Electron and Optical Physics Division

Technical Highlights

• First Two-Photon Doppler-Free Measurement of the He 1¹S - 2¹S Transition. As a complement to our EVU and VUV efforts, we have just completed the first 2-photon Doppler-free measurement of the 1 ¹S - 2 ¹S transition in He at 120 nm. The 120 nm radiation was generated by harmonic up-conversion of a well-characterized near infrared laser. This measurement is the first Doppler-free laser measurement in this wavelength range, representing a significant advance for high accuracy (1 part in 10⁸) VUV pulsed laser spectroscopy. This project is continuing in collaboration with the University of Connecticut, under the supervision of Prof. Edward Eyler. The upcoming second generation measurement will be more accurate by nearly an order of magnitude. This measurement is of fundamental importance to the atomic physics community, as it provides a stringent test of QED calculations in the simplest 3-body, 2-electron system. (S. Bergeson, T.B. Lucatorto, and J. Wen)
• In-situ Optical Constants. To broaden our EUV optics calibration program, we have recently installed a thin-film deposition system with a load-lock to the sample chamber of our beamline. This allows us to make accurate measurements of the complex index of refraction of films that have not incurred any contamination from exposure to air. These measurements allow developers to improve the design and make more accurate predictions of the performance of multilayers. We have begun our activity by making measurements on the most important multilayer constituent materials. (C. Tarrio, T.B. Lucatorto).

Figure 1. Reflectance of a 92 nm thick Si film on float glass as measured at 50 eV photon energy (squares) and the fit to the data (solid line).

• Infrared Spectromicroscopy Facility Established at SURF. A new infrared (IR) beamline and spectromicroscope has been installed at SURF. The NIST facility consists of a Fourier Transform IR (FTIR) spectrometer coupled to an IR microscope, both housed in a clean room to provide freedom from sample contamination. As has been demonstrated several years ago at the first operational spectromicroscopy facility on the National Synchrotron Light Source at Brookhaven, synchrotron sources can provide brightness in the IR region of the spectrum that is several of orders greater than that obtained from conventional thermal sources. The NIST facility will be available to both internal and external user to provide microscopic chemical analysis on samples of industrial, forensic, and scientific interest. The superior brightness afforded by synchrotron radiation will allow a large increase in the sensitivity of the analysis. (U. Arp, T. B. Lucatorto, and J. Wen)

Figure 2. A 5.4 × 103 kg (6 ton) segment of the lower magnet yoke of SURF II is removed. R. Madden operates the lifting crane as L. Hughey (left) prepares to guide the segment onto an "aero-plank" load-moving system, which then transports it from the SURF beam hall out of the Radiation Physics Building through the back loading bay. During October and November 1997, the SURF operations team disassembled and removed the 1.3 × 105 kg (140 ton) magnet system by this technique.

• SURF II Being Converted to SURF III. The present SURF II facility, built in 1974, is undergoing a major overhaul which is designed to improve its performance for radiometry and other scientific applications. The light at SURF II was officially turned off on September 5, 1997 to begin the dismantling of the vacuum systems and the electromagnet. As of November 1, 1997, 1.3 × 10⁵ kg (140 ton) electromagnet system has been completely removed.

  Pieces of the new magnet core iron are due to arrive in 1997. We hope to finish assembly of the magnet core and make exacting magnetic field measurements in March and April 1998. Commissioning of the SURF III storage ring is scheduled to begin in May 1998.

  The most important aspect of the SURF III concept is improved radiometric accuracy. The magnitude and angular distribution of the flux radiated by SURF III will be much more accurately characterized through a better knowledge of the electron energy and trajectory in the storage ring and improved methods of determining the electron current. The improved accuracy of the electron orbit will be achieved by more stringent mechanical tolerances on the magnet, better steel, and optimized magnet design. As a result of the improved iron in the magnet, a significant gain in field strength will also be realized, allowing the maximum achievable energy of the electrons in the storage ring to be increased from the 300 MeV for SURF II to close to 400 MeV for SURF III. This will allow radiometry and scientific applications to be carried out down through the water window to a wavelength of about 2.5 nm (~500 eV). (M. Furst, A. Hamilton, L. Hughey, R. Madden, A. Raptakes, and R. Vest)
• Activity in Transfer Standard Detector Calibrations. New photodiodes utilizing materials not presently in wide use have been investigated for possible use as transfer standard detectors. A new apparatus for the SURF II detector calibration beamline has enabled the measurement of detector spatial uniformity or the profile of the incident radiation. This system will also allow the employment of a new calibration technique, which will extend the calibration capabilities further into the soft x-ray region with the advent of SURF III.

