ALSNews is a biweekly electronic newsletter to keep users and other interested parties informed about developments at the Advanced Light Source, a national user facility located at Lawrence Berkeley National Laboratory, University of California. To be placed on the mailing list, send your name and complete internet address to ALSNews@lbl.gov. We welcome suggestions for topics and content.

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ALSNews           Vol. 63           October 16, 1996

Table of Contents


    1. OPERATIONS UPDATE
    2. MAGNETIC PHASE TRANSITIONS PROBED WITH SPIN-POLARIZED PHOTOELECTRON DIFFRACTION
    3. THANK YOU!
    4. NEW USER ADVISORY ISSUED ON THE USE OF TOXIC AND CORROSIVE GASES AT ALS

1. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

Beam availability for the last two weeks was 85.2% overall and 85.4% for user shifts. Causes of lost beamtime included continuing problems with the longitudinal and transverse feedback systems, storage-ring rf heater power-supply interlock trips, and failure of a booster-to-storage-ring bend magnet power supply.

Operations Summary for October 16 - November 4

Oct 16, 00:00-08:00           User scrubbing & special operations
                              (1.1-GeV/400-mA/320 bunch requested)
Oct 16, 08:00- Oct 21, 07:15  1.9-GeV/260-mA/320-bunch user operations
Oct 21, 07:30- Oct 22, 24:00  Maintenance & startup
Oct 23, 00:00-08:00           User scrubbing & special operations
Oct 23, 08:00- Oct 28, 07:15  1.9-GeV/260-mA/320-bunch user operations
Oct 28, 07:30-24:00           Maintenance & startup
Oct 29, 00:00-24:00           Accelerator physics
Oct 30, 00:00-08:00           User scrubbing & special operations
Oct 30, 08:00- Nov 04, 07:15  1.9-GeV/260-mA/320-bunch user operations

2. MAGNETIC PHASE TRANSITIONS PROBED WITH SPIN-POLARIZED
PHOTOELECTRON DIFFRACTION
(contact: tober@akamai.lbl.gov)

A group of researchers here has found a new way to probe changes in the short-range magnetic order near the surfaces of ferromagnetic materials: spin-polarized photoelectron diffraction (SPPD). Their recent work with gadolinium (Gd) provides proof of SPPD's usability for studying magnetic phase transitions in ferromagnetic materials. Getting a clearer picture of surface magnetic behavior has important implications in the study of materials for the computer and data storage industry, where the decreasing size of magnetic components increases the proportion of each component's atoms that are at surfaces or buried interfaces.

Results obtained with the advanced photoelectron spectrometer/diffractometer on Beamline 9.3.2 show a 60- to 80-degree difference in the temperatures at which bulk and surface Gd atoms in a thin layer lose their long-range magnetic order (i.e., the bulk and surface Curie temperatures are 60 to 80 degrees K apart). In these magnetic phase transitions, ferromagnetic materials become paramagnetic. The researchers studied the transitions by observing magnetic-order-dependent differences in the scattering behavior of Gd photoelectrons with opposite spins. This work took advantage of the ALS's high brightness to resolve the two spin states. The materials scientists studied photoelectrons emitted from the 4s and 5s levels of Gd(0001) grown as a thin layer on tungsten(110). Their work confirms measurements by other researchers suggesting the unusual result of a higher Curie temperature for the surface atoms. This verification enhances materials scientists' understanding of how changes in temperature affect the magnetic properties of surfaces. The work was carried out by scientists from the University of California, Davis; Instituto de Ciencia de Materiales de Madrid (ICMM-CSIC); and Berkeley Lab.

The SPPD approach makes use of the fact that, in transition metals, core electrons of opposite spin are released from their atoms with slightly different energies, a phenomenon known as multiplet splitting. With high-resolution photoelectron diffraction, therefore, one can measure separate photoelectron intensities for each of the two spin states. In addition, the two types of photoelectrons scatter differently from neighbor atoms in the sample, depending on the magnetic orientation (or net spin direction) of the neighbor atoms: photoelectrons with the same spin direction as a neighbor atom are more strongly scattered than those with opposite spin. Thus, the ratio of the two intensities changes as the magnetic order of the sample changes. By measuring the intensity ratios across a range of temperatures, the scientists were able to determine, in a surface-sensitive way, the temperatures at which magnetic order in the sample changed. Intensity measurements for emission perpendicular to the sample surface showed one change at the known Curie temperature for bulk Gd and another change, corresponding to a separate phase transition for surface atoms, at 60 to 80 degrees K higher temperature.

Experiment performed by C.S. Fadley (principal investigator), E.D. Tober, R.X. Ynzunza, and Z. Wang (University of California Davis and Berkeley Lab); F.J. Palomares (Instituto de Ciencia de Materiales de Madrid); and Z. Hussain (Berkeley Lab) using the advanced photoelectron spectrometer/diffractometer at Beamline 9.3.2.
Funding: U.S. Office of Naval Research and Office of Basic Energy Sciences of the U.S. Department of Energy.

Publications about this experiment:
C.S. Fadley et al., Progress in Surface Science, 54, 341-386 (1997).

3. THANK YOU!
(contact: alsnews@lbl.gov)

We, the editors, thank all of you who took the time to complete the ALSNews reader survey in the September 18 issue. 93% of the responders rated the technical level as just about right and 85% confessed to reading every issue - of course this makes us feel good about our level of customer satisfaction. We note a significant increase since our last survey on the number of readers using the ALS Web pages, which isn't surprising given the increase in internet use during this time period. The feedback we received indicates there are many of you that would like more specific information on the Web about the machine itself and the beamlines, and we will implement this suggestion within the next several months. Of course, we invite your input about ALSNews at any time (email: alsnews@lbl.gov) if you see something you like or if you have suggestions for improvement.

4. NEW USER ADVISORY ISSUED ON THE USE OF TOXIC AND CORROSIVE GASES AT ALS
ENDSTATIONS (contact: alsuser@lbl.gov)

The ALS has recently issued a new User Advisory "The Use of Toxic and Corrosive Gases at ALS Endstations (ALS Advisory 15)." The advisory provides essential information for users intending to use toxic or corrosive gases in experiments, and includes requirements for the safe transportation, storage, and use of these gases at the ALS. Also included are safety guidelines relating to the experimental set-up when gases are in use, such as the use of pressure-reducing regulators, ventilated cabinets, and appropriate tubing to and from the experiment chamber.

Other advisories recently updated include Affixing Beamline and Endstation Components to the ALS Floor (#1), Laser Safety Policies (#6), Vacuum Policy for User Endstations (#9), and ALS Chemistry Laboratory (#11). All advisories are available from the ALS User Office [Tel: (510) 486-7745; Fax: (510) 486-4773, Email: alsuser@lbl.gov], and are posted on the World Wide Web [www-als.lbl.gov/als/user-advis/user_advis_index.html].