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. 36           September 19, 1995

Table of Contents


    1. OPERATIONS UPDATE
    2. INVESTIGATIONS OF MAGNETIC BEHAVIOR IN THIN FILMS
    3. REGISTRATION FOR ALS USERS' ASSOCIATION ANNUAL MEETING
    4. LONG-TERM OPERATIONS SCHEDULE

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

The ALS is in a scheduled shutdown for major installations and maintenance (a list of major shutdown activities appears in ALSNews Vol. 35, September 5, 1995). User operations are scheduled to resume on November 1. A tentative long-term schedule appears in item #4 below.

2. INVESTIGATIONS OF MAGNETIC BEHAVIOR IN THIN FILMS
(contact: tobin1@llnl.gov)

Scientists at the ALS recently examined the behavior of magnetic films only a few atomic layers thick -- research with possible implications for future generations of magnetoresistive computer components. The researchers used magnetic x-ray linear dichroism (MXLD).

In recent years, there has been a shift from inductive to magnetoresistive technology in the heads used to read magnetic data bits from computer hard disks. Magnetoresistance is the tendency of certain materials, composed of many magnetic and non-magnetic layers, to change their electrical resistance depending on their magnetization. As a magnetoresistive read head connected to an electric circuit moves over the surface of a disk, the magnetic fields from the patterns recorded on the disk change the magnetization (and thus the resistance) of the read head's magnetic layers. Thus the current flowing through the head changes, providing an electronic signal corresponding to the magnetically recorded data. Ideally, this electronic signal should be large, and the head should be sensitive to small changes in magnetic field; promising magnetoresistive materials to meet these needs are giant-magnetoresistance (GMR) and spin valve materials, respectively. Magnetoresistive technology has already made it possible to produce much smaller read heads than were available using inductive components, but the layers are still "thick" (hundreds of atomic layers!).

The researchers studied magnetic layers only a few atoms thick, to see if they exhibited different behavior. When layers are thicker than about 10 atoms, the atoms tend to assume their lowest-energy configuration relative to each other -- their bulk structure. However, when an overlayer is grown slowly on a single-crystal substrate, the first few atomic layers can be made to align themselves with the substrate's crystalline structure (this is called pseudomorphic growth). Because this is not the lowest-energy configuration for the overlayer's atoms, they are subject to some strain, which may change their magnetic behavior. For one example, in bulk iron/nickel (FeNi) alloys, if the concentration of nickel is below about 25%, the alloy loses its magnetic properties in a phenomenon known as invar quenching. A key question in this study was whether invar quenching occurs in thin, pseudomorphic layers.

To find out, the researchers used FeNi and cobalt iron (CoFe) samples -- magnetic films only about 5 atomic layers (10 angstroms) thick -- grown pseudomorphically on a single-crystal copper (001) substrate. (A 001 crystal face has atoms arranged in squares with another atom in the center of each square, resembling the 5-face on a six-sided die.) They magnetized each sample using pulses from an electromagnet, illuminated it with linearly polarized x rays from Beamline 7.0, and measured the energy spectrum of photoelectrons emitted from the surface. Then they reversed the sample magnetization using electromagnet pulses with the opposite field and took another spectrum. The difference between these two spectra divided by their sum is the magnetic asymmetry, or magnetic linear dichroism. By performing this entire procedure at x-ray energies preferentially absorbed by one element or the other in a sample, the researchers were able to separate the contribution of each element to the magnetic behavior of the alloy.

The preliminary results from this study show that the same invar quenching occurs in pseudomorphic layers as in bulk materials, but the elemental contributions may differ in the thinner layers. Further experiments will explore the latter possibility. The research group for this experiment included scientists from Lawrence Livermore National Laboratory, Pennsylvania State University, and the ALS, working at Beamline 7.0's Spectromicroscopy Facility.

3. REGISTRATION FOR ALS USERS' ASSOCIATION ANNUAL MEETING
(contact: alsuser@lbl.gov)

October 6 is the deadline for advance registration for the ALS Users' Association Annual Meeting on October 23-24, 1995 at LBNL. Everyone on the ALS mailing list should have already received registration materials; anyone who didn't receive them and wishes to pre-register should contact the ALS User Office (510-486-7745 or alsuser@lbl.gov). The registration fee of $125 covers attendance at the meeting, two working lunches, and the banquet Monday evening. Pre-registration is encouraged but participants may also register on the day of the meeting.

Most of the meeting's program will be devoted to presentations on recent experiments conducted at the ALS to provide a glimpse at what scientific opportunities the first third-generation synchrotron sources offer. The preliminary agenda is:

Monday Morning: Report from ALS Management, Update on Funding Prospects
Monday Midday: Vendor Exhibit during an extended lunch break
Monday Afternoon and Tuesday Morning: New Results from the ALS
Tuesday Midday: Poster Session and Vendor Exhibit
Tuesday Afternoon: Participating Research Team (PRT) Meetings

4. LONG-TERM OPERATIONS SCHEDULE
(contact: fred_schlachter@lbl.gov)

The schedule for ALS user operations through early February 1996 appears below in table form (best viewed in a monospaced font). We expect the spring shutdown to be delayed until at least mid-March, and that part of the schedule remains to be determined.

The first part of each heading is the storage ring energy in GeV. The second part specifies the bunch pattern: MB for multibunch (320 electron bunches) or 2B for two-bunch. For example, 1.9/2B refers to 1.9-GeV operation with two electron bunches. Accelerator physics time, maintenance/startup, and holidays occupy the time not accounted for in the "Dates" column, as well as some shifts on the first and last days of most periods listed.

Dates (MM/DD/YY)    1.3/MB    1.5/MB    1.9/MB    1.9/2B    Shutdown

09/19/95-10/29/95                                           Shutdown
11/01/95-11/06/95             1.5/MB
11/08/95-11/13/95                       1.9/MB
11/15/95-11/22/95             1.5/MB
11/29/95-12/04/95                       1.9/MB
12/06/95-12/11/95             1.5/MB
12/13/95-12/18/95   1.3/MB
12/20/95-12/22/95             1.5/MB
01/03/96-01/08/96             1.5/MB
01/10/96-01/14/96             1.5/MB
01/17/96-01/22/96                                 1.9/2B
01/24/96-01/29/96             1.5/MB
01/31/96-02/04/96                       1.9/MB
02/07/96-03/17/96   --------------- To be determined ---------------