Technical Highlights
- First Measurements of Electron Scattering From Laser-Excited
Chromium. Scattering of electrons from atoms has been an important
component of atomic and molecular physics -- and a traditional emphasis of the
Electron Physics Group -- for many years. The understanding of a wide variety
of phenomena relies on modeling electron-atom scattering processes in a very
detailed way. Yet our ability to predict even the simplest electron-atom
scattering process is surprisingly poor. To address this problem, we have
concentrated on making precision measurements in a pure, state-selected
fashion to provide exacting tests of theoretical models.
In a recent publication in the Journal of Physics B, we have presented a
series of measurements of spin-polarized electron scattering from
laser-excited chromium. These represent the first electron scattering
measurements from chromium ever performed, and they have been done with
complete resolution of all the quantum states of the system. The
spin-polarization of the electrons allows resolution of the spin channels of
the scattering, and the laser excitation, which is done with circularly
polarized light, allows selection of a single energy level and angular
momentum state of the chromium atom.
The chromium atom provides a particularly interesting -- and theoretically
challenging -- target for electron scattering. The ground state configuration
consists of a half-filled 3d-shell and a half-filled 4s-shell.
Thus there are six unpaired electrons, all of which (by Hund's rule) have
their spins aligned. This gives chromium the largest ground state net spin
found in the periodic table. Because of this unique spin configuration,
comparison of the new NIST measurements with state-of-the-art calculations is
expected to lead to new insights into the role played by spin-exchange in
electron scattering from a multi-electron atom. These measurements represent
the culmination of our program of electron-atom collision studies.
(J.J. McClelland, R. Scholten, and R.J. Celotta)
- Fermi Surface Properties for Magnetic Multilayers. The geometrical
properties of the Fermi surfaces of a broad range of metals used as spacer
layers in magnetic multilayers were calculated and presented in a key review
paper. These properties are important to obtaining an understanding of the
exchange coupling between magnetic layers that allows these structures to be
used as detectors of weak magnetic fields. The use of such detectors, based on
the Giant Magneto-Resistance (GMR) effect, promises to have an enormous impact
in the magnetic recording and information storage industries.
In multilayer structures consisting of thin magnetic layers separated by
nonmagnetic spacer layers, exchange coupling can lead to anti-ferromagnetic
alignment of the magnetic moments in successive layers. If the layers are
anti-ferromagnetically aligned in zero field, they can be ferromagnetically
aligned by application of a small magnetic field. This change in alignment
leads to a change in the resistance of the films which can be easily measured.
Thus, these structures can be sensitive detectors of magnetic fields. The
field that is required to change the alignment is determined by the exchange
coupling between the magnetic layers. Understanding the exchange coupling
should allow design of devices with extremely small switching fields.
Models for the exchange coupling in magnetic multilayers predict that all
oscillatory components of the coupling derive from geometrical properties of
the Fermi surface of the spacer layer material. The calculations presented in
this publication demonstrate that all measured oscillation periods agree with
the geometric properties of the appropriate Fermi surfaces. This agreement
allows the pursuit of more detailed understanding of the exchange coupling,
and the use of this understanding to predict the optimal design of devices
based on these structures. Figure 1 shows the extremal spanning vectors
of the Cr Fermi surface for the (001) direction of Cr. These explain the
periods measured in experiments done in this Division's Electron Physics
Group. The spanning vectors are labelled by the period (in nm) of the
oscillatory coupling they lead to. (M. Stiles)
Figure 1: Calculations of the geometrical properties of the Fermi
surfaces were made for a range of metals used as spacer layers in magnetic
multilayers, illustrated (above) by Cr. These calculations provide an
understanding of the exchange coupling that allows these structures to be used
as detectors of weak magnetic fields, for example in data storage
applications.
- New Approach to the Role of Phonons in High Tc
Superconductivity. There is much debate on the microscopic mechanism
causing high temperature superconductivity. It is, however, well known that
there is a strong interaction between electrons and phonons in these systems
and that phonons are responsible for superconductivity in "normal"
metals. It is very possible that phonons play an important role in high
Tc superconductivity and it must be true that they are affected by
the electronic part of the high Tc materials.
We have introduced a new approach to the study of phonons in metals of complex
structure that takes proper account of local field effects and treats the
electrons and phonons on an equal footing. The problem is formulated in terms
of the concept of a total dielectric function that includes both the
electronic and lattice polarizabilities.
