Showing posts with label COMPUTING. Show all posts
Showing posts with label COMPUTING. Show all posts

Friday, August 30, 2013

THE ARTIFICIAL SPIN GIVES A LOOK AT MAGNETIC CHARGED CRYSTALS

FROM:  LOS ALAMOS NATIONAL LABORATORY 
Magnetic charge crystals imaged in artificial spin ice

Potential data storage and computational advances could follow

LOS ALAMOS, N.M., August 28, 2013—A team of scientists has reported direct visualization of magnetic charge crystallization in an artificial spin ice material, a first in the study of a relatively new class of frustrated artificial magnetic materials-by-design known as “Artificial Spin Ice.” These charges are analogs to electrical charges with possible applications in magnetic memories and devices; in describing this class of materials, the new work demonstrates their utility.

Los Alamos National Laboratory staff scientist Cristiano Nisoli explained, “Magnetic technology generally concerns itself with manipulation of localized dipolar degrees of freedom,” he said. “The ability of building materials containing delocalized monopolar charges is very exciting with possible technological implications in data storage and computation.”

Honeycomb configuration helps disassemble magnetic islands

“The emergence of magnetic monopoles in spin ice systems is a particular case of what physicists call fractionalization, or deconfinement of quasi-particles that together are seen as comprising the fundamental unit of the system, in this case the north and south poles of a nanomagnet,” Nisoli said. “We have seen how arranging magnets in a honeycomb configuration allows for these charges to be sort of ‘stripped’ from the magnetic islands to which they belong and become relevant degrees of freedom.”

Nanoscale magnets prevent freezing

The unique properties of spin ice materials have fascinated scientists since they were first discovered in the late 1990s in naturally occurring rare earth titanites. The material is aptly named: the highly complex ordering of nanoscale magnets in spin ice obey the same rules that determine the positional ordering of hydrogen and oxygen atoms in frozen water ice. Both have “spin”—degrees of freedom—with frustrated interactions that prevent complete freezing, even at absolute zero.

In 2006, an interdisciplinary team of physicists and materials scientists designed the first artificial spin ice, a two-dimensional array of magnetic nanoislands that are fabricated to interact in complex ways, depending on the chosen design of the array. The islands were lithographically printed onto a substrate, arranged in a square-lattice pattern, with the north and south poles of each nanomagnet meeting and interacting at their four-pronged vertices.

New annealing process allows polarity flip

Now the same research team has developed a new annealing protocol that allows the artificial material’s full potential for highly complex magnetic interactions to be realized. The new protocol was applied to two artificial spin ice materials, one configured in a square-lattice pattern, the other in a hexagonal-honeycomb pattern with three-pronged vertices.

In the honeycomb pattern, where three magnetic poles intersect, a net charge of north or south is forced at each vertex. The magnetic “monopole charge” at each vertex influences the magnetic “charge” of the surrounding vertices. The team was able to image the crystalline structure of the magnetic charges using magnetic force microscopy.

University of Illinois physicist Peter Schiffer, who led the team, explained, “Nanomagnets are so small that their behavior becomes relatively simple. We can arrange the magnets in a particular lattice pattern—square or honeycomb—and they interact in a way that we can predict and control.”

Schiffer added, “The challenge—you have to get the nanomagnets to flip their north and south poles to show how they interact. It’s hard to force them to show the effects of interaction, since they get stuck in one particular arrangement.”

The research team’s new annealing protocol—heating the material to a high temperature where their magnetic polarity is suppressed (here, about 550 degrees Celsius) —allows the nanomagnets to flip their polarity and freely interact. As the material cools, the nanomagnets are ordered according to the interactions of their poles at the vertices.

Engineered material allows study that’s impossible in natural crystals

The collective thermal behavior of the arrays is studied through statistical mechanics, a branch of fundamental physics. As theorized, the monopole charge of each vertex was found to contribute to the order of the entire system in a manner analogous to the interactions of electric charges at the atomic scale during water ice crystal growth.

An advantage of artificial spin ice is that it can be designed in different topologies, and examined subsequently to see the effects of those topologies. That allows physicists to explore a wide range of possible behaviors that are not accessible in natural crystals.

“This work demonstrates a direction in condensed matter physics that is quite opposite to what has been done in the last six decades or so,” said Nisoli. “Instead of imagining an emergent theoretical description to model the behavior of a nature-given material and validating it indirectly, we engineer materials of desired emergent properties that can be visualized directly.”

The team’s research, led by Schiffer, also of the University of Illinois’ Frederick Seitz Materials Research Laboratory, has published its findings in the Aug. 29 issue of the journal Nature. The theoretical work for this research was performed at Los Alamos National Laboratory under Nisoli and LANL Oppenheimer Fellow Gia-Wei Chern, and at Penn State University under Vincent Crespi and Paul Lammert. The synthesis of the magnetic materials and the high temperature treatment was performed at the University of Minnesota’s Department of Chemical Engineering and Materials Science under Chris Leighton. The magnetic measurements and lithography were performed at Penn State University and the Frederick Seitz Materials Research Laboratory by graduate students Sheng Zhang and Ian Gilbert under the direction of Schiffer.

This research was supported by the U.S. Department of Energy and the National Science Foundation.

Wednesday, September 26, 2012

RELIABLE, SAFE NUKES WITHOUT EXPLOSIVE TESTING

Photo:  Nuclear Bomb Test.  Credit:  U.S. Army Signal Corps

FROM: U.S. STATE DEPARTMENT
Maintaining the U.S. Nuclear Stockpile in the Absence of Nuclear Explosive Testing
Fact Sheet

Bureau of Arms Control, Verification and Compliance

September 26, 2012

The leading methods used to maintain the United States nuclear weapons stockpile include:
The
Stockpile Stewardship Program (SSP), run by the National Nuclear Security Administration (NNSA), maintains the continued safety, security and reliability of the nation’s nuclear weapons in the absence of nuclear explosive testing. A key goal of the SSP is to increase scientific understanding of nuclear device performance, as well as the aging behavior of weapon materials and components to ensure a safe and effective nuclear deterrent.

Life Extension Programs (LEPs) extend the service life of the current weapons in the stockpile by using only nuclear components based on previously tested designs thereby eliminating the need to conduct nuclear explosive tests. NNSA, in coordination with the Department of Defense (DoD), also performs alterations and modifications to the stockpile in order to sustain the warheads that underpin the U.S. nuclear deterrent.

Advanced Simulation and Computing capabilities provide greatly increased confidence in the ability to model and evaluate the performance and safety of nuclear weapons without nuclear explosive testing. Computers have become at least a hundred-thousand times more powerful, and modern integrated design codes now more realistically capture the behavior of real nuclear devices.

Enhanced Surveillance tools and models play critical roles in providing information essential to assessing weapon safety, security, and performance changes that would affect military effectiveness. The use of data from surveillance of our nuclear weapons enables us to predict how the weapons will perform over time without using underground nuclear explosive testing.

The Annual Assessment process of the U.S. Nuclear Weapons Stockpile is the authoritative method for the DoD and NNSA to evaluate the safety, reliability, performance and military effectiveness of the nuclear weapons stockpile, and it is a principal factor in our ability to maintain a credible nuclear deterrent without nuclear explosive testing.

Infrastructure Modernization is in accordance with the Nuclear Posture Review; NNSA has identified a path for sustaining the nuclear deterrent while modernizing the supporting infrastructure without nuclear explosive testing. This modernization is implemented by focusing on recapitalization and refurbishment of existing infrastructure for plutonium, uranium, tritium, high-explosive production, non-nuclear component production, high-fidelity testing and waste disposition

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