to develop techniques for fabricating nanostructures and measuring their electronic and magnetic properties.
INTENDED OUTCOME AND
BACKGROUND
The intended outcome of this work is the continuous improvement of methods for fabricating and characterizing nanometer-scale electronic and magnetic structures, as required to meet current and future needs of the semiconductor and data storage industries.
Our main tools for pursuing this program are scanning electron microscopy with polarization analysis (SEMPA) and the scanning tunneling microscope (STM). The Electron Physics Group in the Division has been a leading innovator in both of these methods, which are outgrowths of work begun at NIST in the 1970s. SEMPA enables us to use conventional scanning electron microscopy (SEM) to image nanometer-scale magnetic structure, through spin-polarization analysis of secondary electrons ejected from the sample. It has several unique capabilities that distinguish it from other magnetic imaging techniques: it is a highly sensitive, nonperturbative method, and thus is especially well suited for in situ studies of surface and nanostructure magnetization; it provides a direct measurement of the magnetization of a material region, rather than of a magnetic field; it has the high spatial resolution (about 10 nm), long working distance, and large depth-of-field characteristic of SEM; and it facilitates simultaneous measurements of the magnetization and the topography. SEMPA studies have led to a number of breakthroughs in understanding the basic mechanisms of magnetism on the micro-and nanoscale, and have also addressed near-term measurement issues faced by the magnetic data storage industry.
Our STM program is focused on understanding the electronic and magnetic properties of nanostructures on surfaces. In recent years the STM program has been particularly concerned with the magnetic multilayer materials that have been investigated by SEMPA. The complementarities of SEMPA and STM measurements have elucidated many connections between conditions of layer growth and magnetic device performance.
The main, current direction of the STM program is the course of research made possible by the recently completed Nanoscale Physics Laboratory. This laboratory permits us to measure quantum electronic structures with atomic-scale imaging resolution and high electron-energy resolution. Samples grown in situ can be measured in an ultrahigh vacuum environment with magnetic fields of up to 10 T at temperatures down to
2.3 K. Additionally, a program in Autonomous Atom Assembly is underway that allows us to fabricate highly complex and perfect nanostructures on demand.
The work reported below was performed by the Electron Physics Group. In 2006, the NIST Director selected the Electron Physics Group to serve as the nucleus for a new NIST Center for Nanoscale Science and Technology (CNST), which would have primary responsibility for activities associated with the new nanofabrication facility in the NIST Advanced Measurement Laboratory complex. In May 2007, CNST was formally established as a NIST operating unit, at the same level as the Physics Laboratory and separate from it. We are proud to see the Electron Physics Group’s fine work recognized at this level. Subsequent reports on their activities will be issued by CNST.
Accomplishments
Changing the Rings: A Key Finding for Magnetics Design
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Figure 7. Damped precessional motion of the magnetization represented on the surface of a sphere. The red curve shows the direction of the magnetization precessing around its equilibrium direction. On the left, the amplitude of the precession steadily decreases due to the presence of magnetic damping. On the right, a steady state precession amplitude is reached due to the presence of an alternating applied magnetic field balancing the damping. |
We have made the first theoretical determination of the dominant damping mechanism that settles down excited magnetic states—“ringing” in physics
parlance—in some key metals. These results point to more efficient methods to predict the dynamics of magnetic materials and to improve the design of
key materials for magnetic devices.
The ability to control the dynamics of magnetic materials is critical to high-performance electronic devices such as magnetic field sensors and
magnetic recording media. In a computer’s magnetic storage—like a hard disk—a logical bit is represented by a group of atoms whose electron “spins”
all are oriented in a particular direction, creating a minute magnetic field. To change the bit from, say, a one to a zero, the drive’s write head
imposes a field in a different direction at that point, causing the electrons to become magnetically excited. Their magnetic poles begin precessing—
the same motion seen in a child’s spinning top when it’s tilted to one side and begins rotating around a vertical axis. Damping is what siphons off
this energy, allowing the electron spins to settle into a new orientation. For fast write speeds—magnetization reversals in a nanosecond or faster—a
hard disk wants strong damping.