There are various conditions on the dielectric function that can influence the
possibility for a material to be superconducting such as the requirement that
the electron-electron interaction be attractive while the system is stable
with respect to lattice or electronic distortions. It may ultimately be
possible to place limits on the maximum temperature of superconductors and it
will certainly be possible to better understand the role of phonons in high
Tc materials using our approach. The first calculations using this
new approach have been carried out in collaboration with Prof. M.L. Cohen
(University of California, Berkeley) and Dr. Steven P. Lewis (State
University of New York). (D.R. Penn)
- Magnetic Domain Imaging of AlliedSignal Metglass Ribbons. The
Electron Physics Group SEMPA Facility was used to image the magnetic domain
structure of amorphous ferromagnetic ribbons produced by AlliedSignal. The
SEMPA images provided researchers at AlliedSignal with their first look at the
domain structure of the as cast Metglass material which is used extensively as
the ferromagnetic core in high efficiency and high frequency electrical
transformers. The AlliedSignal researchers were especially interested in
seeing the correlations between the domain structure and the physical
structure, especially processing defects in the Metglass. These defects affect
the magnetic anisotropy of the Metglass and the domain wall motion, both of
which can, in turn, influence the overall energy conversion efficiency of the
transformer.
SEMPA is ideally suited for looking at these correlations, because SEMPA
images the magnetization and topography simultaneously and independently. An
example of a SEMPA image from an AlliedSignal Metglass is shown in
Figure 2. The images show the intensity and magnetization from a pit-like
defect in the ribbon. Note the finer scale domain structure within the defect.
Since the initial work on static domain wall structures has been so promising,
AlliedSignal is currently negotiating a CRDA with NIST in order to expand the
current work into investigations of domain wall dynamics. The proposed work
will look at how Metglass domain structures behave in applied magnetic fields
that are oscillating at transformer frequencies. (J. Unguris,
D.T. Pierce, and R.J. Celotta)
Figure 2: SEMPA measurements show a processing defect in an amorphous
ferromagnetic ribbon (left), and its effect on the magnetic microstructure
(right). Such defects are suspected sources of energy loss in high efficiency
electrical transformers using these materials.
- SEMPA Measurements of Oscillatory Exchange Coupling in Magnetic
Multilayers. Electron Physics Group members used SEMPA to investigate the
oscillatory exchange coupling in Fe/Au/Fe(100) multilayers. Along with the
group's previous measurements of the Fe/Cr/Fe and Fe/Ag/Fe multilayer systems
the current Fe/Au/Fe results are the most precise determinations of
oscillatory coupling periods to date. The measured coupling periods are a
sensitive test of theories used to explain the exchange coupling in
ferromagnetic multilayers. These multilayers are currently of intense
interest, because of their magnetoresistive properties which make them
excellent candidates for such uses as magnetic storage read heads, low cost
magnetic field sensors and magnetoresistive random access memories.
The atomic scale ordering in the multilayers used in the SEMPA measurements
was carefully controlled by using molecular beam epitaxy to deposit the films
onto nearly perfect Fe crystal whisker substrates. The high spatial resolution
of SEMPA allows wedge shaped interlayers to be examined so that a wide range
of coupling phenomena could be observed simultaneously.
The coupling measurements for Au, Ag, and Cr interlayers are summarized in
Figure 3. This figure shows the magnetic coupling, which oscillates
between ferromagnetic (positive) and antiferromagnetic (negative), as a
function of the interlayer thickness. The advantage of present technique is
clear, when compared with coupling strength measurements from three other
leading labs. The Fe/Cr/Fe work was recently honored by being selected for
The Scientist's "Hot Paper" column which profiles work in science that
has generated an unusually high number of citations in a limited period of
time. (J. Unguris, D.T. Pierce, and R.J. Celotta)
Figure 3: NIST measurements of the oscillation of the magnetic
coupling in magnetic multilayers over a wide range of interlayer thickness are
a sensitive test of theories of these systems. Magnetic multilayers show
promise for magnetic storage read heads, low cost magnetic field sensors, and
nonvolatile random access memories.
