Researchers at Bell Labs, NEC Research, and Argonne have found a way to create an image of antiferromagnetism within a solid material. It could lead to a more cost-efficient evolution of advanced magnetic recording materials and technologies. As with other sensing application in our shrinking electronics, the trick is in seeing what we are doing.
In building denser electronic memories it is important to map out individual antiferromagnetic domains, regions in which the atomic magnetism lies along a particular direction. This is what the images produced using Argonne’s Advanced Photon Source do.
"Visualizing the organization of atoms and molecules in solids allows scientists to learn more about the possibilities of the materials," Bell Labs’ Eric Isaacs tells us. "The physics and chemistry of submicron devices need to be understood to take full advantage of their potential. These are crucial building blocks for technology."
NEC’s Gabriel Aeppli adds, "Historically, there have been very few practical applications of antiferromagnets because until now they have been extremely difficult to image. The new microscope makes it dramatically easier to map out antiferromagnets and analyze their structures for practical purposes."
To get their images, the team combined magnetic X-ray diffraction with X-ray focusing optics. The combination gave them images of individual antiferromagnetic spin density wave domains in a chromium single crystal at the micron scale. The Advanced Photon Source at Argonne is the most brilliant source of X-rays in the United states and it let them watch changes in chromium, the most common metal in which antiferromagnetism is observed, as it cooled below room temperature.
"We used the Advanced Photon Source to build an X-ray microscope," Isaacs tells us, "allowing us to look inside materials at dimensions below one micron."
Here’s how it works. The cross-section for nonresonant magnetic X-ray scattering depends on the antiferromagnetic modulation vector and spin polarization direction. That means that these quantities can be extracted independently. Body centered cubic chromium is a near-perfect example of a structurally simple metal showing complex magnetism. Electron spins in it order in a long-period density wave and on cooling not only undergo a transition between a disordered paramagnetic phase and an ordered Neel state but also display a transition between Neel states of mutually perpendicular spin polarizations.
This spin-flip transition dominates many physical properties of chromium below room temperature, but has been impossible to see or map. The team working at Argonne used its magnetic X-ray diffraction technique exploiting focusing optics to visualize the antiferromagnetic domains. It showed that the spin-flip transition begins at the walls between domains with orthogonal propagation directions for the spin modulation and coarsens inward.
The technique is an important first step toward understanding the relationship of microscopic domains to the bulk properties of such elements as chromium. The team sees it going beyond chromium, because being able to image antiferromagnetic domains as easily as magnetic force microscopy images ferromagnetic domains will get us into antiferromagnetic engineering.
Interplay between macroscopically observable phenomena and the configuration of domains and domain walls, magnetic and ferroelectric, is becoming crucial as feature sizes move toward the dimensions of individual domains.
In its images of antiferromagnetic domains, the team found that new
types of domains appear through growth from the walls between domains of
a type already present at room temperature. Now they want to learn how
the walls affect the passage of electric current. Learn this and they will
be onto new types of nanoscale devices.
The height of a continuous, one-dimensional column of liquid gallium in the tube (about 75 nm diameter) varies linearly and reproducibly in this temperature range. The expansion coefficient is the same as for gallium in the macroscopic state.
Gallium was chosen because it has one of the greatest liquid ranges of any metal, staying in this state from 29.78 to 2403 degrees C, and has a low vapor pressure even at high temperatures.
They determined how dependable a temperature indictor gallium-in-a-tube would be with a microscope equipped with a Gatan holder and twin heating system. Their gallium column had a continuous length up to 7560 nm. Increase or decrease temperature in the 50-500 degree C range and the level moves consistently.
The Japanese team calculated how to calibrate the nanothermometer. Take it as 58 (the level at this temperature was used as a reference) plus the difference in height in nanometers of the gallium column at 58 degrees C and at the present temperature, all divided by 0.753.
Changes in the length and diameter of the carbon nanotube itself can be disregarded because of the minute linear expansion coefficient of graphite. The height of the gallium column is determined strictly by its volume at that temperature.
Because the gallium meniscus level in a carbon nanotube moves linearly and reproducibly with temperature within the given range, it meets the requirements of a filled-system thermometer. It should be feasible to read the temperature recorded by the nanothermometer in situ with the help of a scanning electron microscope. The walls of the carbon nanotube and the space inside as well as the gallium level can be clearly seen, even if you are using an instrument operated at 10 keV.
The nanothermometer would extend temperature measurement in the small
world beyond the 4-80 K range attainable by resistance micrometer-sized
cryogenic thermometers. It should be easy to use since the gallium meniscus
is almost perpendicular to the inner surface of the carbon nanotube and
the liquid column is continuous and long.
Seeing fuel injection is critical to efforts to produce cleaner and more efficient engines. Engineers must be able to understand the structure and dynamics of the fuel sprays if they are to optimize the injection process, increase fuel efficiency and reduce pollutants. Twenty years of substantial improvement in laser diagnostics still leaves the region close to the nozzle obscure.
A team of US-German researchers got a good enough look to discover that high-pressure fuel sprays are supersonic under typical fuel injection conditions--something that would not have been recognized without the quantitative information they derived. The team of researchers at Argonne, Cornell, and Germany's Robert Bosch used monochromatic X-radiography to probe high-speed fuel sprays and show the generation of shock waves.
It captured a series of images of the spray at 5 microsecond intervals as it propelled outward, generating supersonic shock waves. It studied a high-pressure common-rail diesel injection system of the sort used in a passenger car with a specially fabricated single-orifice nozzle. The orifice was 178 micrometers in diameter and the injection pressure could be set between 50 and 135 MPa.
What the shock waves mean for dispersal of the fuel and its combustion properties is still up in the air, but how they got their images is useful.
