which focus the x-ray beam down to a spot size of 10s of nm. The sample is then scanned perpendicular to the optical axis, while the intensity of the transmitted x-rays is detected at the same time. Such a microscope is currently commissioned at SSRL BL13-1. It will initially provide a spatial resolution of about 35nm, while operating under UHV condition, at temperatures between 25K and 450K, as well as in applied magnetic fields.
BL11-3 is used for diffraction experiments using a large area detector. It is used for fast data collection when angular (Q space) resolution is not necessary. It is used for rapid screening of samples, study of texture and strain, and preliminary characterization of thin films and multi-layers. Also because of fast data collection rate, it is often used for study of kinetics of chemical reactions. It can accommodate several different types of sample stages and has a built-in motorized xyz state. The main user communities are materials science, environmental science and archaeometry.
The imaging station at BL 10-2 is used to perform rapid imaging on larger samples with larger beam sizes. The sample positioning stage has a total travel limit of 600 mm horizontally and 300 mm vertically. The beam size can be determined by pinhole apertures (50 to 250 microns) or glass capillary (~10 microns). Future upgrades may allow for the installation of a K-B mirror pair to attain beam sizes of ~2-5 microns. BL 10-2 uses a wiggler for the x-ray source and has x-ray fluxes approximately 10-50 times greater than BL 2-3.
