NIST Tech Beat - August 3, 2007

A layer of ruthenium just a few atoms thick can be used to fine-tune the sensitivity and enhance the reliability of magnetic sensors, tests at the National Institute of Standards and Technology (NIST) show.* The nonmagnetic metal acts as a buffer between active layers of sensor materials, offering a simple means of customizing field instruments such as compasses, and stabilizing the magnetization in a given direction in devices such as computer hard-disk readers.

In the NIST sensor design, ruthenium modulates interactions between a ferromagnetic film (in which electron “spins” all point in the same direction) and an antiferromagnetic film (in which different layers of electrons point in opposite directions to stabilize the device). In the presence of a magnetic field, the electron spins in the ferromagnetic film rotate, changing the sensor’s resistance and producing a voltage output. The antiferromagnetic film, which feels no force because it has no net magnetization, acts like a very stiff spring that resists the rotation and stabilizes the sensor. The ruthenium layer (see graphic) is added to weaken the spring, effectively making the device more sensitive. This makes it easier to rotate the electron spins, and still pulls them back to their original direction when the field is removed.

NIST tests showed that thicker buffers of ruthenium (up to 2 nanometers) make it easier to rotate the magnetization of the ferromagnetic film, resulting in a more sensitive device. Thinner buffers result in a device that is less sensitive but responds to a wider range of external fields. Ruthenium layers thicker than 2 nm prevent any coupling between the two active films. All buffer thicknesses from 0 to 2 nm maintain sensor magnetization (even resetting it if necessary) without a boost from an external electrical current or magnetic field. This easily prevents demagnetization and failure of a sensor.

The mass-producible test sensors, made in the NIST clean room in Boulder, Colo., consist of three basic layers of material deposited on silicon wafers: The bottom antiferromagnetic layer is 8 nm of an iridium/manganese alloy, followed by the ruthenium buffer, and topped with 25 nm of a nickel/iron alloy. The design requires no extra lithography steps for the magnetic layers and could be implemented in existing mass-production processes. By contrast, the conventional method of modulating magnetoresistive sensors—capping the ends of sensors with magnetic materials—adds fabrication steps and does not allow fine-tuning of sensitivity. The new sensor design was key to NIST’s recent development of a high-resolution forensic tape analysis system for the Federal Bureau of Investigation (see Magnetic Tape Analysis “Sees” Tampering in Detail).

* S.T. Halloran, F.C. da Silva, H.Z. Fardi and D.P. Pappas. Permanent-magnet-free stabilization and sensitivity tailoring of magneto-resistive field sensors, Journal of Applied Physics. August 1, 2007.

It’s a truism that the world looks very different through the eyes of quantum mechanics. In a new paper appearing in Science,* an international team of scientists working at the National Institute of Standards and Technology’s Center for Neutron Research (NCNR) and the Rutherford Appleton Laboratory in Britain have detailed a particular striking example—a system that looks completely random to classical physics but that at the quantum level displays a hidden, relatively long-range, coherence. Their work demonstrates a unique solid-state device—basically a row of nickel atoms in a ceramic crystal—that might form the basis for a key element in a future quantum computer.

Classical magnetism is a function of the orientation of electron “spins” in a material that has a discrete set of quantum states. If the spins in a material tend to line up in the same direction, it’s ferromagnetic, and if they tend to alternate, cancelling out any magnetic field, then it’s antiferromagnetic. These new experiments studied magnetism in linear chains of up to 100 nickel atoms that, because of their arrangement in the ceramic, have the curious property of forming a “spin fluid” where the spin directions remain in constant flux rather than assuming a fixed configuration as in a ferromagnet or an antiferromagnet.

It’s seemingly a completely disorganized system, but, according to team member Collin Broholm, a physics professor at the Johns Hopkins’ Krieger School of Arts and Sciences, the chain has “a beautiful, underlying quantum order.” Although the spin directions may be random, the underlying wave functions that describe the quantum mechanical nature of the atoms are coherent—perfectly synchronized over the span of almost 100 atoms and 30 nanometers. In the quantum mechanical world, that’s a significant distance for coherence, hitherto seen only in exotic materials such as superconductors, superfluids and Bose-Einstein condensates.

The team was able to demonstrate this by pinging the chains with neutrons that induce small packets of magnetic excitation to propagate along the coherent segment—which can be measured indirectly by observing the scattered neutrons. They also demonstrated that they could limit the coherent region by introducing defects in the chain, either by adding chemical impurities or raising the temperature to induce thermal breaks (the experiments were conducted from 10 degrees to 120 degrees above absolute zero). Their results show that relatively large-scale regions in a solid-state material can be placed in a coherent quantum magnetic state, which might function as a “q-bit” (quantum bit) in a quantum computer. (For more on quantum computing, see www.nist.gov/public_affairs/quantum/quantum_computing.html.)

The research team included scientists from Johns Hopkins, the Department of Energy’s Brookhaven National Laboratory, the NIST Center for Neutron Research, Dartmouth College, University College London, Louisiana State University, the Rutherford Appleton Laboratory, Japan’s National Institute of Advanced Industrial Science and Technology, and the University of Tokyo.