On the other hand, damping is associated with noise and loss of signal in the same drive’s read heads—and other magnetic field sensors—so they need
materials with very weak damping. The design of improved magnetic devices, particularly at the nanoscale, requires a palette of materials with
tailored damping rates, but unfortunately the damping mechanism is not well understood. Important damping mechanisms have not been identified,
particularly for the so-called intrinsic damping seen in pure ferromagnetic materials, and no quantitative calculations of the damping rate have been
done. So, the search for improved materials must be largely by trial and error.
To address this, we calculated the expected damping parameters for three commonly used ferromagnetic elements, iron, cobalt, and nickel, based on
proposed models that link precession damping in a complex fashion with the creation of electron-hole pairs in the metal that ultimately dissipate the
magnetic excitation energy as vibration energy in the crystal structure. The calculation is extremely complex, both because of the intrinsic
difficulty of accounting for the mutual interactions of large numbers of electrons in a solid, and because the phenomenon is inherently complex,
with at least two different and competing mechanisms. Damping rises with temperature in all three metals, for example, but in cobalt and nickel it
also rises with decreasing temperature at low temperatures.
By comparing the calculated damping effects with experimental measurements, we were able to identify the dominant mechanisms behind intrinsic damping
in the three metals, which at room temperature and above is tied to electron energy transitions. The results point to materials design techniques that
could be used to optimize damping in new magnetic alloys.
Speed Bumps Less Important Than Potholes for Graphene
For electrical charges racing through an atom-thick sheet of graphene, occasional hills and valleys are no big deal, but the potholes—single-atom
defects in the crystal—are killers. This conclusion comes from detailed maps of electron interference patterns in graphene that show how defects in
the two-dimensional carbon crystal affect charge flow through the material. The results have implications for the design of graphene-based
nanoelectronics.
A single layer of carbon atoms tightly arranged in a honeycomb pattern, graphene was long thought to be an interesting theoretical concept that was
impossible in practice—it would be too unstable, and crumple into some other configuration. The discovery, in 2004, that graphene actually could exist
touched off a rush of experimentation to explore its properties.
Graphene has been described as a carbon nanotube unrolled, and shares some of the unique properties of nanotubes. In particular, it’s a so-called
ballistic conductor, meaning that electrons flow through it at high speed, like photons through a vacuum, with virtually no collisions with the atoms
in the crystal.
This makes it a potentially outstanding conductor for wires and other elements in nanoscale electronics. Defects or irregularities in the graphene
crystal, however, can cause the electrons to bounce back or scatter, the equivalent of electrical resistance. So one key issue is just what sort of
defects cause scattering, and by how much?
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Figure 8. Comparison of an STM topographic image of a section of graphene sheet (top left) with spectroscopy images of electron
interference at three different energies. Strong interference patterns are generated by atomic scale defects in the graphene crystal (red arrows)
but only modest disturbances are caused by larger scale bumps in the sheet (blue arrows.) Analysis of the ripples shows that the electron energy in
graphene is inversely proportional to its wavelength, just like light waves. The area imaged is approximately 40 nm square.
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To answer this, we grew layers of graphene on wafers of silicon carbide crystals and mapped the sheets with a custom-built scanning tunneling
microscope (STM) that can measure both physical surface features and the interference patterns caused by electrons scattering in the crystal.
(Graphene on silicon carbide is a leading candidate for graphene-based nanoelectronics.)
The results are counter-intuitive. Irregularities in the underlying silicon carbide cause bumps and dips in the graphene sheet that lies over it rather like a blanket on a lumpy bed, but these relatively large bumps have only a minor effect on the electron’s passage. In contrast, missing carbon atoms in the crystal lattice cause strong scattering, the interference patterns rippling around them like waves hitting the piles of a pier. From a detailed analysis of these interference patterns, the team verified that electrons in the graphene sheet behave like photons, even at the nanometer scale.