- Chromium Atoms Focused by Lasers to Create Nanostructures on a
Surface. Researchers in the Electron Physics Group have successfully
demonstrated a new process using lasers to fabricate nanometer-size metallic
structures on a surface. Reporting their work in the November 5 issue of
Science, they describe experiments in which a chromium atomic beam is
collimated using laser cooling, and then focused in a laser standing wave
which grazes across the surface of a silicon wafer. The nodes of the standing
wave act as an array of "atom lenses," focusing the chromium atoms into a
series of lines as they deposit onto the surface. Examination of the surface
with an atomic force microscope after removal from the vacuum system has
revealed an array of chromium lines 34 nm high and 65 nm wide
(Figure 4). The lines are spaced by 213 nm, or exactly half the
wavelength of the laser light, and uniformly cover an area of almost a square
millimeter.
Figure 4: Successful laser focusing of an atom beam to deposit narrow
lines of Cr atoms on Si illustrates the possibility of fast, massively parallel
fabrication of nanostructures.
The research is the first step in the development of a wide range of
extensions and applications of nanostructure fabrication by laser focusing of
atoms. A major advantage that this technique has over other methods such as
electron beam lithography is the fast, parallel nature of the fabrication --
an entire square millimeter can be patterned in about 10 minutes. In
addition, theoretical calculations show that the ultimate feature size could
be as small as 10 nm. Large arrays of structures could also be fabricated
by using a two-dimensional standing wave to make an array of dots and scanning
the substrate. Eventual applications may include the fast, accurate
fabrication of nanostructured materials or devices for microelectronics or
micromagnetics, and the fabrication of length standards on a microscopic
scale. (J.J. McClelland, R. Scholten, R. Gupta, and
R.J. Celotta)
- Controlling Thin Film Growth at the Single Atomic Layer Level.
Present day technological demands the fabrication of structures in both the
semiconductor and magnetic industries with tolerances approaching thicknesses
of a single atomic layer of material. This results in the need for
semiconductor and magnetic films in the 5 nm to 10 nm thickness
range. Roughness in these structures, of just a few atomic layers
(approximately 1 nm) yields a 20 % thickness variation in the
structure which becomes unacceptable in the performance of the resulting
device. In the Electron Physics Group, scientists have been studying the
processes of the growth of magnetic materials and have obtained the first
atomic real space images of the growing surfaces of iron crystals.
To grow materials, we have used Molecular beam epitaxy (MBE). MBE is used to
produce artificial structures with abrupt interfaces at the single atomic
layer level. It has had great success in producing complex multi-layer
semiconductor structures, and has recently been extended to produce multilayer
metallic structures. To obtain single layer precision, the MBE technique
requires the counting of the individual atomic layers during growth. The most
common way to count these layers has relied on
reflection-high-energy-electron-diffraction (RHEED), in which the reflected
electron beam intensity is monitored in real time. One typically observes
cyclic oscillations in the RHEED intensity, which are interpreted as
corresponding to one atomic layer per cycle. To date, little has been known as
to the actual physical structure corresponding to the RHEED intensity
behavior, or even to the exact cause of the intensity oscillation themselves.
Some of the questions plaguing MBE growers are: Is one oscillation really one
layer? Do oscillations mean the material is growing one layer at a time? How
does one improve the quality of the growing material?
Our experiments have answered many of these questions. For example, it was
shown that one RHEED oscillation does correspond to one atomic layer of
material in MBE growth, but only in special circumstances do oscillations
imply that material is growing one layer at a time. Figure 5 shows a case
of poor growth (with damped RHEED oscillations) of an iron crystal
(200 nm × 200 nm) at an intermediate growth temperature of
180 °C, where individual atomic levels (atomic steps) are observed as
different shades of grey. These measurements show in detail how the mechanisms
of surface diffusion, nucleation, and growth affect the quality of the
resulting thin film. As a result of the NIST work, the RHEED measurements that
are widely used to monitor MBE growth can now be used more knowledgeably to
optimize growth processes and control film quality. (J. Stroscio and
D.T. Pierce)
Figure 5: Rough growth of an Fe film with several layers in the growth
front is illustrated.
- Influence of Thin Film Roughness on Magnetic Devices. We have
recently completed STM measurements of thickness fluctuations in chromium
films deposited on iron crystals. These data enable us to understand the
strikingly different magnetic coupling between iron layers separated by such
films. Layers of magnetic materials separated by layers of nonmagnetic
materials may couple magnetically and exhibit a giant magnetoresistance, a
property currently being exploited to obtain improved read heads for magnetic
recording. Neither the giant magnetoresistance nor the magnetic exchange
coupling mechanism is well understood, but both have been seen to be
influenced by the character of the interfaces between the layers.