  Studies have been conducted to understand the relationship between photoemission and the proper use of semiconductor photodiodes, an important consideration in far ultraviolet detector radiometry.

  Filter radiometer photodiodes useful in solar physics and in plasma diagnostics have been extensively characterized in collaboration with industry and academia. Studies of special silicon photodiodes have led to the determination of the internal yield of silicon in the 160 nm to 254 nm spectral region, important in the development of absolute detectors for lithography and medical applications in this region.

  Twenty-eight calibrations of transfer standard detectors were performed during the year for applications in astronomy, aeronomy, solar physics, and plasma diagnostics. A number of special radiation optical filters were also characterized in research collaborations. (L.R. Canfield and R. Vest)

Construction of Nanoscale Physics Facility. Laboratory renovations in the Metrology building were completed this year for a new Nanoscale Physics Facility in the Electron Physics Group. The new facility will allow a new level of nanoscale measurements to be performed to elucidate the physics of electron confinement and transport in nanoscale structures and devices. Our measurement capability will be based on a cryogenic scanning tunneling microscope operating at temperatures as low as 2 K. A vector superconducting magnet surrounding the microscope will generate rotating magnetic fields up to 10 T. Facilities for MBE (Molecular Beam Epitaxy) fabrication of metal and semiconductor structures are incorporated in this system with in-vacuum transfer of fabricated samples, as shown in Fig. 3.

Figure 3

This new measurement capability will allow very high energy-resolution measurements together with atomic scale positional resolution that will be exploited to study electron states and conduction on an atom-by-atom basis. The new measurement capability together with new MBE fabrication facilities will allow the study of a wide variety of systems, including 2-D electron gases, quantum wires and dots, and magneto-transport devices The laboratory renovation consisted of the construction of a four-office module laboratory in the Metrology basement with new air and electrical services. The renovation included the construction of a 1 m diameter pit for the superconducting cryostat system and an acoustically and electrically shielded room for the sensitive measurements. The laboratory renovations were completed this year along with the initial assembly of the vacuum systems. The construction of the cryogenic tunneling microscope will be undertaken in FY98. The facility is expected to be fully operational by the end of 1998. (J.A. Stroscio, D.T. Pierce, A.D. Davies, and R.J. Celotta)

Figure 4. Comparison between measured and modeled exchange coupling strengths. The model uses measured roughness data along with ideal coupling strengths that are very close to theoretical predictions.

• Fe/Au/Fe(100) Magnetic Exchange Coupling. The magnetic coupling between ultrathin magnetic films is an essential element in the latest generation of magnetic sensors and magnetic storage devices. However, ever since the discovery of indirect oscillatory exchange coupling between magnetic films separated by nonmagnetic spacer layers, researchers have been puzzled by the large, order of magnitude, discrepancy between measured exchange coupling strengths and theoretically predicted values. We have found that there is nothing wrong with the theory, but it requires growing nearly ideal, atomically precise, multilayers in order to attain the large predicted coupling strengths. To achieve this high quality of growth, lattice matched Fe/Au/Fe sandwich structures were grown on nearly perfect Fe single crystal whiskers. The samples were grown with a wedge shaped Au spacer layer of varying thickness so that the thickness dependence of the exchange coupling could be captured in a single image. Coupling strengths were measured by taking images of the magnetic structure for various applied magnetic fields using a unique confocal magneto-optic microscope developed in the Electron Physics Group specifically for this project. The measured coupling strengths were an order of magnitude larger than previously measured for Fe/Au/Fe, but the values were still slightly less than predicted. This remaining difference was due to a small amount of residual roughness. Reflection high energy electron diffraction (RHEED) measurements of this roughness showed that the measured exchange coupling was the average of the coupling strengths from the various Au film thicknesses. The final result showed excellent agreement with theortical prediction and should lead to a better understanding of how these values are affected by the atomic scale roughness of the films. (J. Unguris, R.J. Celotta and, D.T. Pierce)
• Polarized Light Emission from the Metal-Metal STM Junction. Macroscopic magnetic properties that one can measure are controlled by microscopic properties for which there are insufficient direct measurement techniques. Technological advances in higher density magnetic information storage and smaller magnetic devices are driving magnetic measurement requirements further toward nanometer resolution. The addition of magnetic contrast to scanning tunneling microscopy is a widely recognized challenge that would not only allow high spatial resolution magnetic imaging, but would also allow correlation of magnetic microstructure with topographic and spectroscopic properties measured by the STM (Scanning Tunneling Microscope).