‘Atomic Switch’ Experiments Expand Nanoscale Toolkit
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Figure 9. (a) An STM moves a singel cobalt atom (blue sphere) in a small molecule back and forth between two positions on a crystal surface. (b) Switching is most likely when the STM tip is positioned to the left of the cobalt atom (blue and white speckled area). |
We have used a beam of electrons to move a single atom in a small molecule back and forth between two positions on a crystal surface, a significant step toward learning how to build an “atomic switch” that turns electrical signals on and off in nanoscale devices.
A custom-built, cryogenic scanning tunneling microscope (STM) was used to perform several different types of atomic scale measurements and manipulations. A molecular chain of one cobalt atom and several copper atoms was constructed, atom by atom, on a surface of copper atoms using the STM to move the atoms. Then the STM was used to shoot electrons at the molecular chain, and its effect on the switching motion of the cobalt atom was measured. Theoretical calculations of the atoms’ electronic structure confirmed the experimental results.
We used a new technique called “tunneling noise spectroscopy”—based on our 2004 discovery that an atom emits a characteristic scratching sound when an STM is used to move the atom—to determine how long the atom stays in one place. We found that a single electron boosts the molecule above a critical energy level, allowing a key bond to break so the cobalt atom can switch positions. The cobalt atom was less likely to switch as the molecular chain was extended in length from two to five copper atoms, demonstrating that the atom switching dynamics can be tuned through changes in the molecular architecture.
We also found that the position of the STM tip is critical—switching is most likely when the STM tip is positioned to the left of the cobalt atom. This insight raises the possibility that molecular orbital analysis may be used to guide the design and control of single atom manipulation in nanostructures.
Laser Trapping of Erbium May Lead to Novel Devices
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Figure 10. A purple laser beam slows erbium atoms (the purple beam traveling right to left) emerging from an oven at 1300 °C, in preparation for trapping and cooling. |
We have used lasers to cool and trap erbium atoms, a “rare earth” heavy metal with unusual optical, electronic, and magnetic properties. The element has such a complex energy structure that it was previously considered too wild to trap. This demonstration might lead to the development of novel nanoscale devices for telecommunications, quantum computing, or fine-tuning the properties of semiconductors.
Laser cooling and trapping involves hitting atoms with laser beams of just the right color and configuration to cause the atoms to absorb and emit light in a way that leads to controlled loss of momentum and heat, ultimately producing a stable, nearly motionless state. Until now, the process has been possible only with atoms that switch easily between two energy levels without any possible stops in between. Erbium has over 110 energy levels between the two used in laser cooling, and thus has many ways to get “lost” in the process. We discovered that these lost atoms actually get recycled, so trapping is possible after all.
Erbium atoms were produced by an oven at 1300 °C. Magnetic fields and six counter-propagating purple laser beams were then used to cool and trap over a million atoms in a space about 100 μm in diameter. As the atoms spend time in the trap, they fall into one or more of the 110 energy levels, stop responding to the lasers, and begin to diffuse out of the trap. Recycling occurs, though, because the atoms are sufficiently magnetic to be held in the vicinity by the trap’s magnetic field. Eventually, many of the lurking atoms fall back to the lowest energy level that resonates with the laser light and are recaptured in the trap.
The erbium atoms can be trapped at a density that is high enough to be a good starting point for making a Bose-Einstein condensate, an unusual, very uniform state of matter used in NIST research on quantum computing. Cold trapped erbium also might be useful for producing single photons at wavelengths used in telecommunications. In addition, trapped erbium atoms might be used for “doping” semiconductors with small amounts of impurities to tailor their properties. Erbium—which, like other rare earth metals, retains its unique optical characteristics even when mixed with other materials—is already used in lasers, amplifiers, and glazes for glasses and ceramics. Erbium salts, for example, emit pastel pink light.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents |
