X-ray absorption spectroscopy (XAS) utilizes the energy dependent absorption of x-rays to obtain information about the elemental composition of the sample and the chemical environment of each element (see also NEXAFS Spectroscopy). Typically XAS is measured by detecting the intensity of an x-ray beam before and after it is transmitted through a sample. This approach provides information about bulk properties. However, one can also achieve excellent sensitivity to interfaces, buried layers and diluted impurities or dopants by detecting electrons or photons that are emitted from the sample due to secondary processes instead. Finally, one can employ the polarization dependence of the x-ray absorption process (dichroism) to obtain information about low symmetry charge distribution or long range magnetic order. For example ferromagnetic materials like Cobalt will exhibit X-Ray Magnetic Circular Dichroism (XMCD) while antiferromagnetic materials like Nickeloxid will show X-Ray Magnetic Linear Dichroism (XMLD). By using microscopes that are capable of imaging the spatial variation of the X-ray absorption one can finally obtain images with chemical, elemental or magnetic contrast as shown in the figure on the left.
SPEAR3 is a third generation synchrotron and therefore an excellent source of highly brilliant soft x-rays beams produced by state of the art insertion devices in the straight sections of the synchrotron. Since it is important for most soft x-ray dichroism experiments to have full control over the incident polarization of the incoming x-rays it is necessary to use so called elliptical polarizing undulators as a source. These devices also provide coherent x-rays, which makes it also possible to also use holography like imaging techniques like Speckle.
Two Soft X-ray Magnetic Dichroism Spectroscopy beamlines are currently operational at the SSRL. A third beamline is currently under development and will be available in late 2007.