  A few years ago, it was reported that STM induced luminescence was emitted from a tunnel junction, formed by a W tip and a ferromagnetic Co thin film. Further, the radiation had a circularly polarized component the sign of which was related to the direction of magnetization. The promise of this intriguing result for magnetic imaging on the nanometer scale led us to make further measurements of this type. Our goals were 1) to test the generality of this effect by measuring a different ferromagnetic material, 2) to eliminate surface roughness as a possible source of changes in circular polarization, and 3) to fully characterize the polarization of the STM induced luminescence to try to understand the underlying mechanism.

  To accomplish the first two of these goals we use an Fe(001) whisker which we have shown with RHEED and STM to be a high quality single crystal that has a very flat surface with terrace widths of approximately 1 µm. The polarization was fully characterized by measuring the Stokes parameters of the luminescent radiation. The results for Fe(001) showed that the luminescence light was fully polarized within experimental uncertainty. It was primarily linearly polarized in the plane defined by the tip axis and light emission axis. There was a small, circular polarization component, but there was no change in the circular polarization that could be associated with a change in magnetization within an experimental uncertainty of approximately 2 %. We attribute the small circular polarization of the luminescence to tip asymmetries.

  Our result is in direct contrast to previous measurements of Co. Assuming the validity of the measurements on Co, the absence of an effect on Fe does not make this a useful means to obtain magnetic contrast in general. Further, a number of factors make the measurement of the circular polarization of the luminescence vs. sample magnetization very difficult: 1) low count rates, 2) circular polarization from tip dependent asymmetries, and 3) necessity of protecting the tip during sample magnetization reversal. Other means will have to be found to obtain high resolution images in scanning tunneling microscopy. (D.T. Pierce, A.D. Davies, and J.A. Stroscio).
• Fabrication of Nanowires and Nano-Trenches by Reactive-Ion Etching of Laser-Focused Chromium. In a nanostructure fabrication technique recently developed in the Electron Physics Group, chromium atoms are focused in the nodes of a laser standing wave as they deposit onto a substrate. The result is a large, highly coherent array of lines or dots with a spacing of 213 nm and width as small as 30 nm. As part of our ongoing research in this field, we have been exploring ways to refine the nanostructures created in this process, and also broaden their applicability. One potential drawback to the structures as deposited is that there is some degree of background chromium deposition between the features. We have recently shown that this background can be removed through a relatively simple reactive-ion etch process. Once the background is removed, further etching transfers the pattern into the silicon substrate. By carefully controlling the etching process and the contrast of the chromium lines, we have found that some very striking structures can be made. These range from narrow (~80 nm) trenches in the silicon, to well-defined chromium nanowires with diameter ~68 nm atop sharp silicon ridges. This extension to our basic process demonstrates the possibility of making more interesting and useful structures, and also extending the process to other materials. (J.J. McClelland)

  

  Figure 5. Nanowires and nanotrenches created by reactive-ion etching of laser focused chromium. (a) Cr wires ~68 nm wide on narrow Si ridges; (b) highly regular array of Cr ribbons on Si; (c) nanotrenches ~80 nm wide cut into Si.
• Atomic-Scale Electronic Structure Measurements of Magnetic Surfaces. Tunneling spectroscopy measurements with the scanning tunneling microscope yield information about the electronic structure of surfaces on the atomic scale. We have taken advantage of unique tunneling spectroscopy characteristics of transition metal bcc(001) surfaces to study local electronic properties of magnetic systems. These surfaces support a surface state near the Fermi energy that gives rise to a sharp tunneling conductance peak. The state originates from a nearly unperturbed d orbital that extends into the vacuum region and is exchange split on magnetic surfaces, leading to two spin-polarized states at different energies. The energy position of the spin-polarized states reflects the strength of the local exchange splitting and thus provides some information about the local magnetic properties. We have investigated this state on Fe(001), Cr(001), and FeCr(001) alloy surfaces.