The work was funded by the Office of Basic Energy Sciences within the U.S. Department of Energy’s Office of Science, the National Science Foundation, the Wolfson-Royal Society (U.K.), and by the Basic Technologies program of the U.K. Research Councils. For additional details on this work, see “Discovery of ‘Hidden’ Quantum Order Improves Prospects for Quantum Super Computers” www.jhu.edu/news_info/news/home07/jul07/quantum.html.

* G. Xu, C. Broholm, Y.-A. Soh, G. Aeppli, J. F. DiTusa, Y. Chen, M. Kenzelmann, C. D. Frost, T. Ito, K. Oka and H. Takagi. Mesoscopic phase coherence in a quantum spin fluid. ScienceExpress. Published on-line 26 July 2007 (10.1126/science.1143831).

Physicists at the National Institute of Standards and Technology (NIST) have induced thousands of atoms trapped by laser beams to swap “spins” with partners simultaneously. The repeated exchanges, like a quantum version of swinging your partner in a square dance but lasting a total of just 10 milliseconds, might someday carry out logic operations in quantum computers, which theoretically could quickly solve certain problems that today’s best supercomputers could not solve in years.

The atomic dance, described in the July 26 issue of Nature,* advances prospects for the use of neutral atoms as quantum bits (qubits) for storing and processing data in quantum computers. Thanks to the peculiarities of quantum mechanics, nature’s rule book for the smallest particles of matter and light, quantum computers might provide extraordinary power for applications such as breaking today’s most widely used encryption codes. Neutral atoms are among about a dozen systems being evaluated around the world as qubits; their weak interactions with the environment may help to reduce computing errors.

The NIST experiments demonstrated the essential part of a so-called swap operation, in which atom partners exchange their internal spin states, trading an “up” spin (notionally a binary 1) for a "down" spin (binary 0.) Unlike classical bits, which would either swap or not, quantum bits can be simultaneously in an unusual state of having swapped and not swapped at the same time. Under these conditions, spin swapping has the effect of “entangling” the pairs, a quantum phenomenon that links the atoms’ properties even when they are physically separated. Entanglement is one of the features that make quantum computers potentially so powerful. The swapping process is a way of creating logical connections among data, crucial in any computer.

The NIST experiment was performed with about 60,000 rubidium atoms trapped within a three-dimensional grid of light formed by three pairs of infrared laser beams arranged to create two horizontal lattices overlapping like mesh screens, one twice as fine as the other in one dimension. This created many pairs of energy “wells” for trapping atoms. The scientists attempted to place a single atom in each well, each pair with opposite spins. They then merged the paired wells to force each pair of atoms to interact with each other. Due to the rules of quantum mechanics, the merged atoms oscillate between the condition in which one atom is 1 and the other is 0, to the opposite condition. As they swap spins, the atoms pass in and out of entanglement, the key feature that enables quantum computation. This is believed to be the first time that quantum mechanical symmetry (“exchange symmetry”) has been used to perform such an entangling operation with atoms.

In these experiments the same spin-swap was done in parallel for all pairs of atoms. The next step, according to the researchers, is to develop ways to address and manipulate any pair of atoms in the lattice, which should allow for scalable computer architectures. The NIST group is continuing to work on improving the reliability of each step and on completing the logic operation by separating atoms after they interact. The research was funded in part by the Disruptive Technology Office, the Office of Naval Research and the National Aeronautics and Space Administration. The authors are affiliated with the Joint Quantum Institute, a collaboration of NIST and the University of Maryland.

*M. Anderlini, P.J. Lee, B.L. Brown, J. Sebby-Strabley, W.D. Phillips and J.V. Porto. Controlled exchange interaction between pairs of neutral atoms in an optical lattice. Nature. 448, 452-456 (26 July 2007).

Researchers at the National Institute of Standards and Technology’s Center for Nanoscale Science and Technology (CNST) have done the first theoretical determination of the dominant damping mechanism that settles down excited magnetic states—“ringing” in physics parlance—in some key metals. Their results, published in the Physical Review Letters,* 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, CNST researchers 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, the team was 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, they say, point to materials design techniques that could be used to optimize damping in new magnetic alloys.

* K. Gilmore, Y. U. Idzerda and M. D. Stiles. Identification of the dominant precession-damping mechanism in Fe, Co, and Ni by first-principles calculations. Physical Review Letters 99, 027204 (13 July 2007).

The National Institute of Standards and Technology (NIST) is making available “virtual machine images” of secure configurations of the Microsoft Windows XP and VISTA operating systems (OSs) to assist federal agencies in complying with computer security requirements mandated by the government’s Office of Management and Budget (OMB). The OS images allow federal agencies to simulate what will happen, and how critical applications will perform, when they move from their current operating environment to either of the two Microsoft OSs using security configurations mandated under OMB’s Federal Desktop Core Configuration (FDCC).

