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ALSNews           Vol. 115          November 11, 1998

Table of Contents


    1. X-RAY RESONANT SCATTERING FROM MAGNETIC MULTILAYERS
    2. INDEPENDENT INVESTIGATOR PROPOSALS DUE DECEMBER 1
    3. WHO'S IN TOWN: A SAMPLING OF ALS USERS
    4. OPERATIONS UPDATE

1. X-RAY RESONANT SCATTERING FROM MAGNETIC MULTILAYERS
By Art Robinson (Contact: M. Sacchi, sacchi@lure.u-psud.fr)

Magnetic multilayers are currently the focus of considerable activity because they have potential applications in high-density magnetic data-storage systems. Working at the ALS, an international team of researchers from France, Italy, Sweden, and the USA has used resonant scattering of circularly polarized soft x rays to measure the index of refraction of iron near iron absorption edges in iron/vanadium magnetic multilayers. The dispersive and absorptive components of the index of refraction measure the response of a material to excitation by light and are therefore needed to fully reconcile theoretical models with the results of experiments aimed at understanding and eventually controlling the behavior of magnetic multilayers.

In their x-ray resonant scattering experiments on Beamline 6.3.2, the researchers used a model system composed of alternating iron (ferromagnetic) and vanadium (normally nonmagnetic) layers, each about 15 angstroms thick. According to the Bragg equation for x-ray diffraction, the intensity of light reflected from the multilayers reaches a maximum (Bragg peak) at an angle determined by the thickness of an iron/vanadium layer pair (30.6 angstroms) and the wavelength of the light used. Resonant scattering occurs when the wavelength is near an absorption edge, resulting in an enhanced intensity of reflected light and a shift in the position of the Bragg peak relative to that expected from the Bragg equation.

Detailed measurements of the intensity of reflected light as a function of the angle and wavelength near the Bragg peak can give a direct measure of the dispersive part of the index of refraction for wavelengths around the absorption edges. Known as anomalous diffraction when applied to crystals, this technique has also been used for multilayers for many years. Absorption measurements not only can directly yield the absorptive component but can also indirectly reveal the dispersive component via a mathematical technique known as the Kramers-Kronig transformation. Unfortunately, this transformation requires approximations and the introduction of arbitrary parameters. With x-ray resonant scattering, however, both components can be determined without resorting to Kramers-Kronig analysis.

What the international group working at the ALS did was use circularly polarized light on a magnetized sample with the magnetization vector in the plane of the layers. Working at wavelengths around the absorption edges for 2p core electrons in the iron (the L3 and L2 edges), they made measurements with the magnetization either parallel or antiparallel to the helicity of the light. From this data, as well as absorption data obtained under the same conditions, they obtained both dispersive and absorptive components of the index of refraction for both orientations. Knowing the constants then allowed them to reconstruct all the components of the dielectric tensor, which quantifies the response of the material to electromagnetic fields. In particular, the diagonal elements give the response of the material to unpolarized radiation, while the off-diagonal elements are directly related to the magnetic properties of the sample.

Research conducted by M. Sacchi, C. F. Hague, and A. Mirone (LURE, Orsay); L. Pasquali (INFM, Modena); J.-M. Mariot (Université Pierre et Marie Curie, Paris); P. Isberg (Uppsala University); and E. M. Gullikson and J. H. Underwood (Berkeley Lab).

Funding: Centre National de la Recherche Scientifique (France), Istituto Nazionale di Fisica della Materia (Italy), and the U.S. Department of Energy, Office of Basic Energy Sciences.

Publication about this experiment: M. Sacchi et al., Phys. Rev. Lett. 81, 1521 (1998).

2. INDEPENDENT INVESTIGATOR PROPOSALS DUE DECEMBER 1

The deadline for independent investigator proposals for beamtime from April to September 1999 is fast approaching. Proposals from scientists who wish to conduct research as independent investigators during that time must reach the ALS by December 1, 1998. All proposals for beamtime submitted by that date will be considered for the next two proposal cycles. The number of eight-hour shifts requested in this proposal submission will automatically be considered for both the April to September 1999 and the October 1999 to March 2000 running periods. Proposals arriving after December 1 will only be considered for beamtime from October 1999 to March 2000. Scientists wishing to renew a previous proposal should notify the ALS User Administrator, Ruth Pepe (contact information below), before the December 1 deadline.

The proposal form for independent investigators is available in Portable Document Format (PDF) on the Web (http://www-als.lbl.gov/als/quickguide/independinvest.html). Information on the proposal process and a summary of the proposal deadlines for both general sciences and protein crystallography are available at the same location. Data sheets on beamlines at the ALS (available in Portable Document Format on the Web at http://www-als.lbl.gov/als/als_users_bl/datasheets.html) provide information that may be useful to prospective ALS users.

To request a proposal form by mail, contact:

Ruth Pepe, ALS User Administrator
Tel: (510) 486-5268
Fax: (510) 486-4773
Email: alsuser@lbl.gov

Gary Krebs, ALS User Services Group Leader
Tel: (510) 486-7727
Fax: (510) 486-4773
Email: g_krebs@lbl.gov

3. WHO'S IN TOWN: A SAMPLING OF ALS USERS

To highlight the richness of our user community and help introduce recent arrivals, we offer this listing of some of the experimenters who will be collecting data during the next two weeks at the ALS.

Beamline 1.4.3: Satish Myneni (Berkeley Lab) will study coordination chemistry of silicon in alkaline materials. Hoi-Ying Holman and Regine Goth-Goldstein (Berkeley Lab) will continue in-vitro studies of toxin uptake in human cells.

Beamline 7.0.1: Z. Q. Qiu (Univ. of California, Berkeley) will conduct photoemission studies of quantum confinement in thin metallic films. Satish Myneni (Berkeley Lab) will use spectromicroscopy for studies of bacteria and soils. Claus Pecher (Univ. of Wisconsin, Milwaukee) will conduct spectromicroscopy studies of rusts in environmental samples.

Beamline 7.3.3: Ernie Glover (Berkeley Lab) will be working on the development of a femtosecond photoelectron gate.

Beamline 10.3.2: Andy Smith (Daresbury Laboratory) and Mike Henderson (University of Manchester, UK) will investigate the spatial and chemical relationships of zinc and iron in growth-banded sphalerite. William F. Stringfellow and Mark Conrad (Berkeley Lab) will investigate the speciation of various phases of copper and other metals taken up from basalt rock by lichens. Art Chester (Mobil Corporation) and Geraldine Lamble (Berkeley Lab) will look at a vanadium-containing catalyst and its distribution and chemistry with other trace elements within a largely aluminosilicate matrix.

4. OPERATIONS UPDATE
(Contact: rmmiller@lbl.gov)

Beam reliability for the last two weeks was 97% overall and 97.9% for user shifts. All outages were of short duration.

Long-term and weekly operations schedules are available on the Web (http://www-als.lbl.gov/als/accelinfo.html). Weekly operations scheduling meetings are held on Fridays at 3:30 p.m. in the Building 6 conference room. The Accelerator Status Hotline at (510) 486-6766 (ext. 6766 from Lab phones) features a recorded message giving up-to-date information on the operational status of the accelerator.