  Figure 6. The top two tunneling conductance spectra in Fig. 6 (on the left) are representative of spectra from Cr(001) and Fe(001) surfaces. The sharp conductance peak in these spectra is due to the surface state. We identified the surface-state origin through a collaboration with M. Weinert at Brookhaven National Laboratory. The surface state is spin-polarized for both Cr and Fe, however in both systems only one of the states is close enough to the Fermi energy to be observed in tunneling spectroscopy measurements. Because of the unique energy positions of the states, we can use these conductance signatures has a means of chemical identification.

  Tunneling spectroscopy measurements of a Fe/Cr alloy surface allow us to measure the electronic structure around a single isolated magnetic impurity. Deposition of Cr on Fe at elevated temperatures leads to a surface alloy. Submonolayer Cr coverages results in embedded single Cr impurities decorating the Fe surface. The bottom two spectra in Fig. 6 compare the tunneling conductance characteristics on top of and away from an embedded Cr impurity. When the tip is over the Fe region of the surface and away from a Cr impurity, a conductance peak is observed characteristic of the Fe(001) surface, as expected. The tunneling conductance spectra over a Cr impurity show an attenuated Fe(001) peak and an additional peak at a negative sample voltage. The energy position of this peak is shifted to lower energies compared to the peak position on the pure Cr(001) surface, reflecting the presence of neighboring Fe rather than Cr atoms. Experiments are underway to measure the atomic-scale lateral variation of these spectroscopy features at impurity sites. (A.D. Davies, D.T. Pierce, and J.A. Stroscio)

• Magnetic Nanowire Arrays. We have fabricated and characterized arrays of nanoscale magnetic wires of iron. Using our previously created arrays of chromium lines as a self-shadowing mask, we deposited about 20 nm to 40 nm of iron. Because the iron beam was incident at a glancing angle, only one side of each 50 nm high chromium line was coated with iron. The other side remained bare. The glancing angle was chosen so that the deposited iron wires were about half as wide as the chromium line spacing. The resulting iron wires are about 100 nm wide, 20 nm to 40 nm thick, and 150 µm in length. We observed the magnetic domain structure of these wires with SEMPA (see Fig. 7) and found the wires in single domains magnetized along the length of the wires. From the observed transitions between the two possible magnetization directions, we estimate the mean domain length to be about 140 µm. Because the width of the wires is small compared to the width of a magnetic domain wall in bulk iron, these nanowires are effectively one-dimensional objects and provide an excellent opportunity to study magnetization in systems with reduced dimensionality. (J.J. McClelland, M.H. Kelley, J. Unguris, D.A. Tulchinsky, and R.J. Celotta)

Figure 7. SEMPA image of iron nanowire array. Black/white regions are domains magnetized down/up in the figure. The bare chromium regions between the iron wires is not magnetized and appears gray.

• Quantitative SEMPA/MFM Comparison. In collaboration with the Electromagnetic Technology Division of EEEL we have begun an effort that ultimately will allow quantitative comparisons between various magnetic imaging techniques. To facilitate such quantitative comparisons, the Electromagnetic Technology Division has developed a magnetic imaging reference sample. The sample is fabricated from thin-film high density magnetic recording media onto which a test pattern is written and onto which navigation marks are lithographically patterned. The navigation marks allow repeated measurements of the same precise area of the sample, for example to check the long-term stability of a given magnetic imaging system or to make detailed comparisons between different magnetic imaging techniques. The accompanying figure shows a comparison between magnetic images of the sample obtained by both SEMPA and MFM. As a result of this work, one can use the quantitative magnetization information obtained from the SEMPA image to work toward a quantitative understanding of the magnetic contrast observed in the MFM. (M.H. Kelley, D.A. Tulchinsky, and J. Unguris)

Figure 8. SEMPA and MFM images of the magnetic imaging reference sample.