These images were created through a collaborative effort between Microsoft, OMB, NIST, the Department of Defense (DoD) and the Department of Homeland Security (DHS), and are available for download on a new Web site established by OMB. The images contain pre-configured security settings for agencies to use when testing and evaluating their applications to ensure they function effectively and securely during migration to these new operating systems.

“This resource facilitates agencies’ efforts to implement common security configurations which will boost government’s information security, improve system performance and decrease operating costs,” said Karen Evans, administrator of OMB’s Office of E-Government and Information Technology.

In addition, NIST’s National Checklist Program is working with a number of information technology providers on standardizing security settings for a wide variety of products and environments. NIST maintains more than 120 common security configuration guides used by agencies.

Frequently asked questions about the Web site, the virtual machine images and other technical information for adopting the secure Windows XP and VISTA configurations may be found at: http://csrc.nist.gov/fdcc.

The documents on which the FDCC is based are two OMB memoranda: M-07-11 of March 22, 2007, “Implementation of Commonly Accepted Security Configurations for Windows Operating Systems,” and M-07-18 of June 1, 2007, “Ensuring New Acquisitions Include Common Security Configurations.” Both may be accessed at www.whitehouse.gov/omb/memoranda/index.html.

Lead in goat blood might not be on the top of your shopping list, but for U.S. medical personnel who each year perform more than 2 million human blood measurements, Standard Reference Material (SRM) 955c from the National Institute of Standards and Technology (NIST) can’t be beat.

SRM 955c is an improved version of SRM 955b, a material clinicians already relied on heavily to provide quality assurance for lead blood measurements. Significant changes in material composition, lead levels and expanded uncertainties of the certified lead concentrations make SRM 955c an even more effective tool for use in addressing lead poisoning, a condition particularly harmful to the developing nervous systems of fetuses and young children, causing learning disabilities and behavior problems and, at high levels, seizures, coma and death.

Children can be exposed to lead from lead-based paint in older buildings, or from contaminated soil near highways where vehicles once used leaded gasoline. Lead levels in children have dropped since lead was banned from both paint and fuel, but they remain significant. In 1990 the U.S. Department of Health and Human Services (DHHS) established as a national goal reducing lead blood levels to no greater than 25 micrograms per deciliter (the equivalent of 250 parts per billion) by 2000 and no greater than 10 micrograms per deciliter (100 parts per billion) for 2010. The department’s Centers for Disease Control and Prevention (CDC) currently estimates that 300,000 American children, aged one to five years, have lead blood levels greater than the 2010 objective. Research reports also provide evidence of adverse effects at an even lower lead blood level than that of the 2010 target among children younger than 72 months.

SRM 955c is packaged as four vials of frozen blood at four progressively elevated lead concentration levels. Unlike previous issues of SRM 955 that were based on hog or cattle blood, SRM 955c is based on blood obtained from goats. The red blood cell system of an adult goat is much closer to that of a human, making it a better model for assessing proficiency for erythrocyte protoporphyrin, a biomarker of lead exposure. NIST’s partner in developing 955c, the New York State Department of Health’s Wadsworth Center Lead Poisoning Laboratory, dosed adult goats with lead acetate to produce blood pools containing lead physiologically bound to red blood cells.* The new SRM provides a better match than its predecessor to the blood samples clinicians analyze. The lead concentrations were determined by NIST using highly specialized methodology that resulted in high accuracy and low measurement uncertainty.

The changing goal line for blood lead concentration makes SRM 955c especially useful to laboratories. The lowest lead concentration level in the previous standard was 4 micrograms per deciliter or 40 parts per billion. In contrast, the lowest lead concentration level of SRM 955c is 0.4 microgram per deciliter or four parts per billion, the level of lead in an undosed animal. It is intended to represent the natural level of lead in an unexposed human population (although it is not yet known if any lead is naturally present in human blood). The lowest concentration in the SRM is 25 times lower than the DHHS 2010 goal, and will enable the development of next generation clinical methods that will be needed to accurately measure blood lead levels in children as progress is made toward eliminating lead exposure.

In addition to lead, levels two through four of SRM 955c contain added amounts of arsenic, cadmium, mercury, methylmercury and ethylmercury to facilitate future efforts to develop clinical methods to measure these toxins in human blood. At present, NIST provides information values only for the concentrations of cadmium and total mercury (including methylmercury and ethylmercury.) As values become available for arsenic, methylmercury and ethylmercury, the certificate of analysis will be updated to reflect the new information. Information values are considered “useful,” but lack sufficient data for NIST to be able to assign an uncertainty to the measurement.

*The procedure for dosing the animals and collecting blood pools is covered by an active protocol approved by the Wadsworth Center’s Institutional Animal Care and Use Committee (IAUC).

The National Institute of Standards and Technology (NIST) has released three draft Special Publications (SP) documents for public comment.

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Editor: Michael Baum

Date created: August 2, 2007
Date updated: August 3, 2007
Contact: inquiries@nist.gov