The team got its time-resolved radiography in two ways. In one, it scanned with a focused, small X-ray beam and a fast point detector in a line-of-sight manner at a bending magnet beamline at Argonne’s Advanced Photon Source.
In the other way, it used a beam of 1 percent band pass and extended size along with a microsecond framing area detector (Cornell’s Pixel Array Detector) at a beamline of the Cornell High Energy Synchrotron Source (CHESS). The area detection method proved to be the only practical technique to visualize the shock waves induced by the fuel spray.
Fuel was injected into a spray chamber filled with an inert gas (sulfur hexafluoride) at 1 atm pressure and room temperature. The heavy gas allows simulation of the dense gas environment of a diesel chamber without resorting to high-pressure X-ray windows. The No. 2 diesel fuel was blended with an X-ray contrast-enhancing cerium-containing compound.
With single-wavelength X-rays, the analysis of the transmission of the attenuating material--the fuel--gives an exact measure of the mass in the X-ray beam. The researchers saw a distinct boundary between the ambient gas and the leading edge of the fuel spray because the fuel mass abruptly increased from zero to nearly five micrograms. The interface was sharp enough to ease calculation of the apparent speed of the leading edge. A highly concentrated fuel region just after the leading edge (an extremely sharp peak) showed an accumulation of droplets as they impacted on the ambient gas at the leading edge.
The team is sure the method will lead to broader applications. It paves
the way to directly study the complete range of fluid dynamics inside and
close to high-pressure liquid sprays. Time-resolved X-radiography should
also be useful in characterizing highly transient phenomena in optically
dense materials such as dense plasma and complex fluid-gas interactions.
Researchers at the Northwestern University are currently working to come out with a technology which has the ability to detect pollutants in air and water. Efforts are under way to bring out a diffraction based chemosensors. This invention offers an inexpensive, versatile, environmentally robust, and chemically specific approach to real-time pollutant monitoring.
The technology is based on the selective recognition of targeted environmental contaminants (organic compounds and trace level metals) using micropatterned mesoporous sensor materials coupled with quantitative detection through refractive index modulation. Micro-patterned thin-film sensors are fabricated from a wide range of chemoresponsive materials, using soft lithographic techniques. Interaction of coherent light with the photonic-lattice sensors produce diffraction patterns which report on the local environment of the lattice. Changes in the "diffraction efficiency" of a lattice induced by interaction with a volatile or condensed-phase analyte provides the sensing modality. The technology enables determination of contaminants in air and aqueous media. Selection of appropriate sensor materials permit target contaminant discrimination in mixtures, for organic and metal species. Significant increases in detection limits are achieved by employing resonance-enhanced diffraction and sensing. Experimental measurements indicate detection limits on the order of 100 nM.
Researchers Joseph Hupp and Keith Stevenson believe that this technology provides a versatile framework of sensor design and optical detection that can be tailored to any atomic or molecular analytes. Application of these methods to assay biological materials such as proteins and oligonucleotides should also prove valuable. These devices can be configured and be made portable, robust and inexpensively.
The invention provides a remarkably simple and low-cost means for sensing any atomic or molecular analytes at low levels in air and aqueous media. It finds its application in water and air quality assurance testing of effluents from manufacturing, water treatment and other facilities, chemical sensing, biological assay, and medical diagnostic screening.
The technology has been demonstrated with organic and metal species.
Enhanced detection selectivity and sensitivity protocols have been established.
Patent application is in process and Northwestern is seeking corporate
partners to develop and commercialize this technology to its fullest commercial
potential.
Genetic and proteomic screening with gene-chips and proteomic arrays allow researchers to peer into the genetic code of individuals and develop leads for important therapeutic agents in the pharmaceutical industry. Current technology uses arrays of either proteins or DNA on the micrometer level as screening tools for analyzing DNA, protein-protein interactions and for drug testing. Miniaturizing these arrays could dramatically improve their capabilities.
A research team at Northwestern utilized a process invented at the university called Dip-Pen Nanolithography. With this technology, the team was able to make arrays of proteins with features more than 1,000 times smaller than those used in conventional arrays. This leads to nanoarrays with more than 1 million times the density of current commercial microarrays.
Led by Chad Mirkin, director of Northwestern's Institute for Nanotechnology, the research team joined forces with Professor Milan Mrksich of the University of Chicago and his group. The combined teams showed that the novel arrays could be used to study important biological processes, such as cell adhesion.
"Our technology opens up many new possibilities for detection and understanding the interactions of biomolecules with each other and synthetic agents," says Mirkin. "We have developed a simple way of recognizing complex materials. Once the pattern of protein dots that matches a particular biomolecule or structure is known, we can build a detector for that biomolecule. This means that instead of testing for anthrax DNA, which requires a lot of processing, we might be able to test for the anthrax spore itself. Mirkin adds "creating patterns on a sub-micrometer level is important. More detailed questions can be asked and answered when working on the nanometer scale. This is a fundamental advance in biorecognition."
Mirkin's method of Dip-Pen Nanolithography allowed the researchers to use an atomic force microscope tip as a nano-pen to write out a tiny protein array on a gold surface. With an array of protein ‘dots’ as small as 100 nanometers in diameter, the gold surface in between the dots was processed to prevent it from absorbing target proteins and disturbing the readings. When an array on a chip was exposed to protein targets in solution, the protein on the substrate (16-mercaptohexadecanoic acid or MHA) bound its complementary proteins (lysozyme and rabbit immunoglobin). The atomic force microscope then read the chip and recorded a match where a change in height was detected.
A report on the protein nanoarrays was web-published on Feb. 7, on the
Science Express website. The research was supported by the Air Force Office
of Scientific Research, the Defense Advanced Research Projects Agency,
the National Science Foundation and the National Institutes of Health.
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