In our SEMPA measurements of iron layers separated by a chromium layer of
variable thickness, we found the coupling oscillated from ferromagnetic to
antiferromagnetic as a function of chromium thickness with two oscillation
periods. Which period dominated the SEMPA measurements depended on the growth
temperature of the chromium film, as shown in Figure 6 on the next page
(100 °C growth bottom left figure, 350 °C growth bottom right
figure). The role of the deposition temperature was investigated by STM
measurements of the surface morphology of chromium films grown on iron at
various temperatures. Layer-by-layer growth of chromium films was observed by
STM for deposition at temperatures greater than 300 °C (top right figure,
600 nm × 600 nm), and rougher growth, limited by
diffusion kinetics, at lower growth temperatures (top left figure,
100 nm × 100 nm).
Figure 6: Scanning tunneling microscopy measurements of Cr thin film
thickness fluctuations resulting from different growth conditions (top) provide
the explanation for the strikingly different periodicity of the magnetic
coupling in Fe/Cr/Fe (bottom). The magnetic coupling must be optimized for
magnetic sensor applications based on giant magnetoresistance.
Taking into account the thickness fluctuations resulting from the rougher
growth of the chromium spacer layer, we can explain the widely divergent
magnetic coupling of the iron layers observed by SEMPA. Ultimately, the
knowledge of this coupling will allow the optimization of the thickness of the
nonmagnetic spacer layer to engineer a sensor using giant magnetoresistance
which responds as required for the magnetic field of a particular application.
(J. Unguris, D.T. Pierce, and R.J. Celotta)
- Eighth National Synchrotron Radiation Instrumentation Conference
(SRI-8) Convened at NIST Gaithersburg Campus. The biennial National
Synchrotron Radiation Instrumentation Conference was first organized by and
held at NIST in 1979, and has since been hosted by each of the U.S. national
synchrotron radiation facilities. This cycle has taken 14 years for the
conference to return to NIST, where it was held 23-26 August 1993. Growth in
the field of synchrotron radiation during this period is indicated by a
tripling of the number of papers presented at the conference. At the eighth
meeting, the first reports were made of operations of three new U.S.
facilities, and progress and planning reports were made concerning several
more that have been proposed. There were over 200 attendees at the meeting,
and 20 industrial firms exhibited equipment.
Synchrotron radiation sources cover the entire electromagnetic spectrum. They
are of particular interest as sources of x-ray radiation, where they provide
the practical basis for a wide range of research in biology, materials
science, chemistry, and physics. Most research on applications of x rays also
utilizes synchrotron radiation, such as x-ray angiography, microscopy, and
lithography. NIST maintains an in-house synchrotron radiation source, the
SURF II Synchrotron Ultraviolet Radiation Facility, which is a national
standard for absolute radiometry, and also supports measurement and research
programs in atomic, surface, and condensed-matter physics. In addition,
scientists from other Divisions in the Physics Laboratory and in the Chemical
Science and Technology and Materials Science and Engineering Laboratory also
make extensive use of the higher-energy radiation available at other
synchrotron facilities, both in the United States and abroad.
(R.P. Madden, SRI-8 Chair)
- Calibration Services and Device Development for Far-Ultraviolet
Radiometry. The new dual-grating monochromator system was used on the
SURF II instrument calibration beamline on three occasions during 1993.
As a result, alignment and detailed operation of this instrument have been
reduced to routine operations. With this additional experience, new
enhancements were devised to further improve the performance on this system.
Several NIST standards were calibrated in this new system in 1993.
A long-term study of Schottky-Effect photodiodes of both GaP and GaAsP was
continued. Additionally, a collaborative effort with industry has resulted in
further major advances in far UV silicon photodiode technology. A number of
new filtering materials were incorporated into the fabrication of a group of
photodiodes, resulting in novel wavelength-selective detectors for this
region. Another new processing technology was developed and optimized,
resulting in the highest resistance to radiation damage yet reported in
silicon devices. In view of this development, these detectors are being
evaluated for use as transfer standards over the entire far-UV, and will
likely supplement the present NIST standards in the near future.
Thirty-eight calibrations of transfer standard detectors were performed in
1993, for applications in aeronomy, plasma diagnostics, solar physics, and
astronomy. In addition to these calibrations, a number of special-purpose
detectors and filters were characterized as research collaborations.
(L.R. Canfield and R. Vest)
- Calibrations and Instrumentation Development at the NIST/ARPA National
EUV Reflectometry Facility. The NIST/ARPA National EUV Reflectometry
Facility at SURF II, the only such facility in the U.S. open to all
members of the EUV optics community, entered its fourth year of operation.
Over 100 calibrations were performed on this facility, on a variety of
multilayer mirrors, gratings, and photocathodes, for customers in industry,
national laboratories, and universities. Design was completed for a new
reflectometer system and construction was begun. The new reflectometer will be
able to accommodate optics up to 35 cm in diameter and 40 kg in mass.
This capability, which will be unique in the world, is necessary for the
characterization of the large optical elements required for EUV projection
lithography systems (T.B. Lucatorto, C.S. Tarrio, and
R.N. Watts).
- Design of Nanodetector Completed. The "nanodetector," a
unique soft
x-ray microscope for high-resolution imaging, is being developed in the Photon
Physics Group as a tool for the real-time visualization of EUV images. It is a
conversion microscope which converts an EUV image into a photoelectron image
which is then magnified by a low-energy electron microscope. Such a device
would be useful in EUV lithography for the testing and alignment of projector
optics and for mask inspection; it will also make it possible to perform
photoelectron spectroscopy with a spatial resolution of about 20 nm. During the
past year a special purpose ray-tracing code was developed and used to design
the electron optics, and construction of the nanodetector was
initiated (T.B. Lucatorto, F. Pollack and R.N. Watts).
- Facility Renovation Completed at SURF II. Major electrical and
architectural changes were completed at SURF II in the early spring of 1993.
The wall between the Control Room and the Power Room was removed along with a
false wall in the corner of the Control Room. A new doorway was cut through
from the present Control Room into recently acquired space in the A-wing. Prior
to the wall demolition, an entirely new electrical supply system was installed,
nearly tripling the power available at SURF as well as providing for a more
logical distribution of power. All of the work was accomplished with the loss
of only 11 user days from January 25 to February 10 during which it was
impossible to occupy the Control Room.
In addition to affording additional electrical power, the renovations made
possible the extension of the new beamline 7, the new reflectometer
beamline, into space formerly occupied by a wall and provides room for the
future construction of a beamline 10 which would end in the present Control
Room. The new doorway leads into 230 m2 of space in the A-wing.
About 75 m2 of this space will eventually be used for a new
control station and SURF staff work area. The remaining area is already in use
as office and work space for users. (A. Hamilton and R.P. Madden)
- SURF II Calibrations for Space Missions. During 1993 there were
13 instruments calibrated by seven user groups at the Spectrometer
Calibration Facility (BL-2) at SURF II. Some examples include:
A team from Johns Hopkins University calibrated the spectrograph from the
Hopkins Ultraviolet Telescope. This shuttle based experiment will be launched
in early 1994. The instrument is used to obtain spectra in the range
41.5 nm to 185 nm from faint astronomical sources such as quasars,
planetary nebulae, and white dwarfs. Temperature, density, and chemical
composition of the sources are determined from these spectra.
A Boston University-Berkeley collaboration performed the first absolute
calibration of a unique optics-free EUV spectrometer called the Gas Ionization
Solar Spectral Monitor (GISSMO). The instrument couples a neon gas ionization
chamber with an electron energy spectrometer. A position sensitive
microchannel plate detector is to determine the wavelength of the incident
ionizing photons over a wavelength range from 7 nm to 40 nm.
The Naval Research Laboratory performed a post-flight calibration of the
shuttle based SUSIM-ATLAS 2. This was the second of ten missions for the
Atmospheric Laboratory for Applications and Science which is studying solar
irradiance and atmospheric conditions over a complete 11 year solar cycle.
Another solar instrument, the Solar EUV Irradiance Comparison Experiment was
calibrated by a group from NCAR. This sounding rocket based spectrograph is
calibrated over a spectral range from 25 nm to 120 nm. The solar EUV
full-disk irradiance is being measured during the declining phase of solar
cycle 22. (M.L. Furst and R. Graves)
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